2.4.1 Plant Nutrients in the soil:
You need to learn the following terms (words) used to describe plant nutrition:
- Chemical element
- Inorganic salt
- Solution
A plant uses chemical elements in the soil as nutrients. These elements are present if the form of salt – sometimes called inorganic or mineral salts. The salt dissolves in water and is absorbed by the roots.
Macro nutrients in plant nutrition:
Macro-nutrients are the main chemical element that plants need from the soil. Macro means large. Macro-nutrients are those that the plant need in large amounts. Macro-nutrients are found in the form of salts in the soil, when the salts are diluted with water – they become soluble and plants can absorb them. Crops absorb macro-nutrients from the soil. When crops are harvested, the nutrients they have absorbed are removed from the soil and fertilisers thus need to be applied to prepare the soil for the next crop.
The following elements are important macro-nutrients and can be supplied by fertilisers:
- N –Nitrogen
- P –Phosphorus
- K –Potassium
When you buy fertiliser, for example 2:3:2(30), this means that the ratio of the macro-elements in that fertilizer are represented as: N P K = 2: 3: 2.
Other important macro-nutrients are Calcium (Ca), Magnesium (Mg) and Sulphur (S). Most soils normally have enough of these, so there is sometimes no need to add them.
1. Nitrogen
Nitrogen is the protein of the plant and is responsible for plant growth and green leaves. A shortage of Nitrogen can easily be seen when the leaves of the plant turns yellow.
Nitrogen helps plants to:
- Form chlorophyll, the green substance of their leaves.
- Photosynthesis and produce sugars and starch
- Increase the size of stems and leaves
- Build up proteins
- Grow quickly
- Produce bigger crops, including “fodder crops” for feeding animals
- Control and regulate the uptake of P and K
Sources of Nitrogen:
- Nitrogen fertilizers
-Chemical fertilizers like: Ammonium Sulphate (NH3 S04)
-Lime stone Ammonium Nitrate (LAN)
–Urea
-Organic fertilizers like: Growmore, Kraal manure, Chicken manure, Green manure
- Deficiency symptoms of Nitrogen
Plants:
-Are small or stunted in their growth, as they cannot make the proteins they need.
-Grow very slowly
-Have yellow, weak-looking leaves
-Mature too quickly, and produce flowers and grain before they have reached a normal size.
- Effects of too much nitrogen
Plants:
-Show lush growth but they are soft and weak
-Have week stems
-Are subject to attacks by insect pests and diseases
-Produce leaves but not much grain
-Take too long to reach maturity
-Store little food – root crops and potatoes have low yields
-May die as young seedlings
- Other sources of Nitrogen
-Nitrogen can enter the soil from sources other than fertilisers. These are some of the ways in which this can happen.
i) Nitrogen fixation during thunderstorms
Nitrogen and oxygen, present in air, combine when a lightning flash occurs. Nitrous oxide gas is formed, and this dissolves in rainwater. When it reaches the soil, it is converted to nitrates the plant can absorb.
ii) Nitrogen fixation legumes (peas and beans)
These plants have swellings of nodules on their roots where Rhizobium bacteria live. The bacteria use nitrogen from the air to build up ammonium compounds and make their own proteins. The plant uses some of the nitrogen compounds that the bacteria produce. This is an example of symbiosis where two organisms live together and help each other. The plant shelters the bacteria and gives them carbohydrate food; the bacteria supply nitrogen to the plant.
iii) Nitrogen fixation by soil bacteria
Azotobacteria is a bacterium that lives in the soil, not inside a plant. It makes compounds using nitrogen from the air. The bacteria do not live long, and when they die, they decompose. The nitrogen in their proteins then becomes available to plants.
Figure 2.39: Nitrogen fixation on a plant.
2. Phosphorus
Phosphorus enhances root development and at the same time accelerates earlier maturity of the plant for example, flowering and fruit development.
Sources of Phosphorus
- Phosphorus fertilizers
-Chemical fertilizers like: Super Phosphate, Raw Phosphate (Rock)
-Organic fertilizers like: Chicken manure
- Deficiency symptoms of Phosphorus
Plants:
-Grow slowly and remain small and stunted.
-Have poorly developed roots
-Wilt quickly in dry weather because their roots are small.
-Have few flowers and sum may drop off.
-Have leaves and stems that show red or purple patches.
- Effect of too much Phosphorus
Plants:
-Too much Phosphorus may cause micro-nutrients such as zinc, iron and copper to be deficient.
3. Potassium
Potassium integrates with Nitrogen in various plant processes, for example the forming of protein. Potassium is important for the growth of root crops and tuberous crops like carrots and potatoes.
K – helps plants to:
Photosynthesis produces sugars and starch (which is very important for crop plants that store these food substances, e.g. Cassava, sugar-cane, potatoes and cereal grains). And thus, potassium helps plants to:
- Grow strong stems that stands well in high winds
- Set seed and grow well developed fruits
- Resist plant diseases
Sources of Potassium
- Potassium fertilizers
-Chemical fertilizers like: Potassium Chloride
-Potassium Nitrate
-Potassium Sulphate
-Organic fertilizers like: Burnt plant rest (coal)
- Deficiency symptoms of Potassium
-Plants cannot photosynthesize well.
-Plants remain small and stunted.
-Plants will have weak stands that may fall over.
-Leaves maybe discoloured and scorched (withered). They may have orange margins.
-The plants will fail to development fruit and seeds properly.
Figure 2.40: Potassium deficiency on maize.
- Effects of too much Potassium
Plants:
-Plants cannot absorb other nutrients properly, especially Calcium.
-Have small leaves and fruits, which do not develop normally.
4. Calcium, Magnesium and Sulphur
As mentioned before, calcium, magnesium and sulphur are also macro-elements, but plants do not need very much of each. They are usually enough in the soil and need not to be added by fertilizer, except where serious deficiency symptoms occur (normally through chemical soil analysis reports).
Figure 2.41: Deficiency of calcium on cabbage.
|
Macro- Nutrients |
Effect on plant Growth |
Signs of deficiency |
Sources of nutrients |
|
Calcium (Ca) |
Plants grow strong cell walls. Calcium is present in the cell walls of plants and is needed especially when they are growing quickly. Calcium is a base (alkali). This is why lime, which contains calcium, is applied to the soil to correct soil acidity. Calcium raises the pH value. Lime causes clay particles to form a crumb structure, which improves the drainage of clay soils. |
Plants fail to grow properly, especially at the tips of their roots and shoots, where new cells are being formed. They may also die back at the tips of roots and shoots. |
Calcium is present in limestone and chalk rocks. It is supplied in the form of agriculture lime. Witch may be ground limestone, dolomite limestone, or chalk. These all contain the salt calcium carbonate. |
|
Magnesium (Mg) |
Magnesium help plants to make chlorophyll. Helps in photo photosynthesis. |
Plants are yellow in colour; they cannot make chlorophyll properly and they cannot photosynthesize well. |
Magnesium is present in many rocks. It is released as rocks are weathered and that is why there is usually enough of it in the soil. |
|
Sulphur (S) |
Sulphur helps to make proteins and chlorophyll. It also helps legume plants to fix nitrogen by helping bacteria to form nodules on the roots. |
Plants are yellow in colour; they cannot make chlorophyll properly and they mature too quickly. |
Sulphur is present in organic material (dead plants and animals) and in some fertilizers, such as ammonium sulphate. |
Table 2.6: Effect on plant growth, deficiencies and sources of calcium, magnesium and sulphur
Micro Nutrients in plant nutrition.
Plants need tiny amounts of many other elements from the soil. These are called micro-nutrients. Micro means very small and are those the plant needs in very small amounts – a few parts by million by weight in the soil.
Examples of micro-nutrients are:
- Zinc – Zn
- Boron – B
- Manganese – Mn
- Copper – Cu
- Molybdenum – Mo
- Iron – Fe
Most soils contain enough micro-nutrients for healthy plant growth, but sometimes there can be a shortage of some elements. When this happens, signs of deficiency can appear in the plants.
|
Micro-nutrients |
Effect on plant growth |
Sings of deficiency |
|
Zinc (Zn) |
Helps: To form chlorophyll and proteins Seed formation Legumes to fix nitrogen in photosynthesis |
Yellowing of leaves Twisted growth in leaves |
|
Boron (B) |
Helps: Roots and shoots to develop Plants to absorb calcium |
Yellowing of leaves Die back at tips of roots and shoots |
|
Manganese (Mn) |
Helps: To form chlorophyll In photosynthesis |
Yellowing of leaves |
|
Copper (Cu) |
Helps: To form chlorophyll In photosynthesis |
Yellowing of leaves |
|
Molybdenum (Mo) |
Helps: To form proteins Fix nitrogen |
Leaves are narrow and deformed |
|
Iron (Fe) |
Helps: To form chlorophyll and proteins |
Yellowing of leaves |
2.7: Effect on plant growth and deficiencies of zinc, boron, manganese, copper, molybdenum and iron
2.4.2 Soil and Leaf Samples
Although soil samples can be taken at any time, except after an application of fertilizers, it has little value in diagnosing or confirming a nutrient deficiency symptom. In commercial citrus production, for instance, fertilizers are applied from July to December and, in certain cases, as late as February. Soil sampling is therefore only reliable between February and June, which is the time to take soil samples to formulate the fertilization program for the coming season. In annual crops, the timing of soil sampling depends on whether double cropping is practiced or not.
Double cropping implies the cultivation of one crop in the autumn/winter and one in spring/summer in succession to each other on the same field. In this case, soil sampling can only be done after the previous crop has been harvested. The time before planting the next crop is often less than two months and sampling the soil in good time is of the essence. If only one crop is being produced on the field, then there is more time, but soil sampling should commence shortly after the existing crop has been harvested, in preparation for the following crop. There are specific prescriptions for leaf sampling for individual crops. Consult with a fertilizer consultant and the analytical laboratory on the procedure for the specific crop you are working with. A specific example is that of leaf sampling in sunflower to detect a B deficiency. According to the guidelines, the uppermost mature leaf, without the petiole, has to be taken. If the leaf sample was taken a month after plant, it should contain at least 60 ppm B, while a sample taken at the onset of flowering should contain at least 40 ppm B. Norms to evaluate the results of leaf analyses are developed for a specific leaf type, at a specific position on the plant, taken at a specific physiological stage. This is called the diagnostic leaf. Deficiency symptoms seldom develop on these leaves.
Deficiency symptoms mostly develop on young or old leaves and reference norms are usually not available for the symptomatic leaves. Leaf analyses therefore sometimes fail to detect the deficiency and taking leaf samples have limited value. Other factors such as total biomass produced can complicate the interpretation of the results.
Soil and Leaf analysis
In veiw of what we have discussed regarding plant growth, the last two subjects addressing soil and leaf analysis, are a good way to know for sure that as a commercial farmer that you are on the right track.
Soil Analysis:
This is an analysis or a test done on the soil to determine the available nutrients for plant growth and nutrition within the soil. This includes chemical, physical and biological soil properties which make up soil health. a soil analysis can be done by a professional, or a farmer who has cultivated his land for a while and understands the soil requirements of his/her soil.
Figure 2.42: Know your soil
Leaf analysis:
This practice is one of the most precise methods to determine plant health. The plant reacts to nutrient deficiencies and exhibits that via a change in colour, stunted growth and development. It is also a way for a grower to see what the plant has successfully absorbed. Leaf analysis entails the examining of the leaf to determine the nutritional needs of the plant. Leaf colour is a good indication of plant stress or any deficiency. Also, knowing how to examine leaves is a primary tool which all farmers need in order to know what farming practices to do and when. Knowing you plant will help you to better understand what and when to do what practice. Leaf analysis shows the grower exactly what the plant has successfully absorbed. Leaf analysis also includes the analysis of tissue analysis or foliar analysis.
2.4.3 Micro and macro nutrients recommendations on plants and taking leaf and soil samples for lab analysis
Nitrogen
Citrus Nitrogen is a mobile nutrient in plants and is translocated to areas where it is required the most. The nitrogen in older leaves is moved to new leaves shortly before leaf drop. This is a natural process, which starts when the leaves are about twenty-four months old. Leaves of under the age of twenty-four months can, however, be dropped prematurely when the tree experiences a nitrogen deficit. The leaves receiving the translocated nitrogen will contain elevated levels but at the cost of the older leaves. If the leaves with elevated nitrogen concentration are picked during sampling, the leaf analysis will present an incorrect reflection of the nitrogen status of the trees.
The symptoms of this process are:
- The entire tree is a slightly lighter green than normal;
- The oldest leaves turn deep yellow in colour and drop;
- Twigs have leaves at the tips and few or none at the middle or base;
- The tree is sparsely foliated with only one or two generations of leaves;
- Excessive leaf drop occurs a week before and during a vegetative flush. The nitrogen concentration in the leaves decreases but stabilizes at a level in the “below normal” range, not indicating that this status is sustained by relocated nitrogen from leaves dropped prematurely. The leaves on the tree have a reasonable nitrogen status but this “fairly good” status is applicable to much fewer leaves.
Additional information is required to interpret the analytical data if the trees are growing under abnormal conditions or showing abnormal symptoms such as sparse foliation.
Tomato
Figure 3.1 provided a photograph of chlorosis (yellowing) developing due to nitrogen deficiency. A light red discolouration can also be seen on the veins and petioles.
Under nitrogen deficiency, the older mature leaves gradually change from their normal characteristic green appearance to a much paler green. As the deficiency progresses these older leaves become uniformly yellow (chlorotic). Leaves approach a yellowish white colour under extreme deficiency. The young leaves at the top of the plant maintain a green, but paler colour, and tend to become smaller in size. Branching is reduced in nitrogen deficient plants resulting in short, spindly plants.
The yellowing in nitrogen deficiency is uniform over the entire leaf including the veins. However, in some instances, an interveinal necrosis (between veins) replaces the chlorosis commonly found in many plants. In some plants, the underside of the leaves and/or the petioles and midribs develop traces of a reddish or purple colour. In some plants, this coloration can be quite bright. As the deficiency progresses, the older leaves also show more of a tendency to wilt under mild water stress and become senescent much earlier than usual. Recovery of deficient plants to applied nitrogen is immediate (days) and spectacular.
Magnesium
Citrus Magnesium is a mobile nutrient and the same process as with nitrogen occurs when the supply of magnesium is too low. Magnesium deficiency symptoms are more prominent on seeded cultivars. The symptoms are:
- No change in the green appearance of the entire tree;
- The oldest leaves develop yellowing from the margins and tip towards the petiole, leaving a unique inverted V-shaped green area with its broadest side at the petiole;
- Twigs have leaves at the tips and few or none at the middle or base;
- The tree is sparsely foliated with only one or two generations of leaves;
- Excessive leaf-drop occurs during the onset of spring and/or during a vegetative flush;
- Excessive leaf-drop can also occur after a foliar application of potassium. Applications of potassium will aggravate the hidden magnesium deficiency. The magnesium concentration in the leaf decreases but stabilizes at a level in the “below normal” range, not indicating that this status is sustained by relocated magnesium from leaves dropped prematurely.
Tomato
Figure 3.2 shows a photo of Mg-deficits in tomato leaves indicating advanced interveinal chlorosis, with necrosis (browning dying leaf tissue) developing in the highly chlorotic tissue. In its advanced form, magnesium deficiency may superficially resemble potassium deficiency. In the case of magnesium deficiency, the symptoms generally start with mottled chlorotic areas developing in the interveinal tissue. The interveinal leaf tissue tends to expand proportionately more than the other leaf tissues, producing a raised puckered surface, with the top of the puckers progressively going from chlorotic to necrotic tissue. In some plants such as the Brassica or mustard family, which includes vegetables such as broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, kohlrabi, mustard, rape, rutabaga and turnip, tints of orange, yellow, and purple may also develop.
Copper
Citrus Copper moves slowly through plants. When a strong vegetative flush develops, the supply of copper to the leaves formed on new shoots might be too low and copper deficiency symptoms develop. The symptom will be more pronounced on the lower leaves on a shoot and becomes less obvious towards the tip of the shoot. The typical leaf symptom of copper deficiency is abnormally large leaves. The leaves at the base of a shoot are two to four times the normal size and are sometimes the shape of a boat. When copper deficiency symptoms appear on leaves of a twig exceeding 50 cm in length, it can be attributed to a natural slow supply of copper. However, if symptoms appear on shorter twigs, the copper supply must be supplemented.
Gumming is often connected to a copper deficiency, but gumming is a natural response of the tree to many adverse conditions and not only a copper deficiency.
Tomato
Figure 3.3. provides a photograph of typical copper-deficiency symptoms in tomato leaves. The leaves are curled, and petioles bent downward. Copper deficiency may be expressed as a light overall chlorosis along with a permanent loss of turgor (wilting) in young leaves. Recently matured leaves show netted, green veining with areas bleaching to a whitish grey. Some leaves develop sunken necrotic spots and tend to bend downward. Trees under chronic copper deficiency develop a rosette form of growth. Leaves are small and chlorotic with spotty necrosis.
Iron
Citrus Iron is present in leaves as both physiologically active and inactive forms. Traditionally leaf analyses do not distinguish between these forms and will only indicate the total iron content. The availability of iron depends on external factors such as soil pH and the concentration of bicarbonates in the soil, the water and the plant.
When the soil pH is high, less iron is available to be taken up by the plant. Once taken up, the iron can be inactivated by bicarbonates. Bicarbonates are present in the irrigation water, but also in the cell-sap in the plant. Inactive iron accumulates in the leaves and forms part of the iron concentration detected during analysis. A plant can, therefore, suffer an iron deficiency although the leaf analysis indicates normal, high or excess concentrations of iron. Leaf analyses are only useful when the results indicate a low to deficient iron concentration. Iron is not relocated in the plant and deficiency symptoms develop on the newly formed leaves. The symptoms are more prominent during winter and at the lower shaded part of the canopy.
Tomato
Figure 3.4 provides a photograph of iron-deficient leaves showing severe chlorosis at the base of the leaves with some green netting. The most common symptom of iron deficiency starts out as interveinal chlorosis of the youngest leaves, which develops into overall chlorosis. These areas often develop into necrotic spots. Up until the time where leaves become almost completely white, they will recover if supplemental iron is provided. In the recovery phase, the veins are the first to recover as indicated by their bright green colour. This distinct venial re-greening observed during iron deficit recovery is probably the most recognizable symptom in all of the classical plant nutrition. Because iron has a low mobility, iron deficiency symptoms appear first on the youngest leaves. Iron deficiency is strongly associated with calcareous soils and anaerobic conditions, and it is often induced by an excess of heavy metals.
2.4.4 Identifying Symptoms of Nutrient Deficiencies
Introduction
The identification of nutrient deficiency symptoms in plants is a very powerful diagnostic tool for evaluating the nutrient status of crops. However, identification of a specific single visual symptom is seldom sufficient to make a definite diagnosis of plant nutrient status. Many of the classic deficiency symptoms such as leaf tip scorch, chlorosis and necrosis are characteristically associated with more than a single mineral deficiency. In addition, other stress factors may also cause similar symptoms. Identification remains a most useful tool.
Deficiency symptoms develop as a result of a specific physiological process that cannot be completed. In general, the deficiency symptoms appear about six weeks after the deficiency develops. In crop production, the development of deficiencies must be avoided, and strategies should be in place to monitor the nutritional status of the plants, especially in the case of trees, continuously. One such strategy is annual leaf and soil analyses. It is, however, possible for the concentration of certain nutrient elements to decline below the threshold for the crop. Supplemental to leaf and soil analyses, the plants in the field must be monitored for these hidden deficiencies. Observations must be reported when supplying information to the person preparing the fertilization program.
Symptoms of some Nutrient Deficiencies
Stress factors such as soil salinity, pathogens, and air pollution induce characteristic symptoms. Often, these symptoms resemble those of nutrient deficiency conditions. Pathogens often produce interveinal chlorosis, whereas many air pollution and salinity stresses may cause tip scorching. Although at first these symptoms might seem similar in their appearance to those of nutrient deficiencies, they do differ in detail and/or in their overall developmental pattern. Pathological symptoms can often be separated from nutritional symptoms by their distribution in the crop. If the plants are under a nutrient stress, all plants of a given type and age in the same fields tend to develop the same symptoms at the same time. If the stress is the result of a pathogen, symptom development will tend to vary between plants until the pathogen is relatively advanced.
At first glance, it would appear as if the distinction between deficiency symptoms for the 13 known essential mineral nutrients should be relatively simple. But such an assumption is incorrect. The deficiency symptoms are however quite complex because each nutrient has a number of different biological functions and each function may have an independent set of interactions. In addition, the expression of these symptoms varies depending on how acute or chronic deficiency conditions have developed. Acute deficiency occurs when a nutrient is suddenly no longer available to a rapidly growing plant.
Chronic deficiency occurs when there is a limited but continuous supply of a nutrient, at a rate that is insufficient to meet the growing demands of the plant. Most of the classic deficiency symptoms described in textbooks is characteristic of acute deficiencies. The most common symptoms of low-grade, chronic deficiencies are a tendency towards darker green leaves and stunted or slow growth. Typically, most published descriptions of deficiency symptoms arise from experiments conducted in greenhouses or growth chambers where the plants are grown in hydroponics or in media where the nutrients are fully available. In these conditions, nutrients are readily available while present, but when a nutrient is depleted, the plant suddenly faces an acute deficiency.
The interaction between nutrient mobility in the plant, and plant growth rate can be a major factor influencing the type and location of deficiency symptoms that develop. Mobile nutrients such as nitrogen and potassium, deficiency symptoms develop predominantly in the older and mature leaves. This is a result of these nutrients being preferentially mobilized during times of nutrient stress from the older leaves to the newer leaves near the growing regions of the plant. Additionally, mobile nutrients newly acquired by the roots are also preferentially translocated to new leaves and the actively growing areas. Thus, old and mature leaves become depleted of mobile nutrients during times of stress while the new leaves are maintained at a more favourable nutrient status.
The typical presence of deficiency symptoms of very fewer mobile nutrients such as calcium, boron, and iron initially develop in the growing regions and new leaves. In plants growing slowly for reasons other than nutrition (such as low light), lower levels of nutrients may be sufficient for the plant to slowly develop, maybe even without symptoms. This type of development is likely to occur in the case of weakly mobile nutrients because excess nutrients in the older leaves will eventually be mobilized to supply newly developing tissues.
In contrast, a plant with a similar supply that is growing rapidly will develop severe deficiencies in the actively growing tissue such as leaf edges and the growing region of the plant. A classic example of this is calcium deficiency in vegetables such as lettuce. In lettuce calcium deficits symptoms develop on the leaf margins (leaf margin tip scorch) and the growing region near the meristems. The maximal growth rate of lettuce is often limited by the internal translocation rate of calcium to the growing tissue rather than from a limited nutrient supply in the. When moderately mobile nutrients such as sulphur and magnesium are the limiting nutrients in the system, deficiency symptoms are normally seen on the entire plant. If the nutrient supply is marginal compared to the growth rate, symptoms will appear on the older tissue, but if the nutrient supply is low compared to the growth rate, or the nutrient is totally depleted, the younger tissue will become deficient first. Prominent nutrient elements that develop hidden deficiency symptoms are nitrogen, magnesium, copper, and iron. In the next section, the deficiency symptoms expected in citrus and tomatoes are provided as examples.
Common deficiency symptoms on tomatoes and other vegetable crops:
Phosphorus
Figure 3.5 provides a photograph of phosphorus-deficient leaves, showing necrotic spots. As a rule, phosphorus deficiency symptoms are not very distinct and thus difficult to identify. A major visual symptom is that the plants are dwarfed or stunted. Phosphorus deficient plants develop very slowly in relation to other plants growing under similar environmental conditions but without phosphorus deficiency. Phosphorus deficient plants are often mistaken for unstressed but much younger plants. Some species such as tomato, lettuce, corn and the brassicas develop a distinct purpling of the stem, petiole and the undersides of the leaves. Under severe deficiency conditions, there is also a tendency for leaves to develop a blue-grey shine. In older leaves under very severe deficiency conditions, a brown netted veining of the leaves may develop.
Potassium
Figure 3.6 provides a photograph of potassium deficiency in tomato plants showing marginal necrosis (tip scorch). Advanced potassium deficiency is seen as chlorosis of the interveinal spaces between the main veins as well as with interveinal necrosis. This group of symptoms is very characteristic of K deficiency.
Calcium
Figure 3.7 provides a photograph of calcium-deficient leaves showing necrosis at the base of the leaves. The low mobility of calcium is a major factor in determining the expression of calcium deficiency symptoms. Classic symptoms of calcium deficiency include blossom-end rot in tomato (browning or rotting of the blossom end of tomato fruit), leaf tip scorch in lettuce, blackheart in celery and death of the growing regions in many plants.
All these symptoms show soft dead necrotic tissue in rapidly growing areas, which is generally related to poor translocation of calcium to the tissue rather than a low external supply of calcium. Very slow growing plants with a deficient supply of calcium may withdraw sufficient calcium from older leaves to maintain growth with only a marginal chlorosis of the leaves. This ultimately results in the margins of the leaves grow more slowly than the rest of the leaf, causing the leaf to cup downward. This symptom often progresses to the point where the petioles develop but the leaves do not, leaving only a dark bit of necrotic tissue at the top of each petiole. Plants under chronic calcium deficiency have a much greater tendency to wilt than non-stressed plants.
Boron
Figure 3.8 provides a photograph of boron-deficient leaves showing a light general chlorosis. The tolerance of plants to boron varies greatly between species. Boron requirements necessary for one crop may be toxic to other boron sensitive crops. Boron is poorly transported in the phloem of most plants.
In plants with poor boron mobility, boron deficiency results in necrosis of meristematic tissues in the growing regions, leading to loss of apical dominance and the development of a rosette condition. These deficiency symptoms are similar to those caused by calcium deficiency. In plants in which boron is readily transported in the phloem, the deficiency symptoms localize in the mature tissues, similar to those of nitrogen and potassium.
Both the pith and the epidermis of stems may be affected, often resulting in hollow or roughened stems along with necrotic spots on the fruit. The leaf blades develop a pronounced crinkling and there is a darkening and crackling of the petioles often with exudation of syrupy material from the leaf blade. The leaves are unusually brittle and tend to break easily. Also, there is often a wilting of the younger leaves even under an adequate water supply, pointing to a disruption of water transport caused by boron deficiency.
Boron also plays an important role in the reproduction of plants. Boron deficiency sunflower will show poor seed set and in severe cases of B deficiency, it may lead to misshapen sunflower heads and even snapping off the stem directly below the sunflower head. It is therefore very important to monitor the B status of the soil and plants when sunflower is cultivated, as B deficiencies can cause severe yield losses.
Sulphur
Figure 3.9 provides a photograph of sulphur deficient leaves, showing a general overall chlorosis while still retaining some green colour. The veins and petioles show a distinct reddish colour. The visual symptoms of sulphur deficiency are very similar to the chlorosis found due to nitrogen deficiency. However, in sulphur deficiency, the yellowing is much more uniform over the entire plant including young leaves. The reddish colour often found on the underside of the leaves and the petioles have a more pinkish tone and is much less vivid than that found in nitrogen deficiency. With an advanced sulphur deficiency, brown lesions and/or necrotic spots often develop along the petiole, and the leaves tend to become more erect and often twisted and brittle.
Recommendation for rectifying Nutrient Deficiencies
Recommendations for Rectifying Nutrient Deficiencies Deficiency symptoms should never be the basis on which fertilization programs are developed. The symptoms described above should be used in conjunction with leaf and soil analyses and should form part of the process that culminates in the formulation of a fertilization management plan
The best policy is to prevent deficiencies by applying the necessary elements before or at plant. As growing conditions are, however, not always ideal, the plant may experience deficiencies in certain elements. Deficiencies often occur during fast/active growth and during the reproduction phase of a plant. Applying the necessary element to the soil at the stage of deficiency will often not resolve the problem as it takes a relatively long time for the plant roots to take up the nutrients and relocating of the nutrients in the plant where it is needed may also take some time (up to a week or more). Therefore, the deficient element(s) are most often applied by foliar application, as the plant will react on the element within 24 to 48 hours after application.
Nitrogen
Nitrogen can be supplemented as soil applications and/or foliar sprays depending on the time of the year and severity of the deficiency. When a hidden nitrogen deficiency is detected in a crop, it should be reported and rectified as part of the overall nitrogen application schedule. Nitrogen cannot be applied to the crop at any time of the year and all corrections should be made during the timeframe for nitrogen applications for the specific crop. In tree crops, where serious deficiencies are identified, one must consider the potential adverse effects on the current and next crop before deciding on a nitrogen application. In vegetable crops and field crops, N will be applied as soon as the deficiency occurs, as one cannot wait as these are annual crops. Applying N too late will reduce the uptake of N as the leaves have started their natural senescence and the produce has started to mature. Before applying N as a foliar application, you should consult with your nutrient supplier if it will still have an economical beneficial effect on the crop or not.
Magnesium
Magnesium is supplemented by soil applications and/or foliar sprays depending on the time of the year and severity of the deficiency. A magnesium deficiency in citrus can be corrected at any time of the year, as long as the correct carrier is used. Do not apply magnesium nitrate during August to 100% blossom or during March to June. Other sources of magnesium have fewer constrictions unless they contain plant-available nitrogen. In annual crops, Mg sprays will only be applied if it is to the financial benefit of the crop. It will thus be restricted to high-value vegetable crops.
Copper
Copper is applied as a foliar spray. When applied to small citrus fruit, the sap from damaged cells react, causing a darker blemish. This accentuated blemish grows with the fruit and such fruit cannot be exported and are culled at picking. Copper products, especially copper suspensions, should therefore not be applied on small and green fruit.
In annual crops, Cu sprays will only be provided if it is to the financial benefit of the crop. It will thus be restricted to high value vegetable crops.
Iron
Iron deficiency in citrus is best rectified by an application of an appropriate chelate to the soil, preferably during August. Iron chelates are expensive and an application is only economically justifiable when more than 20% of the canopy shows iron deficiency symptoms. Where drip irrigation systems are used, much less chelates is applied and the cost can be justified even as a maintenance application. When the irrigation water or nutrient solution is not acidified, use a chelate that is stable in an alkaline environment. On alkaline soils, the preferred chelates is EDDHA (ethylene di-amine, di-hydroxy tetra acetic acid), which is applied at a rate of 30g per m2 at not more than 300g per tree.
In annual crops, Fe sprays will only be provided if it is to the financial benefit of the crop. It will thus be restricted to high value vegetable crops. Q Other deficiencies Where annually crops are grown, it may be late to rectify a deficit if it is identified towards the end of the growing season. Exceptions are where Ca deficiencies are detected in crops like tomato, peppers, lettuce etc. as well as B deficiencies in sunflower. If the plants are, however, still in the seedling stage, then applying applicable foliar or soil-based fertilizers might be of value.
2.4.5 Setting up a Nutritional Program and based on recommendations as well as a Soil Utilization Plan
Nutritional Program
For the benefit of understanding the content, two nutrient programs are provided as examples. The examples provided are for citrus, representing tree crops and sunflower representing field crops. The programmes provided may be relevant to other, similar crops as the basic principles are the same. However, the values provided in the examples are not definite as these may vary between crops as well as between different farms growing the same crop. This is because soil, climate, crop, cultivars etc. vary. It is for this reason that all good fertilizer programmes are based on recommendations from soil and/or leaf samples for the crop and site.
Nutritional programme for tree crops
Citrus production, as is the case with all other crops, has a specific schedule for the application of nutrients. Certain nutrients require application at critical times, while others may be applied over a longer and less specific period. When the fertilizer applications schedule is planned all production practices must be considered. This is especially true for foliar sprays, where the application of crop
protection products must be considered. Fertilizer application must be coordinated with other production practices to ensure that the right fertilizer is applied at the right time. Various institutions will develop fertilization programs for a crop. These may vary from basic, general programs to highly specific programs. General programs may be of little value in a commercial citrus production unit. The ideal programs are formulated for a specific orchard, based on specific data from that orchard.
Such specific programs are developed based on leaf and soil analyses data for the current year, as well as historical data. The leaf and soil analytical data is usually supported by information on the previous fertilizer applications (what, how much and when) and information on the previous as well as current crop information such as yields and fruit quality. All these factors are evaluated in conjunction in order to formulate the best fertilization program. The best fertilization program is one that will result in the best possible fruit volumes and quality, and therefore give the highest economic benefit, without sacrificing sustainability. A good fertilization program will contain the following information for each orchard:
- Orchard number or reference
- Cultivar and variety
Details for soil applications:
- Quantity in grams (g) to be applied per tree
- Name of fertilizer
- Time of application
Details for foliar applications:
- Quantity in grams (g) or millilitre (ml) to be mixed per 100l water
- Name of fertilizer
- Time of application
Additional information or special instructions
Table 2.9: Fertilizer program of an orchard
Once a programme has been provided, the orchard manager needs to develop a schedule for the Implementation of the programme provided. The schedule is developed for soil applications as well as foliar sprays so-as to slot into the rest of the production programme. The programme may require adaptation to suit younger trees. Q Scheduling a Soil Application Program The best practice for citrus is to schedule soil applications on a monthly basis. Most fertilizer programmes provide the fertilizer mass or volume per month.
The fertilization programs will recommend rates per tree or per ha. In order to develop a soil program for the farm as a whole, the applications of all the orchards are summed and presented in one working document or schedule per month. Additional information is added, such as the size of each orchard and the number of trees per hectare. Using the fertilization program in the example above as a starting point, the following Soil Application Schedule can be developed.
Figure 2.10: Soil fertilization plan
It is important to note those N-containing fertilizers are not applied as a single dose, but rather spread the application as 2 to 4 applications during the month. Nitrogen is highly soluble in water and if 500g LAN per tree is applied as a single dosage, the ammonium nitrate, may scorch the roots and reduce absorption. The high concentration of ammonium nitrate can also not be absorbed within a short period of time and subsequent irrigation may leach the nitrogen beyond the reach of the roots.
The solubility and salt index of the fertilizers, the clay content of the soil, and the rooting depth of the plants are the major factors that affect the efficiency of a fertilizer application. Leaching is less of a problem in clay than in sandy soils. The buffer capacity of clay soils is better than that of sandy soils and the temporary increase in salinity due to ammonium nitrate is lower. The salt index of fertilizers indicates the necessity of splitting an application. Fertilizers with a high salt index, such as potassium chloride, should be split into multiple applications. This may not be necessary in the case of calcium nitrate, which has a low salt index.
After the Soil Application Program has been compiled, an application schedule is developed for each month. During such as exercise, it is planned which orchard will receive its application during what time of the month, taking into consideration available manpower and equipment. Please note that if a number of applications are done in the same orchard in consecutive months, care should be taken to ensure that the applications are done on more or less the same day of each consecutive month. In practice, it is difficult to apply exactly the recommended quantity per tree to all of the trees in the orchard. It is, therefore, necessary to calculate the actual average rate of the application once the application has been completed. This is done by dividing the total quantity applied to the orchard by the number of trees per orchard and thus calculating the average volume per tree. The actual average quantity per tree may vary somewhat from the recommended quantity per tree, but it is important to ensure that this variation is not larger than 10%, i.e. 618-683g per tree in the case of orchard A. Soil application programs are developed for August, September and October, with two separate programs for the application of LAN and potassium chloride in August and September.
Developing a Foliar Application Program
A foliar application program is developed in a similar manner, although the calculations are somewhat different. The recommended foliar applications are supplied in g or ml per 100l water. The size of the trees will determine the total amount of spray mixture required. Foliar feeds are applied as medium cover sprays. The larger the tree the higher the volume applied per tree and thus the volume per hectare. On average, between 1250 and 2,500 litres of spray mixture is applied per hectare on mature citrus trees using a medium-cover spray.
Figure 2.11: Foliar application program
The average actual application per hectare is calculated once the application has been done to ensure that the variance between the recommended quantity and the actual applied quantity does not vary by more than 10%. A foliar application program is developed for October as well. Note that it was stated in the fertilization program that the Manganese Sulphate and Solubor® sprays are compatible and can, therefore, be sprayed at the same time.
Maintaining Stock Levels and Placing Orders
The ordering of fertilization must be coordinated with the production manager and the administrative staff.
Ensure that documentation is provided to the responsible person in good time and ensure that the manager is aware of all orders that are to be placed. The fertilization program for the next season (remember that a season is from the beginning of August of one year to July of the next year) is usually prepared during March to June. During this period, leaf and soil samples are analysed and the results used inter alia to formulate the fertilization program. March to June is also the harvesting period for the early cultivars and a good time to evaluate the yield and quality of the late cultivars. Ordering should start during May or June to ensure that stocks are on hand when the very important application of nitrogen starts in July or August. The time required for ordering and delivery depends on the agreement between manager and supplier. Ensure however that the fertilizers will be delivered in time if orders are placed for instance thirty days prior to the application date.
Economic considerations could limit the time between ordering and delivery. If payment is made on delivery, one would not submit an order in March for fertilizer required in July.
Figure 2.12: Ordering fertilizer stock
Requirement reports, such as the one above, are developed for each fertilizer and each month, and are done for the low biuret urea required in July (foliar spray), LAN and potassium chloride required in September, the lime in October and manganese sulphate and Solubor® in October. Substitution of Recommended Fertilizers and chemicals for foliar sprays can be substituted with an equivalent elemental base provided that:
The chemistry of the replacement chemical and its reaction in the soil and on the leaf will not create unwanted side-effects; and
The recommended mass or volume is adjusted to compensate for variations in concentration of the active ingredient/s. The person responsible for the formulation of the fertilization program should be consulted before substitutions are made. Fertilizer manufacturers may also be able to assist in this regard. The rate of application for the replacement fertilizer is calculated as follows:
- The rate of application for recommended product multiplied by % active ingredient in recommended product divided by % active ingredient in the replacement product
- A rate of application for a replacement product
Figure 2.43: Fertilizer substitution
The application of fertilizers for sunflower and field-grown vegetables and other field crops differ in that from citrus, due to the fact that we replant each year. Therefore, most of the fertilizers will be incorporated into the soil during soil preparation, some can be applied with a plant and a third part can be applied after the crop is established. In some cases, especially with high-value crops or crops with specific needs, a farmer can also apply some elements as a foliar application. Learners are referred to the previous section on citrus for information on timing fertilizer orders and replacement fertilizers. In this section we investigate:
- Broadcast application of fertilizers to the soil
- Applying fertilizer during ploughing
- Band application of fertilizers with planter
- Fertiliser top dressing after planting
Foliar feeds
As for citrus, it is important to keep a record of what and when fertilizers have been applied. Some of the information to be recorded includes:
Field number – the same number as the one indicated on the soil analysis x Crop and cultivar x Details for soil applications:
- Quantity (kg) to be applied per ha
- Name of fertilizer
- Time of application
Details for band placing:
- Quantity (kg) to be applied per ha
- Name of fertilizer
- Time of application
Details for top dressing:
- Quantity (kg) to be applied per ha
- Name of fertilizer
- Time of application
Details for foliar applications:
- Quantity in grams (g) or millilitre (ml) to be mixed per 100l water
- Name of fertilizer
- Time of application
- Additional information or special instructions
Determining the amounts of N, P, K and B to apply.
During the course of this section, the information in tables 1.1 to 1.6 will be used. Table 1.1. is the information we received back after the Laboratory did the soil and plant analysis. This information will be used in conjunction with tables 1.4 and 1.5 to determine the amount of phosphorus and potassium to apply per ha of crop field. Table 1.2. provides an indication of the yield potential for sunflowers under a certain set of conditions. The higher the yield potential the more nutrients will be required and vice versa. Table 1.2. will be used in conjunction with tables 1.3 to 1.5, to determine the amount of nitrogen as well as phosphorous and potassium to apply per ha of crop land under sunflower. The information in table 1.6. provided information on the volume of boron required. Table 1.1. Test results received from the Soil Laboratory for the soil sample we took.
Determining yield potential (Table 1.2.)
To determine the yield potential for sunflower we use the information provided in Table 1.2. The first step is to identify the field. Once the field in know, gather information on the soil depth and long-term average rainfall. Let us assume the field of interest has a soil depth of 0.9 m, the average rainfall is 550 mm for the growing season and the clay content between 0 and 10%. Using this information and applying it to the data provided in table 1.2., the sunflower yield potential for the field number is 1.501-ton ha-1.
Determining the amount of nitrogen (N) needed per ha (Table 1.3.)
To determine the N required, the yield potential must be known. If we use the data in example 1.3.1., we assume a yield potential of 1.501-ton ha-1, table 1.3 indicates a nitrogen requirement of 22 kg ha-1.
Determining the amount of phosphorus (P) required (Table 1.4.)
To determine the P requirement, we need to know the yield potential for the crop as well as the amount of available P in the soil. If we again use the data in example 1.3.1., we require a yield of 1.501-ton ha-1. If we assume an available P status of 8 mg kg-1, the P requirement can be determined. Table 1.4 indicates that for a yield of 1.5 ton per hectare with a P status of 8 mg/kg, we will require 11 kg P ha-1. We thus need to apply 11 kg P per ha of sunflower.
Determining the amount of potassium (K) needed per ha for field number one (Table 1.5.)
To determine the K requirement, we need to know the yield potential and the available K in the soil. If we assume the K status of soil is 53 mh/kg and a yield of 1.501-ton ha-1, the corresponding value in table 1.5. is 10 kg K ha-1. We thus need to apply 10 kg K for every ha of sunflower we are going to plant in field number one.
Determining the amount of boron (B) needed per ha for field number one (Table 1.6.) To determine the B requirement, we need to know the clay percentage. If we assume clay content of 0 to 10%, the B requirement is given as (Table 1.6) 9 kg Borax or 6 kg Boric acid or 5 kg Sodium oktaborate per ha.
Compiling a fertilization program
The information in the previous section can now be incorporated into a fertilization program. Below is an example of such a program for sunflower.
Fertilizer Programme:
Table 2.13: Sunflower fertilizer program
Compiling a fertilizer plan for all crop fields allows that the correct type of fertilizer is stocked and available at the relevant time. Keep in mind that you are not the only farmer who is going to need the fertilizers. Place your fertilizer order well in advance time to ensure that the fertilizer will be delivered at least a month before you will need it.
Fertilizers application strategies and timing of application
In this section, the reasons for applying certain fertilizers at a specific time will be explained.
Broadcast application of fertilizers to soil
Broadcasting refers to the even distribution of lime and fertilizer before it is incorporated into the soil. Broadcasting is efficient and often the method of choice in areas with perennial plants. Lime has to be incorporated in the soil, at least two months before planting. This will allow the lime to rectify a pH problem, as the lime has a long reaction period. Phosphorus-containing products have to be applied to the soil before or with the plant, as phosphorus does not easily move in the soil. It can therefore not be applied as topdressing later on, as it will not wash into the soil easily as in the case of nitrogen. Broadcasting can be done with a tractor pulling the fertilizer spreader or by hand (Fig 1.1.). Most of the spreaders can also be used to spread other dry formula chemicals (insecticides, fungicides etc.) or even broadcasting seed. After the fertilizer has been broadcasted, it is ploughed into the soil. The two actions can also be combined into one, whereby the tractor’s spreader and plough are attached at the same time.
Placing fertilizer in the soil while ploughing
With this application, the fertilizer is placed in a continuous band into the furrow during the process of ploughing. Each band is covered as the next band is turned over. No attempt is usually made to sow the crop in any particular location with regard to the plough sole bands, as is the case with band placing of fertilizers. This method has been recommended in areas where the soil becomes quite dry up to a few centimetres below the soil surface during the growing season, and especially with soils having a heavy clay pan a little below the plough-sole. By this method, fertilizer is placed in moist soil where it can become more available to growing plants during dry seasons. This is an alternative to broadcasting. The application of lime is always done by broadcasting, while the application of fertilizers containing N, P, K and other elements are mostly done in this way. An exception on the broadcasting of lime is when the subsoil is quite acidic. The lime will be placed deep into the soil while the soil is being ripped with a heavy tooth implement, which penetrates the soil (Fig 1.2.) to a depth of 20 to 50 cm.
- Band placement of fertilizers at a plant
This method refers to the application of fertilizers into the soil close to the seed or plant. Localized placement is usually employed when relatively small quantities of fertilizers are to be applied, otherwise, it can burn the seed leading to low germination and poor stands. Localized placement reduces fixation of phosphorus and potassium in the soil. Localized placement is done with specialized planters and the fertilizer is placed to the side and often below the seed during the seeding operation.
This practice is done to give the young seedling a boost. The fertilizer is placed close to where its roots will grow. The seedling’s roots need not search for the fertilizer which has been mixed with the soil during tillage.
- Top dressing of fertilizers after planting
Nitrogenous fertilizers containing nitrates, like sodium nitrate, calcium ammonium nitrate etc. is applied as a top dressing to closely-spaced crops. In addition, urea is also top dressed. This helps in supplying nitrogen in readily available form to growing plants. Top dressing can be done with fertilizer broadcasters.
- Foliar feeding
This refers to the spraying of suitable fertilizing solutions on leaves of growing plants. These solutions may be prepared in a low concentration to supply a plant with a single nutrient or a combination of nutrients. It has been well established that all plant nutrients are absorbed through the leaves of plants and this absorption is remarkably rapid for some nutrients.
Foliar application does not result in a great saving of fertilizer but it may be preferred under the following conditions.
- When visual symptoms of nutrient deficiencies are observed during early stages of deficiency.
- When unfavourable conditions (physical and chemical) which reduce the efficiency (FUE) of fertilizers occurs.
- During a drought period where soil application could not be conducted. (soil moisture insufficient) There are certain difficulties associated with the foliar application of nutrients:
- Marginal leaf scorching may occur if concentrations of solutions are too high.
- As solutions of low concentrations (usually three to six percent) are to be used, only small quantities of nutrients can be applied during a single spray.
- Several applications are needed for moderate to high fertilizer rates, and hence
- Foliar spraying of fertilizers is costly compared to soil application unless combined with other spraying operations taken up for insect or disease control