eChapters

5.1. Importance of Secondary Nutrients in Agriculture

Secondary nutrients such as sulphur (S), calcium (Ca) and magnesium (Mg) are the macronutrients that are required in relatively large amounts for crop growth. Among the secondary nutrients, Ca is the 5^th most abundant element in the earth’s crust while, Mg and S are ranked 8^th and 13^th, respectively. The average S content of the earth’s crust ranges from 0.06 to 0.10%; whereas, Ca in soil solution of temperate region soils is 30 to 300 ppm and soils of tropical areas is 8 to 45 ppm (Tisdale et al., 1997). In case of Mg, its’ concentration ranges from 5-50 in temperate region and 120-2400 ppm in tropical region. The Ca and Mg are the most abundant cations occupying the exchange sites of the soil colloids. Thus, most soils with the possible exception of highly weathered, leached acid soils, contain enough Ca and Mg for crop growth. In soils, occurrence and reactions of S are different from those of Ca and Mg. The plant available S is SO₄^2- ion which is relatively mobile in soil solution. Like nitrogen (N), S is also subjected to chemical and biological oxidation-reduction reactions. On the other hand, Ca and Mg are much more stable and they exist in soil as Ca²⁺ and Mg²⁺ cations and present in association with soil colloidal fraction similar to that of K. In general, Ca and Mg are the most abundant cations occupying in the exchange site.

Secondary nutrients in soils may be lost due to several reasons such as (i) crop removal, (ii) leaching, (iii) erosion, and (iv) volatilization. The amount of Ca and Mg attain importance in areas where soils are either strongly alkaline or acidic. These soils are often deficient in Ca and Mg. The Ca deficiency is most widespread in acidic soils. The alluvial soils of the Indo-Gangetic Plains (IGP), being neutral to moderately alkaline in reaction, have sufficient bases on their exchange complex and Ca deficiency is, therefore, rarely found in these soils. However, alkali soils may contain inadequate quantities of Ca due to dominance of Na⁺ on the exchange complex. Magnesium deficiency is found in sandy soils with low exchangeable Mg²⁺, acid soils and those with high in native or applied K. Alkali soils have high exchangeable Na⁺. The Ca²⁺ is a suitable ion to replace this Na⁺ from the exchange complex when Ca²⁺ is supplied through gypsum (CaSO₄.2H₂O). The replaced Na⁺ forms Na₂SO₄ which is then leached down to remove the salt away from the root zone of the crops. Gypsum also reacts with Na₂CO₃ to form CaCO₃ and Na₂SO₄ which is leached down the profile. A thorough leaching of the soil with irrigation water should be done after application of gypsum to free it from Na₂SO₄.

Potassium is the third important essential nutrient element after nitrogen (N) and phosphorus (P) required by all living organisms, including plants and animals. Potassium is essential for many metabolic activities like carbohydrate metabolism, nitrogen metabolism. It is required for the activation of over 60 enzymes in plants. It controls/regulates the activities of various essential nutrients. It is vital to photosynthesis, and synthesis of carbohydrates, starch and proteins in plants. It plays vital role in controlling the opening and closing of stomata in photosynthesis. It also helps in controlling water use by plants by regulating the osmotic pressure of guard cells during transpiration. It plays a major role in transport of water and nutrients throughout the plant in xylem. Potassium improves crop yield and quality of flowers, fruits, vegetables and other field crops in terms of size, shape, colour, taste, shelf-life, and fibre quality. Potassium stimulates the growth of strong stems and provides strength to the plant against some disease by promoting thickness of the outer cell walls. It is required for translocation of assimilates. Under reduced K supply, translocation of NO₃^-, PO₄^3-, Ca²⁺, Mg²⁺ and amino acids is hampered. It also imparts resistance against environmental stresses such as drought, cold and frost. Adequate K can reduce moisture loss from growing plants, thereby giving some drought resistance. It reduces lodging of crops and enhances their winter-hardiness.

It is taken up by the plants as K⁺. Being a basic element, it takes part in neutralization of the acidity in plants caused by several organic acids produced. It does not form organic compounds in the plant system. It is the most mobile element in the plant. Thus, it can move through all the plant system due to osmotic pressure. When any plant sample is dipped into water, some K comes out. This shows that it is highly mobile.

Potassium is also required for animal nutrition and human beings. Potassium is essential for many metabolic functions. It maintains salt balance between cells and body fluids. Adequate K is essential for nerve function and preventing muscle cramps. It is routinely added to many animal feeds. Since K⁺ is not stored in the human body, dietary replacement is required on a regular basis. Government agencies state that diets containing foods that are good sources of K and low in Na may reduce the risk of high blood pressure and stroke.

Phosphorus (P) is the second most important essential nutrient limiting crop/ plant growth, next only to nitrogen (N). Phosphorus is an important component of nucleic acid like deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It is needed for energy storage and transfer, which is the single most important essential function of P. Phosphate compounds act as “energy currency” within plants. The most important common P energy currency is adenosine diphosphate (ADP) and adenosine triphosphate (ATP). The ATP transfers energy for synthesis of ADP. Life is not possible without P. It is the source of energy that powers practically all energy-requiring biological processes in plants. Phosphorus is present in plant as phospholipid such as lecithin and cephalin which appears to be involved in the structural framework of protoplasm. Phosphorus is involved in many important processes such as photosynthesis, cell division, tissue development, and growth in plant. Requirement of P by the crops is less, compared to N and potassium (K). It is present in plant tissues. It is reported that for 5-6 t ha^-1 of wheat grain yield, it removes about 100-120 kg N, 20-30 kg P₂O₅ and 150-180 kg K₂O from soil.

Phosphorus is 11^th most abundant element on the earth crust. However, it is the least mobile element among the major plant nutrients. Its complex chemistry in soil makes it highly deficient nutrient element in most of the arable soils across the globe. Only 0.1% of the total P in the soil is available for uptake by plants because of its fixation as various insoluble compounds depending upon the soil pH. About 43% of the world’s arable soil is P deficient which limits crop growth and production (Roy et al., 2018). In the Indian context, about 80% of the soils are reported to have low to medium available P content (Muralidharudu et al., 2011). Thus, application of P-fertilizers to maintain the soil fertility and sustain crop productivity becomes imperative in today’s input intensive agriculture.

Nitrogen (N) is an essential element for all forms of life because it is a major component of amino acids, which are the building blocks of proteins, nucleic acids such as deoxyribonucleic acid (DNA) and other cellular constituents. It is a structural component of chlorophyll in plants, which is used in photosynthesis to make their food. Nitrogen ranks first among the essential nutrient elements required for plants. Its careful management is very important for crop production.

Nitrogen exists as two main stable isotopes (isotopes of a given element have the same number of protons but different numbers of neutrons in each atom) namely, ¹⁴₇N (abundance 99.63%) and ¹⁵₇N (abundance 0.37%). These can be separated by chemical exchange or by thermal diffusion. Both ¹⁴₇N and ¹⁵₇N isotopes of N are stable isotope which do not disintegrate as in case of other radioactive isotope like ³² ₁₅P. There are four other isotopes of N viz. ¹²₇N, ¹³₇N, ¹⁶₇N and ¹⁷₇N, mostly are radioactive having very short half-life and not used in agriculture. The ¹³₇N has a half-life of about 10 min, while other three have half-life of only few sec. The ¹⁵₇N (tracer) isotope is used as a tool for determination of pathways of transformation of N in soil or plant or animal.

The major part of N comes from atmosphere (air). On the basis of volume by volume (v/v), air contains about 78-79% N₂, 20-21% oxygen (O₂) and 0.03% carbon dioxide (CO₂). Most living organisms cannot utilize N₂ gas present in the atmosphere directly because it is present as di-nitrogen form or di-atomic form (N≡N). It is called as an inert gas. Therefore, to break this triple bond, huge amount of energy is required. After breaking of triple bond of N₂ molecules, it combines with other element particularly with O₂ or hydrogen (H) to form oxides of N (NO, NO₂, NO₃ and N₂O) or ammonia (NH₃). Thus, breaking of triple bond is must for forming NH₃ which further can be used as the key material for almost all fertilizers. Therefore, the fixation of N₂ is vital for production of all nitrogenous and complex fertilizers like NP/NPK and nitrophosphates.

Plant absorbs large number of elements from the soil and other sources during their growth and development. But all these elements are not essential for plant growth. Seventeen mineral elements are known to be essential for the growth and development of all plants. A list of essential nutrients, their source and forms by which they are absorbed by plants are presented in Table 1.1.

Except carbon (C), hydrogen (H) and oxygen (O), all the other nutrients are obtained by plants largely from soils, fertilizers or manures. It has been seen that the number of nutrients obtained from soil is about 5 times the number of nutrients obtained from air and water. However, most of the plant tissue, in general, from 74 to 99.5% is made up of C, H and O i.e. nutrients obtained from air and water and only 0.5 to 26% of the plant tissue is synthesized from soil constituents. In ordinary (normal) conditions of agriculture, except drought, C, O and H are not limited for plant growth. It is the remaining 14 elements from soil that are usually limiting plant development and hence considerable emphasis is placed on the improvement of soil in respect of greater availability of these 14 nutrients. It may be borne in mind that all nutrients are essential for all crops even though their requirements differ from crop to crop and even among varieties of a given crop. Secondly, all nutrients are equally essential regardless of amounts required. For example, to produce one tonne of grain, wheat may absorb 25, 9, 33, 3.7, 5.3 and 4.7 kg of nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg), respectively and 56, 624, 70, 24, 48 and 2 g of zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), boron (B) and molybdenum (Mo), respectively. If the crop (any crop) does not get the smallest amounts of micronutrients needed, it cannot make the best use of abundant NPK.

According to Liebig’s Law of minimum which he enunciated in 1855, the growth of a plant is limited by that nutrient which is in shortest supply. This law in fact lays the ground rules for balanced crop nutrition.

Soil fertility is the resultant of several factors, not the least important of which is the soil itself. Theoretically, the soil only plays the part of a support and could be very easily dispensed with and replaced by sand and even that can be replaced by a wire framework to support the plant mechanically and it will grow quite well if the right amount of nutrient materials have been added to water. ‘Where does the soil come in?” you will ask. The importance of the soil is two-fold: (1) It provides vast reserves of plant food which is slowly made available. (2) It provides limiting factors which restrict plant growth; these limiting factors must be neutralized. We have already discussed these reserve food materials and limiting factors in Chapter II.

It has been shown that if we treat a soil with dilute acid exhaustively we can remove everything from it except the mineral framework. If we try to grow anything in this mineral framework nothing will grow. Yet it is not like sand, for whereas in sand by adding appropriate amounts of nutrient materials in water solution you could grow a crop, nothing will grow if you added the same solution of nutrient materials to the soil. This mineral framework, therefore, is not inert like sand, but it is active and can react with the nutrient salts in such a way that either they are rendered unavailable or something else which is toxic is produced by the interaction between the soil and the nutrient salts. This mineral framework is not acted upon by dilute acids, but it reacts with alkalies in such a way that the alkalies are neutralized. In this respect, therefore, the mineral frame work behaves like an acid. With alkalies it forms salts and different alkalies are required in equivalent amounts for the formation of these salts. Thus we can have salts of a number of bases and if we use sodium hydroxide for neutralizing we shall have sodium soil, similarly we can have a calcium, magnesium or potassium soil. Now in nature, all the various bases principally, calcium, magnesium, sodium, and potassium, are associated with it in different proportions. The nature and amount of these bases determines the physico-chemical properties of the soil affecting crop yield and leading to soil infertility. We have already indicated the method of bringing the soiil to the normal state when it suffers from any of the defects associated with the soil framework.

Soil mechanics as a distinct branch of soil science is comparatively of recent growth. Its need arose from the fact that the requirements of the engineer are very often quite different from those of the agriculturist. Whereas the former would as a rule require hard, compact, impervious soil, the latter would have no use for it for the growth of his plants which require soft, friable and porous soil. The agriculturist generally has to look up to nature to help him out of his difficulties, the engineer has to depend on his scientific skill to modify the soil to suit his requirements. In this chapter an attempt will be made to explain in a simple language the requirements of the building, road, and irrigation engineer for particular types of soils and how they are met by modifying the soil if the required properties already do not exist.

Moisture plays by far the most important role in soils as regards vegetation. It is through the medium of water that the plants obtain their quota of mineral food vital for their growth. If water is gradually withheld from the soil, at a certain stage, the plant will begin to wilt and soon dry up. The soil may still contain some moisture but the plant is unable to make use of it. The minimum moisture in the soil below which wilting will take place is called the Wilting Coefficient and at one time great importance was attached to it. The determination of this Coefficient, however, is tedius. One must grow a number of plants and slowly watch them wilt and determine the moisture content at that point. Moisture content of the soil is determined by heating a weighed quantity of the sample in an oven kept at 100-110 ^oC. The moisture is expressed as percentage on the oven-dry soil.

Soil texture and mechanical analysis: The chemistry of the mineral framework was explained in Chapter II. Its physical properties are quite as important- When all the extraneous matter is removed from the soil we are left with a collection of particles of various sizes. These particles are not spherical in shape, but most of them are nearly so. At any rate the discussion is very much simplified if we suppose that they are spherical and the arguments advanced are equally applicable if the shape deviates from the spherical.

Next to the mineral frame work the salts in the soil deserve consideration. In fact these salts constitute the main food material of plants besides what the root hairs can get out of the mineral framework in the form of basic materials already dealt with. We are, however, not concerned with the plant food materials, but salts that are in excess and which alter the properties of the soil to such an extent as to influence its behaviour profoundly.

If the reader has endeavoured to follow the basic chemistry in the first chapter, he would be in a positions to follow up the various reactions that take place in soils and which hitherto had baffled him. Soils contain particles of various sizes ; some so fine that they would remain suspended in water for a long time; others so coarse that they would be sieved out by even a coarse sieve. Soils contain some salts that can be washed out by leaching with water; they contain some water which can be readily evaporated away by exposing to hot sun or by heating slightly. They also con-tain a little organic matter which can be burnt away or dissolved in alkali. When all the extraneous substances have been completely removed we are left with what we might call soil framework. This framework gives mechanical support to the roots of the plants. Without this framework the plant would not stand up even if it could be supplied with all the food material.

Let us suppose a soil freed from all the extraneous matter by continuous leaching with water or a dilute acid. In the former case the removal of all the extraneous matter is extremely slow, whereas by leaching with dilute acid everything except the soil framework is removed in a short time.

When soils from different localities are subjected to leaching with dilute acid we find that the soil framework in every case, behaves in a similar manner. No matter from what part of the world the soil has been obtained, the properties of its mineral framework only differ in quantity but not in kind.

For the intelligent appreciation and understanding of various reactions and processes associated with soils a little basic knowledge of chemistry is helpful. If the reader follows this chapter carefully, the working knowledge of chemistry thus acquired will enable him to follow all the complicated chemical reactions taking place in the soil.

What is Chemistry? Chemistry is the science that deals with the chemical changes. What is a chemical change? When two or more substances combine to give a substance entirely different in properties to the individual components, the change is called a chemical action to distinguish it from the purely physical action in which the properties of the product are intermediate between the components. The latter naturally can be separated easily by mechanical means. For instance a mixture of sand and water can be separated by passing through a filter, and a mixture of salt and water can be separated by evaporating the water when the salt is left as residue. But water itself is a compound of two gases: oxygen and hydrogen. Its properties are totally different from its components. We can freeze it, evaporate it and perform a host of other operations on it without changing it in the least.

Atoms and molecules: The whole science of chemistry rests on atoms and molecules If these are understood clearly, basic chemistry is reduced to simplicity. Take a familiar object like gold or silver and go on subdividing it into smaller and smaller bits. You will reach a stage that further subdivision will become impossible. You will have reached the primary units, the elementary bricks of which the substance is made up, namely the “Atoms”. An atom in chemistry is the smallest brick from which all material substances in the world are made up. Not that you can see an atom. It is so small that even the most powerful microscope will nut reveal its presence. You could perhaps just see a collection of a million atoms as a tiny spot of light if it is illuminated with a powerful beam of light. Even then you could not say if it was round, square, or oblong. However, you need not look for any such tangible proof to believe in the existence of atoms. Even pure logic would compel you to stop somewhere if you started with an imaginary division and sub-division of a substance. You could not go on ad infinitum. Even the ancients, who knew no modern chemistry, believed in their existence purely from philosophical reasoning.