What is fertile soil?
A fertile soil is one that has the physical and chemical properties that plants need to grow well.
A good soil has:
- A deep enough layer of topsoil for plant roots to spread out.
- A good texture and a good structure for water holding, drainage and aeration.
- An almost natural pH
- A high organic matter content
- Plenty of micro-organisms
All these properties help to make soil fertile. Another very important thing we have to know is the nutrients status of soil to sustain plant growth. In this unit, you will also learn how you can add nutrients to soil. You will also learn how good soil gives plants a sufficient and balanced supply of certain essential nutrients. Farmers can feed their crops wisely when they know exactly what nutrients plants require. They can increase the fertility of their soil by adding the correct type of manure and/or fertilizer in the right amounts. Farmers know a soil is fertile when they see it producing good crops. For a farmer, a good yield of good quality crops is the best sign of fertility. A dark coloured soil, usually brown and darker is a sign of organic matter or carbon in the soil, which thus also indicate a soils fertility status.
Figure 2.29: An example pf a plant growing on fertile soil.
How soil gains fertility
Soil can be more fertile by adding some of the chemical elements the plant needs for growth. Often adding some kind of fertilizer to the soil can increase the nutrient status of soil but so does adding manure or compost. Before adding fertilizers to the soil, you need to learn about the different types of fertilizer and what each type can do or the different ratios of materials to add to your compost heap.
How soil loses fertility
Soil can lose fertility in four ways:
- Nutrients are removed by crops and other plants
The plants absorb nutrients from the soil as they grow. When they are harvested, the nutrients are removed from the soil along with the plants.
- Leaching
Plant nutrients dissolve in water and may be lost when water drains down through the soil. This is called leaching. Leached nutrients may end up in streams and rivers, or they may stay in the ground water, deep below the surface.
- Wrong pH values
Plant nutrients may be locked up by soil which is to acid. Nutrients are present in the soil, but they cannot dissolve in water so plants cannot absorb them. Soil pH can be corrected by adding lime so that nutrients become available to plants.
- Oxidants
Organic matter is lost very quickly in hot climates. It combines with oxygen and oxidizes, forming gasses such as ammonia that are lost to the air.
2.3.1 Soil Chemistry
The Colloidal and Chemical properties of Soil
In the discussion of soil texture, it was shown that the clay fraction consists of extremely small particles. These clay particles have colloidal properties. This means that they will stay in suspension in water for a considerable time.
Because these particles are very small, a given mass of clay will have a very large total surface area. A water molecule in the middle of a beaker of water is subject to attraction by surrounding molecules in all directions. A molecule at the surface is however attracted sideways and onwards. The same principle is valid for solid substances where attraction forces of surface molecules are not fully satisfied. To compensate for this, surface molecules are inclined to attract molecules of other substances onto the surface. This phenomenon is known as adsorption. The smaller the particles of a substance, the bigger the number of surface molecules and consequently, a bigger adsorption capacity. Adsorption is, therefore, an important contribution of the clay fraction in soils.
The clay minerals.
Sand and silt are final products of the weathering of rocks. Clay particles, on the other hand, are secondary particles made up from chemical compounds resulting from the chemical weathering process.
Figure 2.30: Cation exchange in the clay particle and the humus particle.
Two basic substances of which clay particles are built up from are tetrahedron and octahedron. The tetrahedron is a central silicon atom surrounded by four oxygen atoms while the octahedron consists of an Aluminium atom bound to six surrounding hydroxyl or oxygen atoms. Several tetrahedrons bind together to form a tetrahedral layer while a similar octahedral layer is formed. These layers are bound together in a lattice which is then the clay crystal.
Two types of clay crystals are formed, i.e. 1:1 and the 2: 1 type. The former consists of one tetrahedral layer bound to an octahedral layer, while the latter is two tetrahedral layers bound to an octahedral layer in the middle. Kaolinite is an example of the 1:1 clay and illite and montmorillonite are 2:1 type.
The electromagnetic charge on clay minerals
An important characteristic of the clay minerals is that they have negative electric charges on the surface. These charges are created in different ways:
Isomorphous substitution
The basic structure of clay mineral is dominated by the O2 and OH- ions with the cations like Si 4+ and Al 3+ fitting in between them. In this structure, an Al 3+ structure may replace a Si 4+ with the result that a negative charge is created. Similarly, the trivalent Al 3+ may be replaced by a divalent Mg2+, once again creating a negative charge. This phenomenon is known as isomorphous substitution.
Broken Crystal edges
A second manner through which negative charges may be formed is broken crystal edges. In this case, some of the oxygen atoms are bound to only one silicon instead of two. Consequently, only one of the oxygens two negative charges are neutralized, leaving one negative charge
Charge density
The term charge density refers to the relative quantity of charges a specific mass fo clay has. This varies according to the type of clay. In the 1:1 type, lie kaolinite, few isomorphous substitutions occur and therefore these clays have a low charge density.
Other colloidal substances in the soil
The hydrous oxides
Oxides and hydroxides of aluminum and iron occur in most soils and are generally referred to as hydrous oxides. They have colloidal properties and charges similar to that of clay.
Organic colloids
Humus is the final product of organic matter being broken down in the soil. Whereas clay has a mineral (inorganic) composition, humus consists of complex organic compounds(hydrocarbons). Humus particles may even be smaller than that of clay and they are highly colloidal. Due to its complex composition, it is not yet quite clear how the negative charges are formed, but humus does have negative charges and in fact, an exceptionally high charge density.
Cation adsorption
Figure 2.31: Soil particle and cations
Negative charges attract positive charges to create a stable state of neutrality. A cation is an ion with a positive charger. Already, since the 1850s, it is known that clays have the ability to retain cations. The explanation for this was found in the fact that clays have negative charges as described above. Cations in soils come from salts which are formed through weathering of rock minerals, the decay of organic matter as well as the addition of fertilizers. Many of these cations are essential to plant nutrients, while some of them play an important role in soil chemical properties. The most important cations occurring in soils are Ca+, Mg2+, K+, Na+, H+, Al+ as well as smaller quantities of Cu2+, Zn2+, Mn2+.
Cation exchange
A clay particle, also called a micellar, is surrounded by water in which several positively charged cations are dissolved. As explained before, the cations are attracted and held by the negative charges on the micelle. This adsorption is, however, not a static process, as the cations are in perpetual movement around the negative particle. The cation closest to the negative charge will be absorbed, but it can be replaced by another cation, called the intruder cation. This phenomenon is known as the cation exchange.
Figure 2.32: Cation exchange
This exchange can take place between cations of the same type, for example, a calcium ion in solution can replace an adsorbed calcium, which then goes into the solution. But cations of different types can also replace each other (H + replaces Ca+ etc). Several factors determine the degree of adsorption and the strength by which cations are held:
i) The type of cation:
The higher the valency of a cation the higher will obviously be its positive charge and therefore its adsorbing force. The hydrogen ion is an exception to this rule as it behaves like a trivalent cation. According to this rule, cations will be absorbed in the following decreasing order: H+, Al3+, Ca2+ , Mg2+ , K+ , Na+.
ii) The concentration of the ions:
According to the chemical law of the action of mass, the concertation (quantity) of a substance influences a reaction. This law is also obeyed in cation exchange. This means that the higher the concentration of a cation, the higher will its rate of adsorption be. Therefore, by increasing the concentration of a weaker held cation it will be able to replace a stronger held one, e.g. Ca2+ can replace Al3+ and H+ if the concentration of Ca2+ is increased. This principle is employed in the improvement of acid soils
Cation adsorption capacity
The cation adsorption of a soil may be defined as the as the total quantity of cations, expressed in centimole per kilogram that a soil can adsorb. The term cation exchange capacity (C.E.C) is more frequently used and is quantitatively exactly the same as cation adsorption capacity. (A soil can obviously exchange only as many cations as it can adsorb). It should be clear that the C.E.C of a soil firstly depends on the charge density. The charge density is in turn, determined by the percentage and type of clay and also the quantity of humus present in the soil. Because considerable differences among soils, regarding these properties exists, great variation in C.E.C can also be expected. It should now be clear why clay has a better retention of nutrients than sand.
Figure 2.33: Retention capacity of clay.
Determination of the cation exchange capacity
Figure 2.34: Cation exchange capacity
Because the C.E.C of soil has a marked influence on soil productivity it is useful to determine it in the laboratory. The principle of cation exchange is applied in this determination. A certain mass of soil (e.g. 20 grams) is weighed out and shaken with a solution containing an excess of one specific cation, for example, potassium chloride (KCl). These excess K+ ions will replace all the cations from the colloids. The suspension is filtered and further leached with the KCl solution. Excess KCl is leached out with distilled water. The adsorbed K+ can now be replaced and leached out with a solution. Excess KCl is leached out with distilled water.
Figure 2.35: A closer look at cation exchange in the soil and a plant
The adsorbed K+ can now be replaced and leached out with a solution containing another cation, for example, NH4+ in ammonium acetate. The quantity of K+ in this filtrate is then determined and this is equal to the cation exchange capacity.
Cation exchange and plant nutrition
The six macro-elements which serve as plant nutrients may be divided into two categories. Phosphorous, nitrogen, and sulphur are taken up as anion radicals while calcium, magnesium, and potassium are taken up as cations. The former groups can easily bind with organic compounds and is stored in the soil in this form, but calcium, magnesium, and potassium are held in the soil by cation adsorption. If it was not for this, they would have leached out of the soil.
Figure 2.36: Root hair and soil particle cation uptake
The uptake of ions by roots takes place mainly through the excretion of carbon dioxide, which dissolves in soil water to form carbonic acid (H2CO3)
CO2 + H2O -> H2CO3
The carbonic acid ionizes as follows:
H2CO3 -> HCO3- + H+
The H+ now replaces the Ca2+, Mg2+ and K+ from the colloids and they are then taken up by plant roots. Roots can also excrete other organic acids and H+ ions which then react in an identical way to exchange cations. The mechanism of cation exchange, therefore, plays a significant role in plant nutrition.
Base Saturation of soils
The adsorbed cations may be divided into two ways, namely those having an acid reaction and those being alkaline or basic.
Hydrogen (H+) and aluminium (Al3+) belong to the first group and are called acid forming cations. The contribution of H+ ions to acidity is direct. The influence of Al3+ is indirect because it creates the liberation of H+ in this manner:
Al3+ + H2O -> Al (OH)2+ + H+
Al (OH)2+ + H2O -> Al (OH)2+ + H+
Al (OH)2+ + H2O -> Al (OH)3+ + H+
2.3.2 The importance of pH
What is pH?
pH is a measure of soil acidity (or alkalinity). An acid is a bitter substance. If a soil has too much acid in it, plants will not grow well in it. Farmers say a soil like this is sour and seeds don’t germinate.
- Acid soils
Imagine what can an acid soil must be like to a plant! Think of a fruit with a sharp taste, such as a lemon. Lemons taste sour because they contain citric acid. Another food substance with a sour taste is vinegar, which contains acetic acid. Just as we do not like sour-tasting food, so do plants not like sour soils. There are many different acids. The ones used in the laboratories are very strong and dangerous. They could burn your skin, and you must never taste them! Soil is sour when there is too much acid present in it. Plants cannot grow properly in a sour soil.
- Alkaline soils
Some soils are alkaline (the opposite of acid) and usually contain a lot of calcium and magnesium. Plants do not like this either because it hinders the uptake of certain nutrients. Most crops and vegetables grow best when the soil is close to neutral, i.e. not too acid or too alkaline.
How to tell if soil is too acid or too alkaline?
You cannot taste the soil, as you could taste a lemon, but you can test it using a pH indicator. An indicator is a substance that changes colour depending on the pH of the solution you add it to. When we want to test the pH of a soil, or anything rather, we use a pH scale. a field method for testing soil pH is normally by collecting a sample of soil, making the soil wet by adding water, and then dipping litmus paper in the mixture. If the paper changes colour to red or a pinkish colour, it indicates acidity. if the paper turns to a purple-ish or blue colour, it indicates alkalinity. Below is a pH scale indicating soil pH levels, and other household products:
Figure 2.37: A pH scale indicating acidity levels of different household products and soils.
Numbers below 7 means the soil is acid. The lower the number, the more acidic the soil is. A reading of 7 on the scale shows that the soil is neutral, neither acid nor alkaline. A reading above 7 means that the soil is alkaline. The higher the number the more alkaline the soil is. Most soils are usually within the pH range of 3.5 to 8.5.
Factors affecting soil acidity:
Soils are more likely to be acidic than alkaline for several reasons:
- Carbon dioxide in the soil
This gas comes from the respiration of plant roots and soil organisms. It combines with water in the soil, and forms weak carbonic acid which lowers the pH of the soil.
- The breakdown of organic matter
This is done by micro-organisms and causes the formation of carbonic acid and sulphuric acid.
- Leaching of soil bases
Soil bases are alkaline substances in the soil. They neutralize acids and stop the soil becoming too sour. An example of a soil base is calcium, the element present in lime. Normally there is a balance between acids and bases in the soil. Bases are lost by leaching after heavy rain (They are carried away in the drainage water). This makes the pH go down. Areas of high rainfall often have soil with a low pH.
For example:
The formulation for water is H2O. This means that in each water molecule there are 2 Hydrogen cations (H+ H+) which are positively loaded and 1 oxygen ion (O2-) which is negatively loaded. In the soil there are calcium cations (Ca2+) which are positively loaded. The following happens when it rains: For example, imagine this scenario:
H2O H2O H2O (Water when it rains)
Ca2+ O2- O2- Ca2+ (Topsoil profile)
Ca2+ O2- O2- Ca2+
Ca2+ O2- O2- Ca2+
H+ H+ H+ H+ H+
H+ H+ H+ H+ H+
(Through leaching)
Ca2+ Ca2+ Ca2+
O-O- O-O- O-O- (What happens here is that; 2 Oxygen ions leaches out 1 calcium cation from the topsoil and only leaves hydrogen cations in the topsoil which in turn causes acidity).
- Calcium is taken from the soil by plants and animals
Calcium is one of the macro-element’s plants take in through their roots. When calcium is lost, the soil pH goes down. Calcium can be lost from the land:
-When crops are taken off the land at harvest time, the calcium they have absorbed is removes with them.
-When dairy cows eat grass, they utilize the calcium in it for making milk – this calcium is lost from the soil.
Another process of acidity in soils are when excess hydrogen ions are a good characteristic or indicator in acid soils. The question arises then, what are these sources of H+ ions?:
i) Plant roots Excrete carbon dioxide which dissolves in water to form carbonic acid, which in turn ionizes to form H+ ions.
CO2 + H2O H2CO3 -> HCO3- + H+
ii) Al3+ ions also liberate H+ ions
iii) In the decay of organic matter, organic acids are formed to be a further source of H+ ions.
iv) Strong acids like nitric acid (HNO3) and sulphuric acid are also formed during the biological breakdown of organic matter, to be further sources of H+ ions.
v) Application of certain fertilizers may result in the forming of H+ ions.
It is mainly the ammonium fertilizers, given as a source of nitrogen, which is responsible for forming of H+. The ammonium is oxidized through bacterial action to nitrate with the liberation of H+ ions.
vi) These H+ ions now replace the base forming cations from the colloids because they are stronger adsorbed and furthermore there is a continual production of H+ as earlier mentioned. The replaced cations are very soluble in water and therefore leached out of the soil. For this reason, soils will acidify more rapidly under high rainfall.
In arid regions, the forming of hydrogen ions is much slower and little or no leaching takes place. Consequently, base-forming cations will accumulate resulting in a more neutral soil(s). On the other extreme, calcareous soils containing free calcium carbonate (lime) may occur having a pH of above 7.
The term soil leaching is more often used because it includes the whole spectrum from acid to alkaline.
Figure 2.38: Losses of nitrogen from ammonium fertilizers
How to control soil pH
What can you do if the pH of your soil is too high or too low? This is an important question because unless your soil has the correct pH your crops will not grow well.
The importance of correcting soil pH
If the pH is too low, you can raise it by adding lime to the soil before planting. The amount of lime needed is calculated from the pH. A measured amount is spread on the soil with the manure and fertilizer, and dug in. A good indication is to add at least 1 to2 tons of lime per hectare per year in high rainfall areas (750mm per annum and higher).
Crop plants are sensitive to pH changes
A few crop plants can take more acid than others can. Most of them cannot grow well if the pH is too low or too high. For each crop there is:
- A minimum pH -the crop will not grow if the pH is lower than this
- A maximum pH –the crop will not grow at a pH higher than this
- An Optimum pH –this is the best pH for the crop – it is in the middle range of the pH scale
The effect of pH on plant growth
The wrong pH prevents plants from taking in nutrients. If the soil is too acid (low pH), some nutrients form chemical compounds that cannot dissolve in water (i.e. insoluble compounds). Examples include phosphorus and some of the trace elements needed by plants. If the nutrients cannot dissolve in water, plant roots cannot absorb them.
If the soil is too alkaline (high pH), other elements needed by plants (for example potassium) form insoluble compounds in the same way. Plants cannot use elements in these insoluble forms. Vegetables and other crops can then suffer from nutritional deficiency. You can see how this can happen even when soil does contain the elements plants need. The nutrients are there, but they are unavailable because the soil has the wrong pH. If you can correct the pH of the soil, the plant nutrients trapped as insoluble compounds are released into solution again and taken up by plants.
The effect of pH on micro-organisms
Figure 2.39: influence of pH on microbes under a microscope
Soil pH also has a big effect on micro-organisms. Remember that these are the bacteria and fungi that break down dead plants and animals. They release nutrients and from humus, but they cannot work normally if the pH is lower than 5.5 or higher than 7.8. If the pH of the soil lies outside this range, the number of soil micro-organisms will be small. The few that are present will not be very active.
When there are not enough micro-organisms, the fertility of the soil is reduced and its crumb structure can become damaged. Plant growth will be badly affected as plants have a symbiotic relationship with micro-organisms. When you correct the soil pH, the micro-organisms multiply and start working again, and everything returns to normal.
Methods to change soil pH
Increasing soil pH
The first step would be to neutralize the H+ ions in the solution. A suitable source of OH-ions is therefore necessary. Secondly, the adsorbed H+ must be replaced by a favorable action and then also neutralized. Agricultural lime is such a substance which can accomplish both these functions simultaneously. This can happen as follows:
In both these reactions, the H+ is replaced by the Ca2+ and neutralized by the CO2 or OH. Consequently, the concentration of H+
Deposits of agricultural lime are found in several localities in the country and are available at a reasonable price. Acidic soils can therefore effectively and economically be improved.
Lowering the pH
Too high a pH indicates an excess of base-forming cations (in the case of brackish soils) and even the presence of free lime (calcium carbonate) in the soil.
Lowering the pH can be achieved by the leaching o bass and addition of H+ ions. In practice, this is not always easily and economically achieved, especially where free lime occurs. H+ ions can be increased by applying acidifying fertilizers as described earlier. Sulphur may also be applied. Sulphuric acid is eventually formed which will result in a drop of the pH. Sulphur is, however very expensive. It is often more effective to overcome the problems of high pH soils in a direct way, for example, by applying leaf sprays of microelements where deficiencies occur.