2.5 Soil Utilization Plan

Selecting the appropriate Soil

A permanent crop like citrus, which may remain in the same soil for more than fifty years, proper selection of the soil, is essential. One is, however, limited to the soil types that occur on the farm but have to select the best from what is available. Tree crops are often not planted on ideal soils and the subsequent horticultural practices need to be developed to compensate for the deficiencies of the soil. In evaluating soils for crop production, one strives to get as close to the optimum soil characteristics as possible. This is a fairly specialized activity and should ideally be done by an experienced pedologist. This process of soil surveys on a farm should culminate in the creation of a soil map indicating positions of the various orchards overlaid onto the map produced by the soil survey. Once this is done, soil preparation and the irrigation layout can be planned. Slope – Orchard crops require the use of fairly heavy machinery, such as spray carts and picking trailers. During soil preparation, the tractors and implements used to prepare the soil must also be able to drive safely. The slope must therefore not be so severe that workers and equipment are put in danger. A flat surface with a slope of fewer than 2 degrees is however also not suitable, as surface drainage of rainwater will then be too slow. These recommendations also hold true for other crops.

In crop production, other than tree crops, slopes are often contoured (Fig 2.44.) to reduce the flow of water (runoff) across the land surface, thus reducing erosion.

Fig 2.44: Contour Ploughing 

 

Apart from the influence of the slope of the site on safety and ease of vehicle movement in the fields, it is also important for row orientation, surface drainage, and erosion. Planting on contours is no longer done and if the slope poses any potential problems with surface drainage and/or erosion, orchard layout should be adapted accordingly. The orchard roads and vehicle paths between rows are usually protected by grass-cover. If the orchard layout facilitates water movement at a moderate speed, erosion can be avoided. Row orientation is more important in the southern parts than in the northern parts of Southern Africa and can be changed to facilitate surface drainage. Also remember that in South Africa, northern slopes are warmer than southern slopes. The crop selection should, therefore, be done in accordance, namely crops with a lower tolerance to heat on the southern slopes and those crops which growth is encouraged by more heat on the northern slopes.

 

Some of the benefits of contour ploughing are:

• Soil erosion can be reduced by as much as 50%
• Increased water infiltration promotes better water quality.
• Increased soil retention encourages root development, binding the soil and preventing erosion.
• Limits the release of nutrients/particulates into nearby river systems or lakes, minimizing harmful effects such as eutrophication.
• Can disrupt wind currents, therefore reducing wind erosion.

 

Important Soil characteristics to promote crop growth

Soil Depth

In general tree crops will require a minimum soil depth of 30cm to 40cm if micro-jet irrigation is used, and 40cm to 50cm where drip irrigation is used. It is important that the layer below this potential rooting zone does not restrict drainage. The ideal soil depth varies between crops. In the case of crops like onion and cabbage, they have a shallow root system that will be restricted to the top 15 cm soil layer. Such crops can, therefore, be planted on shallow soils. Deep-rooted crops such as tree crops require an effective soil depth of at least 60 to 90 cm, with no restriction layers present. In the case of shallow-rooted crops, an effective soil depth of 20 to 30 cm is required.

In all cases, the soil requires effective drainage to prevent water logging. Where the potential effective rooting depth is limited, it can be improved by ridging. This creates a thicker layer of soil where the crop will be planted. Ridging also demarcate the path for vehicles to drive in, avoiding re-compaction of the soil after planting. Ridging lowers the preparation cost. Energy required in soil preparation increases the quadratic equation with depth; hence, costs will also increase. Where large quantities of lime are required, ridging can reduce the quantity required.

As example, where 10 tonnes of lime have to be mixed in with the top 50 cm of soil, it can be more easily and cheaply done by mixing 5 tonnes into the top 25 cm, and then ridge the field. Keep in mind that ridging makes harvesting more difficult in tree crops. Due to the undulating surface, the pickers have to walk more. Ridging improves drainage in areas with such problems and is often used in irrigation or high rainfall areas. Tomatoes, potatoes and tobacco are some of the crops which are often ridged. Ridging reduces the soil temperature around seed tubers. The deeper one goes into the soil, the cooler it becomes. If seed tubers are to be planted in the summer in very hot soils (> 30°C), they may rot. To prevent high soil temperatures, the soil is ridged to a height of 15 – 20 cm which effectively cools the soil temperature to 20°C around the seed tubers.

 

Soil Clay Content

The optimum clay content varies between crops. In the case of citrus optimum clay content is between 5% to 20% for micro-jet irrigation and 5% to 35% for drip irrigation. In general, the ideal soil type for most crops is sandy loam soils, with a clay content of 10 to 20%. Most crops will adapt to soils with higher clay content provided that no other growth factors are limiting. The use of non-ideal soils may require additional production practices, which could lead to additional costs. One should therefore ideally use a recommended soil type as far as possible. Some crops, such as potatoes and peanuts that bear their produce under the soil surface, may be more sensitive to soil clay content.

With these crops, clay soils may affect the crop in reducing the value of the crop by discolouring the crop or make it difficult to harvest. These crops are also more prone to diseases as the soil tends to remain wet for longer periods. Under dry land production crops soils with higher clay content (>25%) are often preferred, as the heavier clay soils have a better water holding capacity than sandy soils.

The clay content of soil influences its water holding capacity, (volume of easily available water), its cation exchange capacity and its aeration. These are all factors that will influence the crop. The water holding capacity influences the irrigation scheduling, yield and fruit size. The cation exchange capacity influences the frequency of fertilizer application, leaching of nutrient cations and utilisation of potassium. Poor aeration affects the root system negatively. Under anaerobic conditions, roots cannot function properly and diseases like Phytophthora proliferate. As one cannot increase or decrease the clay percentage in the soil in the field, one has to adapt to the conditions or try to improve the soil conditions. Adapting implies the use of adapted crops and applicable irrigation systems and good irrigation scheduling. Improvement may include the incorporation of organic matter to improve aeration or installing a drainage system to improve drainage.

 

Soil Stratification

During soil preparation, all forms of stratification must be removed, i.e. layers in the soil are broken up and mixed. The type and depth of these layers will determine the implements that may be required. The fewer layers present in a soil, the lower the preparation cost, as soil with little or no stratification can merely be loosened. Stratification, or layering, restricts water movement and root development, resulting in soil volumes with few roots. These volumes become water logged and create pockets in the soil where root-rot starts. These unoccupied volumes also contain water and nutrients that are unavailable to the plant due to a lack of roots. During soil preparation, all forms of layering must be eliminated. This can be done by deep ripping the soil. Special implements are needed but are worthwhile in using as they promote root growth and development.

 

Soil Salts

Content Salts may accumulate in a soil for various reasons. If salt accumulation is due to poor drainage and high levels concentrations of sodium are also encountered, such sites should be avoided. It is difficult and expensive to remove salts from soil, because the causes of accumulation also require removal. If high salt content is caused by the lack of leaching, the site may be reclaimed. Consult a specialist to determine the reclamation process, cost and impact. Calcium carbonates accumulate in the subsoil due to limited leaching. Depending on the depth of accumulation, the soil can still be utilised successfully.

Soils with accumulated salt anywhere in the top 60cm to 100cm should however be dealt with caution. The measurement of resistance or EC of the soil may aid in identifying accumulated salts. Clay soils that provide resistance readings below 250 ohms should be investigated intensively before preparation. The same applies to lighter soils where resistance readings are below to 500 ohms.

If such soils are identified, it is best to consult an expert. While accumulated salts also include nutrients, it usually contains high concentrations of sodium, calcium, chlorides, sulphates and carbonates. The chemical conditions in the layers where the salts accumulated restrict root development and function. Where sodium is the dominant cation, the conditions damage the structure of the soil and hence the physical properties required for proper root functions. Where calcium and carbonate are dominant, the pH will reduce the availability of many nutrients such as Fe, Mn, Cu, Zn, P, and K.

Where chloride and sodium are the dominant ions, the osmotic pressure of the water in that zone will restrict utilization of water by the crop plants. Managing soils with high salt concentrations are extremely difficult and expert advice should be called upon to prevent further degradation of the soil. Managing tools can vary from planting salt tolerant crops (example Oldman Saltbush), to adapting the irrigation scheduling to the application of soil amendments.

 

Soil Improvement

Appropriate Soil Preparation Methods

Soil preparation methods are dictated by the properties of the soil profile, being stratification, texture, pH and salinity. Before soil preparation can be done effectively, a soil survey is required. Some basic soil preparations include:

• Primary cultivation
• Secondary cultivation
• Ridging
• Levelling and Contouring

Vegetable and field crops may require seedbed preparation as the most important step in the soil preparation process.

A good seedbed is characterized as being fine, firm and level. This is to ensure good contact between the seed and the moist soil in order to facilitate germination and rapid adaptation to field conditions. To obtain a fine, firm and level seedbed, primary and secondary cultivation is used. Primary cultivation is the first ground breaking of the season which cuts and shatter the soil with relatively deep penetrating tools (15 to 30 cm). The primary cultivation leaves a rough surface texture. Ploughs (mould board or shear ploughs); listers, bidders and rotary tillers are used to mix the soil with plant rests which may still be on the soil surface, lime and other chemicals which need to be applied before plant.

 

Primary cultivation and ripping

Sometimes, repetitive primary tillage can cause compaction layers (15 to 30 cm below soil surface), which prevents root penetration, encourages runoff and water logging. To break up such as compaction layers, rippers are used. It loosens the soil by the lifting of the soil and letting it down without any mixing, in an up-and-down motion.

These implements can penetrate the soil to a depth of up to 90 cm. The principle of this method is to loosen the soil without changing the downwards sequence, or layering. The best loosening is achieved by ripping the soil in two directions, of which the deepest is down-slope. The angle between the two directions should be 60°. To improve the lifting action, wings are attached to the ripper-tine at an angle of 30° with the horizontal at operating depth. This preparation method is also suitable for soils with clayey, acid, saline or carbonate-rich subsoil. Under these conditions, the subsoil must never be brought to the surface. Due to the cost involved when the soil is ripped, other primary cultivation actions will often not be done after ripping.

 

Secondary cultivation

Secondary cultivation follows on primary cultivation or ripping. This involves operations, which pulverize level and firm the top 5 to 15 cm of soil. It leaves the soil with a crumbly top layer of soil 2 to 3 cm on top of a firm, but not “hard panned,” subsoil. The loose top layer is necessary for oxygen supply and temperature regulation for the seed germination and root growth as explained earlier. Implements used during secondary cultivation include disk harrows, cultivators and rotary tillers.

 

Ridging the Soil Inter-row

Spacing The spacing between cropped rows differs between crops. In the case of citrus trees inter-row spacing may vary from 5 to 7 metres. The space required for spray machines and other orchard vehicles is 2.5 metres.

 

Citrus

When the depth of suitable soil is limited, the soil from the inter-row spaces can be moved to the area where the trees will be planted, thus excavating the vehicle paths and filling the planting area with more suitable soil. This creates ridges of suitable soil. The purpose and advantages of ridging are:

• Improvement in surface drainage;

• Increase in soil temperature in the upper layers;
• Increase in the depth of suitable soil;
• Savings in fertilizer and energy cost;
• Facilitation of the mixing of fertilizers; and
• Control of vehicle movement in the orchard. Ridges clearly indicate the inter-row paths and re-compaction of the soil is limited.

Before ridging, the required lime, gypsum or phosphates are mixed with the top 20 to 30 cm soil layer. The ridges are then built with soil containing the right amounts of nutrients.

The mixing process is more effective and cheaper when this method is used, while also saving on fertilizer cost. The height of the ridge is determined by the thickness of the suitable layer of soil. If this layer is less than 20 cm thick, ridging will not improve the potential of the soil sufficiently, although it also depends on the layer below this suitable layer.

 

The disadvantages of ridging are:

• Higher soil temperature, which can be up to 5°C higher compared to a flat soil surface;
• Increased evaporation of water;
• Limitation in irrigation design, as only drip and micro-jets can be used; and
• The undulating surface which makes picking more difficult.

 

Other crops

Ridges are often used in the production of vegetables and field crops to improve drainage and aeration. To ensure the benefits of ridging, the ridges should be at least 30 cm above the normal soil surface. The width of the ridges will depend on the number of rows planted per ridge and the amount of space needed by individual plants, and can therefore be as wide as 1 meter, as but seldom more than that. In most crops, the soil is only ridged once during the life span of the crop. In potatoes, however, it is often done twice. First before plant and the second time after the plants have reached a height of 30 to 50 cm. The bottom 20 cm of the plant is then covered with soil coming from between the rows. In this case ridging (also called earth up) is done to cover the tubers and protect the tubers against attacks from the potato tuber moth and some tuber diseases. By covering the tubers with soil, it also prevents sunlight from coming in contact with the tubers. If tubers are left in direct sunlight, it will turn green. Green tubers do not taste nice, and if one consumes too much green tubers at one go, you may become ill. Additional benefits of ridging at this stage are reducing the soil temperature around the developing tubers and physically controlling of weeds growing between the rows. Special ridge- making implements are available for use in potatoes and one should enquire about this from your local implement dealer.

 

Levelling and Contouring

This involves the shaping of the soil surface within a field to improve surface drainage and eliminates areas where water may pond. The activity requires the use of cultivation and land levelling equipment such as scrapers and heavy tractors. Land grading is generally used to improve drainage but can be used to change the aspect of a site, remove bumps and hollows or provide improved erosion control. See Session 1 of this unit standard for more information on sloping and the handling thereof.

 

Mulching

In areas where water (erosion and losses) are problematic, a layer of plant rests (mulch) are often placed on the soil surface. This layer prevents excessive water losses through evaporation and runoff. Mulching is one type of conservation tillage and the requirement for this is that at least 30% of the soil surface been covered by mulch. The mulch can be from in situ plant rests or applied to the field from another source. Mulching can, however, not be recommended where there is a problem with soil-borne diseases. The mulch will, in this case, provide a habitat for the diseases to flourish in and this will be detrimental to the crop.

 

Minimum and zero tillage

Minimum and zero tillage are also a form of conservation tillage. These types of tillage practices imply that there will be minimum or no soil disturbance. With minimum tillage only, that part of the soil which will be planted to the crop will be disturbed. The disruption of the soil often includes a loosing of the topsoil in the planter furrow, followed by sowing of the seed in the cultivated furrow. The previous season’s plant rest is often disked into smaller units to form a mulch layer on the soil surface (Fig 4.1.).

In zero-tillage practices, the soil will only be disturbed by the drilling of a hole in which the seedling or seed is then placed. The plant rests of the previous season will often still be in place on the field (Fig 4.2.). It is therefore not incorporated into the soil, nor disked into smaller units.

 

2.5.1    Soil Management in a Soil Utilization Plan

Appropriate Soil Maintenance Methods

Maintenance of the soil after establishing the crop includes:

• Protecting the surface against erosion;
• Minimising compaction;
• Guarding against salinization; and
• Limiting acidification.

 

Protection Against Erosion

The best method to protect an orchard-floor against erosion is to use a natural grass cover or cover crop. The orchard floor should not be kept completely free of weeds. The area underneath and between the trees should be weed-free, forming a strip one metre wider than the diameter of the tree canopies. On the rest of the orchard-floor, including the paths between the rows, a natural grass cover must be established as quickly as possible. This is not always possible in low rainfall areas because the paths between the rows are not irrigated. The width of the grass-strip will decrease as the trees grow and must never reach underneath the canopy. The grass must be mowed frequently, and the cuttings blown underneath the canopy. These strips of grass also harbour beneficial insects and play a major role in pest control. As explained in the previous section, by using surface mulch, water infiltration into the soil is improved and it also reduces runoff. If mulch is not or cannot be used, erosion can still be controlled by making sure the soil surface has a crumb structure rather than a smooth structure.

 

Minimising Compaction and the formation of surface crusts.

Compaction of the soil after soil preparation is inevitable but must be minimized as much as possible. Practices that accelerate compaction are vehicle traffic, high precipitation rates, and salinization. Vehicle traffic has the severest impact. All vehicles should be restricted to the 2.5m middle-section in the inter-row paths and must never drive closer to the trees than that. In vegetable and field crops, vehicle traffic should be restricted to fixed pathways (called tram lines) in the field. This will still lead to compaction, but only in certain parts of the field and not the whole crop area. When water is applied at a rate exceeding the infiltration rate of the soil, the soil particles floats and, on drying out, will settle in a more compacted state. This reduces the infiltration rate even more, with the consequences of more surface compaction. This is also referred to as crusting. Salinity, and especially an increase in the SAR of the soil, accelerates crusting. Soil analyses will help detect the development of crusting, but the first symptom is runoff halfway through the irrigation cycle.

 

Guarding Against Salinization

Apart from crusting, which is the first symptom of salinization, accumulation of sodium or reduction of calcium deeper in the profile should be monitored continuously. For this purpose, regular soil analyses are required. Although the ratio of the cations is a good indicator of developing salinity, the subsoil should also be sampled from time to time. The intervals will be determined by the conditions prior to planting when the profile was analysed. As indicators of developing salinity, the calcium to total cation ratio should be 70% to 75% and the sodium 3%

 

Limiting Acidification

Acidification is more active in the sub- than the topsoil. Subsoil sampling is, therefore, more important to monitor the pH. The ammonium form of nitrogen is the cheapest nitrogen source but also one of the greatest sources of acidification. Even in fertilizers like LAN, acidification can be a potential hazard for subsoil pH levels. Liming the subsoil alone is not possible and the pH of the sub-soils must be monitored granularly. The frequency of sampling can only be determined by historical data.

 

Record Keeping

During a soil survey, many soil properties are evaluated, analysed, described and recorded. Many of these properties are stable and will not change. These records are therefore valuable for replanting and for future developments and should not have to be redone.

The stable properties include:

• Slope;
• Aspect;
• Soil depth;
• Clay content;
• Structure;
• Water-holding capacity; and
• Cation exchange capacity

The variable properties of soil are:

• pH;
• Nutrient content; and
• Resistance

It is in this regard that soil and leaf analyses, especially historical data from routine leaf and soil analyses, have its value. This data indicates the direction in which the nutritional status of the soil and trees are moving. Records of the variable data should be kept in an easily accessible format for at least three years. The format may vary but should enable the reader to compare year-on-year figures with ease.