Here is a list of soil fertilizer elements: 1. Nitrogen 2. Phosphorus 3. Potassium.
Nitrogen is most important nutrient for the plant. Nitrogen is an essential constituents of metabolically active compounds such as amino-acids, proteins, enzyme and some non-proteinaceous compounds. Plant growth probably is limited more often by deficiency of nitrogen than of any other element.
Forms of Nitrogen in Soil:
Nitrogen may be present in two forms as follows:
I. Elemental Forms:
Plants are made up mostly of nutrient derived from air, water and soil. Air contains 78 per cent Nitrogen. Plant cannot use it directly. But the plants belonging to Leguminoceae, can play a host to a special group of nitrogen fixing bacteria such as Rhizobium. This bacteria can convert atmospheric nitrogen into organic form that can be used by host plant. Pulses, Dhaincha (Sesbania aculeata), Sannhemp (Crotolaria juncea), Groundnut (Arachis hypogea), Lucern (Medicago sativa), Berseem (Trifolium alexandrinum), Clover (Trifolium sp.), Guar or cluster bean (Cyamopsis tetragonoloba) are some common examples of legumes.
The plant (host) provides the microbes with sugars and the microbes supply the plant with all or atleast part of their nitrogen in soluble compound form. In the soil, the elementary form of nitrogen present in soil atmosphere and soil solution. But this form of nitrogen present in negligible amount. The elementary form of nitrogen is generally utilized by the nitrogen fixing bacteria, such as Azotobacter, Clostridium, Rhizobium etc.
II. Combined Forms:
The combined form of nitrogen are generally present in two form as follows:
(i) Inorganic Form:
The combined nitrogen occurs only in trace quantities in igneous rock, but is required by plant in relatively large quantities. The traces of nitrogen found in .igneous rocks appear to be in ammonium form. The inorganic forms in which the nitrogen present are nitrous oxide (N2O), nitric acid (HNO3), nitrite (NO2), nitrate (NO3) and in ammonical (NH4) form. Out of these, former three present in gaseous form and the rest in ionic form in soil solution. Most of the nitrogen present in soil in inorganic form and varies 1-3 per cent of total nitrogen.
(ii) Organic Form:
Protein is a nitrogenous compound (Protein = N x 6.25) and 50 per cent of total nitrogen present in the soil in the form of protein. Every agricultural soils always contain a considerable amount of protein which are originated from protein in plant residues and soil microbes or from organic manure (i.e. F.Y.M. Compost, green manure, dried blood, guano etc.). The proteins are almost completely insoluble in water. So the plant cannot absorb them directly and the nitrogen reserve in it. The nitrogen becomes available to higher plant after having decomposed to simpler compound by soil organism.
(b) Nucleic Acid of a Nucleo Protein:
Nucleic acid is the combination of organic base phosphoric acid and sugar.
Nucleic acid combine with protein forms nucleo protein. Nucleic acid and nucleo protein will together form 10 per cent nitrogen of total protein.
Chitin is the polysaccharides. Chitin differs from the majority of polysaccharides found in the soil in that its basic unit consists of an amino sugar. Nitrogen is also present in the soil in the form of chitin compound. Chitin is found in cell walls of lower plants and fungi, but the major source in the soil in the form of insect remains. Decomposition of this material is through the elaboration of extracellular chitinase. The amount of nitrogen in the chitin form is not known.
(d) Heterocylic Compound:
The nitrogen present in this form is stable and is not so easily mineralized or hydrolysed. Amount of nitrogen in this form is not known.
(e) Ligno-Protein Complex:
The Lignin molecule undergoes modification in the formation of humus. The main part of humus is composed of lignin and protein. The lignin, the undecomposed part, combines with protein forms ligno-protein.
(f) Fixed Ammonia:
There are some minerals that (e.g. illite type of minerals) can fix nitrogen in their lattice structure and this nitrogen is not easily released. As far nitrogen is concerned, ammonia is the final product of the decomposition of protein and it is a gas which is very soluble in water. Ammonia combines with carbonic acid (H2CO3) and other acid that may be present in the soil solution to form ammonium salt such as ammonium carbonate [(NH4)2CO3], Ammonium salts are readily soluble in water and plant can absorb ammonium as a source of nitrogen. In soil, ammonium compounds are rapidly changed to nitrate by certain bacteria.
(g) Organo-Inorganic Complex:
Clay minerals hold protein in their lattice. Montmorillonite clay contains 8 per cent protein compound by weight.
Nitrate is the final product to which nitrogen from the complicated compound is changed by bacteria. Plant absorbs nitrogen in the form of ammonium (NH4+) and nitrate (NO3–). Nitrate is soluble in water and most important source of available nitrogen for higher plants.
Phosphorus is an essential constituent of every living cells and for nutrition of plant. It takes part in all type of metabolism of plant. It is an essential constituent of majority of enzyme and also structural component of membrane system of cell, the chloroplast and the mitochondria. It is intimately associated with life process.
Phosphorus Content in Soil:
General – 0.02-0.40 per cent
India – 0.03-0.30 per cent
West Bengal – 0.03-0.26 per cent
The availability of total phosphorus is very low. Most of Indian soil (about 70 per cent) is deficient in availability of phosphorus.
Forms of Phosphorus in Soil:
The phosphorus in soil may be divided into two categories as follows:
(i) Organic Phosphorus:
Three per cent of total phosphorus is organic phosphorus. The phosphorus presents in plant body is supposed to be present in soil.
Soil contains some phosphorus in organic combination as follows:
(a) Phytin and its derivatives
(b) Nucleic acid
There is some controversy about the presence of nucleic acid in soil. Bartholomew, Adam and Clark (1954) analysed the soil which is rich in organic phosphorus. But they could not find even a trace of nucleic acid and they came to conclusion that there is no nucleic acid in soil.
(ii) Inorganic Phosphorus:
The inorganic phosphorus compounds which are present in soil are either- (i) compound of calcium or (ii) compound of iron and aluminium.
(a) Calcium Compound of Phosphorus:
The important calcium compound of phosphorus present in soil are as follows:
(b) Iron and Aluminium Compound of Phosphorus:
The iron and aluminium compound of phosphorus are most stable in acid soils and are extremely insoluble.
The availability of inorganic phosphorus is largely determined by some factors as follows:
(i) Soil pH.
(ii) Soluble iron, aluminium and manganese.
(iii) Presence of iron, aluminium and manganese containing minerals.
(iv) Available calcium and calcium minerals.
(v) Amount and decomposition of organic matter.
(vi) Activities of microorganism.
The first four factors are interrelated because their effects are largely dependent upon soil pH.
Potassium is an essential element for plant growth and reproduction.
Forms of Potassium in Soils:
The potassium in soil can be classified on the basis of availability into three general groups as follows:
(i) Unavailable Forms:
Potassium (perhaps 90-98 per cent) remains in unavailable form in mineral soils. The compounds containing most of this form of potassium are the feldspars and micas. These minerals are quite resistant to weathering and probably supply relatively insignificant quantities of potassium during growing season of a crop. The primary minerals (i.e. mica and feldspar) when they are exposed to various weathering process over a period of time, gradually undergoes decomposition.
2KAlSi3O8 + 2H2O + CO2 → H4Al2Si2O9 + K2CO3 + 4SiO2
With this decomposition, there is a release of potassium ion (K+) which may be – (a) lost in drainage water or appropriated by living organism, (b) held an exchangeable ion surrounding clay particles or converted to one of the slowly available form of potassium.
(ii) Slowly Available Form:
Slowly available potassium is generally considered to be that which is not extracted by the procedure generally employed in the determination of exchangeable form of this ions. It becomes available to plants slowly and over a period of time. In the presence of vermiculite, illite and other 2 : 1 type minerals, the potassium of such fertilizer as Muriate of potash not only becomes adsorbed but also may become definitely ‘fixed’ by soil colloid. As such, this element is not readily available to plants. But the fixed potassium is slowly reconverted to available form and it is not subjected to leaching.
(iii) Readily Available Form:
Readily available potassium (K+) is that which occurs in soil solution and on the exchange complex and which is readily absorbed by the plants. The readily available potassium constitutes only about 1 or 2 per cent of the total amount of this element in an average mineral soil.
Factors Affecting Potassium Fixation in Soil:
There are some factors that affect the potassium fixation in soil as follows:
(i) Soil Colloid:
The ability of various soil colloid to fix potassium varies widely. 2 : 1 types clays such as illite and vermiculite fix potassium very readily and in large amounts. On the other hand, 1 : 1 type clays such as kaolinite fixes little potassium. Potassium fixation takes place only in soil that contains montmorillonite or expanding type clays or vermiculite. In this connection, it might be added that organic matter while having a great capacity to retain potassium and other cation in the exchangeable form, has no capacity whatever for the fixation of this element.
(ii) Soil Moisture:
Soil moisture content influences the potassium fixation in soil. When soils containing exchangeable potassium are alternately wetted or dried, a large portion of exchangeable potassium is converted to less available form. The wetting and drying of soils contribute markedly to the fixation of potassium under field condition.
At high soil temperature, fixation of potassium proceeds more rapidly. It has been shown that alternate freezing and thawing result in the release of fixed potassium under certain condition.
(iv) Soil pH:
The effect of soil pH on the fixation and release of soil potassium has been a controversial subject among the soil scientist for many years. Lime is generally applied for the reclamation of acid soil. But the lime application sometimes result in an increase in potassium fixation of soils. If a soil colloid is saturated with potassium and neutral salt such as gypsum. (CaSO4) is added, there will be a replacement of some of adsorbal potassium by calcium.
The soils lose less of their exchangeable potassium by leaching when they are high in base saturation. Liming is the common means by which the base saturation is increased. So it is clear that liming decreases the loss of exchangeable potassium.
Losses of Potassium from Soil:
(i) Crop removal – Plant absorbs potassium from soil and stores them in their different parts. Crop removal of potassium generally exceeds that of other essential elements, with the possible exception of nitrogen.
(ii) Losses of potassium by leaching – Leaching losses of potassium greatly exceeds those of nitrogen and phosphorus. When potassic fertilizer is applied in soil, it becomes soluble and it is subjected to be lost by leaching.
(iii) Losses of potassium by soil erosion – Losses of potassium by soil erosion greatly exceeds those of nitrogen and phosphorus. Soil erosion depletes the potassium along with the soils that are transported from the cultivated land.
Potassium Supply and Plant Behaviour:
1. Deficiency Symptoms:
(i) Plant becomes stunted in growth with shortening of internodes and bushy in appearances.
(ii) A deficiency of potassium brings about chlorosis i.e. yellowing of leaves and leaf scorch in the case of fruit trees. It is also responsible for the ‘dying back tips’ of Shoots. The older leaves show the deficiency symptom earlier.
(iii) Potassium deficient plants show a reduced rate of photosynthesis,
(iv) Potato plants show an abnormal dark green colour of foliage followed by brownish. Deficiency of potassium results in blackening of tubers and damages in storage and transit.
(v) The tips or margin of lower leaves of legumes, maize, cotton, tobacco and small grains are either scorched or burnt.
2. Potassium Supply and Plant Growth:
Potassium improves the health and vigour of the plant and encourages strong root system, thus potassium tends to prevent the undesirable lodging of plants, particularly in cereal crops and to counteract the damaging effect of excessive nitrogen.
3. Potassium Supply and Crop Maturity:
In delaying maturity, potassium works against undue ripening influences of phosphorus. Potassium is important to cereals and potato for grain formation and tuber development respectively.
4. Potassium Supply and Disease Incidence:
Potassium increases the crop resistance to certain diseases and counteracts the damaging effect of excess nitrogen.
Function of Potassium:
(i) Potassium is an essential element for the development of chlorophyll.
(ii) It is essential for photosynthesis i.e. converting hydrogen and carbon dioxide into sugar and for translocation of sugars and also for starch formation. It is, therefore, of special value for crops like sugarcane and potatoes which are rich in sugar and starch respectively. It is absolutely necessary for tuber development.
(iii) It improves the health and vigour of the plant enabling it to withstand adverse climatic condition.
(iv) It strengthens the straw of cereals and keep the plant green and functioning longer that it otherwise do. Thus it reduces lodging in cereal crops. It improves the quality of crop like tobacco, potato, sugarcane, vegetables and fruits.
(v) It increases the crop resistance to certain diseases and counteracts the damaging effect of excess nitrogen.
(vi) It is necessary for the production of best quality grains and fruits. Potassium plays a key role in the production of quality vegetables.
(vii) In delaying maturity, potassium works against undue ripening influence of phosphorus. In a general way, it exerts a balancing effect on both nitrogen and phosphorus and consequently is especially important in mixed fertilizers.
(viii) Potassium is an enzyme activator.
(ix) It increases the plumpness and boldness of grains and seeds.
(x) It improves water balance, promotes metabolism and increases the production of carbohydrates.
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