In this essay we will discuss about the types of fertilisers.
Essay Contents:
- Essay on Nitrogen Fertilisers
- Essay on Phosphatic Fertilisers
- Essay on Potassic Fertilisers
- Essay on Calcium, Magnesium and Sulphur
Essay # 1. Nitrogen Fertilisers:
The ultimate source of nitrogen used by plants is inert gas N2, which constitutes about 78 per cent of the atmosphere. In its elemental form, however, it is useless for higher plants.
Primary pathways of its conversion to forms usable by higher plants are:
1. Symbiotic fixation on roots of legumes by rhizobia and other microorganisms.
2. Fixation by free living microbes and organisms that live on the leaves of tropical plants.
3. Fixation as one of the oxides of nitrogen by atmospheric electrical discharges.
4. Fixation as ammonia, NO3–or CN22- by industrial processes for manufacture of synthetic nitrogen fertilisers.
Nitrogen is lost from soil by plant and microbes uptake, leaching water (NO3–) and volatilisation.
Four general mechanisms of N volatilisation are recognised:
(i) Non-biological loses of ammonia,
(ii) Chemical decomposition of nitrite under acid conditions to yield nitrogen oxides,
(iii) N production by non-enzymatic reaction of HNO2 with ammonia or amino acids and
(iv) Microbial decomposition to liberate N2 and N2O.
On a worldwide basis, total N content of soils range from less than 0.02 to 0.5 per cent. Peat soils, however, contain more than 2.5 per cent. The average N content of major soil groups of India ranges from 0.02 per cent in red soils to 0.06 per cent in deep black soils.
About 2-3 per cent of organic nitrogen is mineralised each year under normal conditions. Ammonium ions may account for 8.0 per cent of nitrogen in surface soil and upto 40.0 per cent in subsoil. Available nitrate and ammonium forms are seldom more than 1-2 per cent of the total soil nitrogen.
Classification of Nitrogen Fertilisers:
Nitrogenous fertilisers are classified into four groups based on the chemical form in which nitrogen is combined with other elements within the fertilisers.
The nitrogen content of commonly used N fertilisers is given in Table 6.13:
1. Nitrate (NO3) Fertilizers:
Sodium nitrate — NaNO3
Calcium nitrate — Ca(NO3)2
Potassium nitrate — KNO3
2. Ammonium (NH4) Fertilizers:
Ammonium sulphate — (NH4)2SO4
Ammonium phosphate — NH4H2PO4
Ammonium chloride — NH4CI
Anhydrous ammonia — NH3
3. Nitrate and Ammonium Fertilizers:
Ammonium nitrate — NH4NO3
Calcium ammonium nitrate — CaNH4, NO3(NH4NO3,CaCO3)
Ammonium sulphate nitrate — (NH4)2SO4, NH4NO3
4. Amide (CN2) Fertilizers:
Urea — CO(NH2)2
Calcium cyanamide — CaCN2
Slowly Available Nitrogen Compounds:
The basic concept of controlled release or slow release fertilisers is that they release their nutrient contents at more gradual rates that permit maximum uptake and utilisation of the nutrient while minimising losses due to leaching, volatilisation or excessive growth.
Controlled or slow release fertilisers are broadly divided into uncoated and coated products. Uncoated products rely on inherent physical characteristics, such as low solubility, for their slow release. Coated products mostly consist of quick release N sources surrounded by a barrier that prevents the N from releasing rapidly into the environment.
1. Uncoated Controlled Release Fertilisers:
Uncoated materials of limited solubility can, for the most part, be manufactured in a smaller particle size than coated products, which makes them good choices for use on most close growing crops.
Another characteristic of uncoated products is that they are homogenous. That is, their composition is the same throughout the particle. By contrast, coated products, which consist of a fertiliser core or substrate, surrounded by a coating are not homogenous. If a particle is damaged due to cultural practices, it will release its nutrient almost immediately.
a. Ureaformaldehyde Reaction (UF) Products:
Urea and formaldehyde are reacted together to various extents to produce polymer chain molecules of varying lengths. The more these products are reacted, the longer the chains tend to be. Chain length, in turn, affects release characteristics.
Ureaform is the oldest class of UF reaction products. It is sparingly soluble and contains at least 35 per cent total N. Urea N content in UF is usually less than 15 per cent of the total N.
Methylene urea products predominantly contain intermediate chain length polymers. Total N content of these polymers is 39 to 40 per cent. The unreacted urea N content, generally, is in the range of 15 to 30 per cent of total N.
UF solutions are clear water solutions. Various combinations of the UF solutions are produced. They contain a maximum of 55 per cent unreacted urea with the remainder as one or more of methylolureas, methylolurea ethers, MDU, DMTU or triazone.
b. Isobutylidene Diurea (IBDU):
Unlike the reaction of urea and formaldehyde, which forms a distribution of different UF polymer chain lengths, the reaction of urea with isobutyraldehyde forms a single type of molecule. Although, similar in chemical structure to methylene diurea (MDU), its physical properties are quite different.
IBDU is a white crystalline solid available in fine (0.5 to 1.0 mm), coarse (0.7 to 2.5 mm) and chunk (2.0 to 3.0 mm) particle sizes. The product contains a minimum of 30 per cent N with 90 per cent N in water insoluble form.
Typical commercialized product contains about 31 per cent N. Nitrogen from IBDU becomes available to plants through hydrolysis. As the N release from IBDU is not microbe dependent, it occurs readily at relatively low temperatures.
2. Coated Slow Release Products:
Coated products have several advantages. Some coated products offer a relatively inexpensive means to exploit slow release characteristics. They also may offer desirable release characteristics in certain conditions.
a. Sulphur Coated Fertilisers:
Sulphur coated urea (SCU) technology was developed in the 1960s and 1970s by the National Fertiliser Development Center. Sulphur was chosen as the principle coating material because of its value as a secondary nutrient.
As the name suggests, SCUs are simply particles of urea coated with a layer of sulphur and usually a sealant as well. The SCUs are typically brown to tan or yellow, depending on the source of urea and whether a sealant is used. Total N content of SCUs varies with the amount of coating applied. Many range from 30 to 40 per cent N.
Mechanism of N release from SCU is by water penetration through micropores and imperfections (cracks) or incomplete sulphur coverage in the coating. This is followed by a rapid release of the dissolved urea from the core of the particle. When wax sealants are used, a dual release mechanism is created.
Microbes in the soil environment must attack the sealant to reveal the imperfections in the sulphur coating. Because microbial activity varies with temperature, the release properties of the wax sealed SCUs is also temperature dependent.
Release rate of a single SCU particle is directly affected by the coating thickness and the coating quality. Particles with thicker coatings may be desirable because they will extend the release rate. Depending on the coating weight, N application rate and environmental conditions, SCUs can effectively provide N for 6 to 16 weeks in turf applications.
b. Polymer Coated Fertilisers:
Polymer coated fertilisers (PCF) represent the most technically advanced state of the art in terms of controlling product longevity and nutrient efficiency. Most PCFs release nutrients by diffusion through a semipermeable polymer membrane and the release rate can be controlled by varying the composition and thickness of the coating. The type of fertiliser substrate also may influence the rate of N release.
Meister Products:
Meister products are produced by using thermoplastic resins as coating materials. The coatings are applied to a variety of substrates including urea, di-ammonium phosphate, potassium sulphate, potassium chloride and ammonium nitrate. Release controlling agents such as ethylene-vinyl acetate and surfactants are added to the coating to obtain the desired diffusion characteristics, while coating thicknesses remain similar for most products.
Release rates can also be altered by blending talc resin into the coating. As with other PCFs, nutrients are released by diffusion through the coating. Typical of most polymer coated fertilisers, the release is largely controlled by temperature.
Reactive layer coating:
A relatively new coating technology known as reactive layer coating (RLC) combines two reactive monomers as they are simultaneously applied to the fertiliser substrate. These reactions create an ultra-thin membrane coating, which controls nutrient release by osmotic diffusion.
The RLC products include coated basic fertiliser materials such as urea, potassium nitrate, potassium sulphate, potassium chloride, ammonium sulphate, ammonium phosphate and iron sulphate, in various particle sizes. Coating weights on urea vary from 1.5 to 15 per cent, depending on the release duration desired.
The coating thickness determines the diffusion rate and the duration of release for RLC products. RLC coated urea with a 4 per cent coating (44 per cent N) will release at twice the rate and will have half the duration as an 8 per cent coating (42 per cent N).
Multicote Products:
In the production of multicote products, fertiliser granules are heated in a rotating pan and treated with materials that create multiple layers of a fatty acid salt. This is followed by application of a paraffin topcoat.
Coating weights are relatively large compared to other technologies, but this is offset by the comparatively low cost of the coating materials. Substrates include potassium nitrate, urea and triple superphosphate. Various coated components are blended together into different grades.
c. Polymer Coated Sulfur Coated Fertilisers:
Polymer/sulfur coated fertilisers (PSCF) are hybrid products that utilise a primary coating of sulphur and a secondary polymer coat. These fertilisers were developed to deliver controlled release performance approaching that of polymer coated fertilisers but at a much reduced cost. Sulphur is employed as the primary coating because of its low cost.
Low levels of a polymer topcoat control the nutrient release rate. Unlike the soft wax sealants of SCUs, the polymers of PSCFs are chosen to provide a continuous membrane through which water and nutrients must diffuse, rather than fill in imperfections. Water permeability characteristics of the polymer control the rate of water diffusion in and out of the particle.
Combination of the two coatings permits a positive cost/benefit value over products with single coatings of either sulphur or polymer. The PSCFs possess excellent abrasion resistance and handling integrity. Because the outer coating is a hard polymer, the products do not leave waxy residues on application equipment.
The nutrient release mechanism of PSCFs is a combination of diffusion and capillary action. Water vapor must first diffuse through the continuous polymer layer. The rate of diffusion is controlled by the composition and thickness of the polymeric film.
Once at the sulphur/polymer interface, the water subsequently penetrates the defects in the sulphur coat through capillary action and begins to dissolve the fertiliser core. The dissolved fertiliser then exits the particle in reverse sequence.
This mechanism provides greater uniformity in nutrient release compared to typical SCU fertilisers. The agronomic advantages of this are reduced surge growth after application and longer residual; up to 6 months. In addition, the combination coating renders the nutrient release rate much less temperature sensitive than most polymer coated fertilisers.
Relative Efficiency of Nitrogen Fertilisers:
A number of experiments have been conducted on different crops under different situations.
General conclusions from these experiments are:
1. Under most conditions urea, calcium ammonium nitrate and ammonium sulphate, on equal nitrogen basis, are equally effective.
2. Ammoniacal fertilisers are more effective than nitrate fertilisers for lowland rice.
3. Nitrate nitrogenous fertilisers are better suited for top dressing.
4. For crops such as tea, ammonium sulphate is more effective than others.
Nitrogen Use Efficiency:
Response to applied nitrogen is very common due to its low content in most soils. Before the introduction high yielding cultivars, traditional crop varieties used to responded to 20-40 kg ha-1 through fertilisers, since organic manures used to meet part of crop nitrogen needs. Improved cereal cultivars are now responding to 120 or even up to 200 kg N ha-1 depending on other management practices.
During crop season, besides uptake by crop, losses through leaching, denitrification, volatilisation and fixing also takes place. Some nitrogen is immobilised by soil microbes. This results in low utilisation by crops.
The average utilisation of applied nitrogen by crops is around 50 per cent, though higher efficiencies around 75 per cent are also possible by adopting appropriate management strategies. On the other hand, efficiency of applied nitrogen is around 30 per cent in rice due to unfavourable soil water situation.
The nitrogen use efficiency can be expressed in different ways such as per cent utilisation of nitrogen (apparent recovery), economic yield per unit of nitrogen applied (agronomic efficiency) or grain yield in relation to nitrogen uptake (production efficiency) as indicated below:
Essay # 2. Phosphatic Fertilisers:
Phosphorus is present in plant tissues and soil in smaller amounts than are nitrogen and potassium and in quantities about equal to that of sulphur. Its tendency to react with soil components to form relatively insoluble compounds restricted its availability to plants.
Total phosphorus concentration in most soils ranges from 90 to 2,225 kg ha-1 with an average of about 890 kg ha-1 in the surface 20 cm of soil. Total phosphorus content of Indian soils varies from 0.03 per cent in West Bengal to 0.23 per cent in Maharashtra.
Availability of Inorganic Soil Phosphorus:
Inorganic soil phosphorus availability is influenced by soil pH, soluble iron, aluminium, and manganese, presence of iron, aluminium and manganese containing compounds, available calcium and calcium minerals, organic matter and activity of soil microbes.
1. Soil pH:
Ionic forms of phosphorus determine its availability to plants. The pH of solution influence ionic forms. In highly acid conditions, only H2PO4– ions will be present. If the pH is increased first HPO42- ions and finally PO43- ions dominate.
At intermediate pH, the two phosphate ions may be present simultaneously.
2. Precipitation by Fe, AI and Mn in Acid Soils:
Soluble iron, aluminium and manganese are usually found in strongly acid soils. The H2P04~ ions react with them leading to formation of insoluble hydroxy phosphates.
Reaction with Hydrous Oxides:
The H2PO4– ions react with insoluble hydrous oxides of iron, aluminium and manganese to form hydroxy phosphates.
3. Fixation by Silicate Clays:
Under moderately acidic conditions, phosphorus is fixed by silicate clays such as kaolinite.
There is a pronounced relationship between the amount of fixation of added fertiliser phosphorus and the R2O3: P2O5 ratio of the soil. This ratio is a measure of the amount of phosphorus present in relation to the iron and aluminium oxide content of the soil.
A wide ratio indicates a small amount of phosphorus present or a low phosphorus saturation value. Under such conditions larger amounts of added phosphorus is fixed than when the ratio is narrow.
4. Inorganic Phosphorus Availability in Soils with High pH:
At higher pH, availability of phosphorus depends on solubility of calcium compounds. If a phosphatic fertiliser containing H2PO4– is added to alkaline soil (pH 8.0), the H2PO4–ion reacts to form less soluble compounds.
Phosphate availability to plants will be maximum when the soil pH is maintained in the 6.0- 7.0 range.
5. Organic Matter and Microbial Activity:
Phosphorus in organic form can be mineralised and immobilised by the same general process pertinent for nitrogen and sulphur.
Intensity and Quantity Factors:
Ability of soil to maintain adequate level of phosphorus in soil solution is important for plant growth. The concentration of phosphorus in soil solution is a measure of the intensity (I) factor of phosphorus nutrition. If this factor is maintained at about 0.2 mg kg-1 or above, maximum yield of most crops will occur.
As plant absorbs phosphorus from soil solution, it is atleast partly replenished from the pool of soil solids that readily exchange phosphorus with soil solution. This source of soil solution replenishment is known as quantity (Q) factor of phosphorus nutrition. The Q level needed to provide a given intensity (I) will vary from soil to soil.
Clay soils with high iron and aluminium need high Q level to assume an l level sufficient for normal plant growth. Sandy soils with low iron and aluminium provide higher intensity (I) with a given quantity (Q) of phosphorus. The clay soil is said to be higher buffered with respect to phosphorus than the sandy soil.
The potential buffer capacity (PBC) of a soil is given by:
PBC = ∆Q/∆l
where, ∆Q is the change is quantity (Q factor) and ∆l is the change in intensity (l factor). The Q/l relations of phosphorus in soils and PBC are important in determining phosphorus fertiliser requirements for high yields.
Relative Efficiency of Phosphate Fertilisers:
From the results of experiments conducted under different situations, general conclusions are:
1. For short duration crops and those with restricted root system, fertilisers with high proportion of water soluble phosphorous are advantageous.
2. Higher water soluble phosphorus is less important for long duration crops.
3. Higher water solubility is desirable for crops requiring quick start.
4. Localised placement of water soluble phosphate fertilisers is more effective when the rate of application is limited.
5. On acid soils, granular fertilisers with high degree of water solubility are more effective than powdered fertilisers when the fertiliser is to be mixed with soil.
6. On acid to neutral soils, band placement of powdered fertilisers with high degree of water solubility will give better results than mixing the fertiliser with soil.
7. On calcareous soils, granular water soluble phosphates are more effective.
8. Fertilisers with low water solubility will give good results when applied in powered form mixed with soil.
9. Mono-ammonium phosphate is better than di-ammonium phosphate on calcareous soils.
10. Rock phosphate and bone meal are ideal for strongly acid soils and for long duration crops.
Essay # 3. Potassic Fertilisers:
Potassium is absorbed by plants in larger amounts than any other element except nitrogen. Unlike phosphorus, potassium is present in relatively large quantities in most soils. On an average, potassium of the earth’s crust is about 1.9 per cent. Its concentration in soils normally varies from about 0.5 to 2.5 per cent with a typical value of 1.2 per cent.
More than 95 per cent of the total potassium in the soil is within the crystal-lattices of silicate minerals. The micas (muscovite and biotite) and the feldspars (orthoclase microcline) constitute the major K bearing minerals. Potash is also found in soils in the form of secondary or clay minerals such as illites, vermiculites, chlorites and interstratified minerals.
Forms of Soil Potassium:
Soil potassium exists in four forms based on their availability to plants: water soluble K (solution K), exchangeable K, non-exchangeable (fixed) K and lattice or inert reserve K. All these forms are in dynamic equilibrium with one another.
1. Water Soluble K:
It is present as soluble cation in solution. Its concentration depends on the type of clay, water content, intensity of leaching, amount of exchangeable K and concentration of other ions. It is in equilibrium with exchangeable K and hence difficult to distinguish.
2. Exchangeable K:
It constitutes the K adsorbed on soil clay complex and replaceable with neutral salts in a relatively short time. In mineral soils, it is usually less than 1.0 per cent of the total K. In organic soils, this fraction represents most of total K. Its content in soils depends on mineralogy of exchange complex, soil texture, moisture content, leaching, liming, fertilisers used and complementary ions.
3. Non-Exchangeable K:
That part of the added K which is firmly bound by the soil and not immediately replaceable with neutral salts is said to be in fixed form. It is bound between the basal planes of chiefly micaceous minerals.
4. Lattice K:
It constitutes major part of total K in mineral soils and is present in primary minerals. Of the clay minerals, illite has substantial K content. Capacity of soils to release K by weathering depends on the content of potash minerals and on soil texture. Higher the specific- surface of the soil, the greater is its power to release K by weathering.
Potassium Fixation:
Potassium fixation is maximum in soils high in 2: 1 clay and with large amounts of illite. Potassium fixation is the result of re-entrapment of K+ ions between layers of the 2: 1 minerals, especially those such as illite. Other cations, excepting NH4+, are apparently too large for entry into openings in the oxygen network of the silicate sheets.
Potassium ions are sufficiently small to enter the silica sheets where they are firmly held by electrostatic force. The NH4+ ion has nearly the same ionic radius as the K+ ion and is subjected to similar fixation by 2: 1 clays.
Since NH4+ can be fixed by clays in a manner similar to that of K+, its presence will alter both the fixation of added potassium and release of fixed potassium. There will be reduction in release of non-exchangeable potassium with increasing amounts of added NH4+.
The NH4+ ions are held in openings in the oxygen network of silicate sheets, thus closing the adjacent sheets and further trapping the K+ ions already present. It is not generally considered to be a serious factor in limiting crop responses to either applied NH4+ or K+.
Fertilisers Containing Potassium:
Main sources of potassic fertilisers are the minerals carnallite (KCl, MgCl2.6H2O), kainite (KCl, MgSO4.3H2O), langbeinite (K2SO2, 2MgSO4) and Sylvite (KCI).
Conversion of K to K2O and vice versa can be with the following expressions:
Per cent K = Per cent K2O x 0.83
Per cent K2O = Per cent K x 1.20
1. Potassium Chloride (KCl):
Potassium chloride (muriate of potash) is produced by dissolving sylvinite or sylvite in hot water and by adding sodium chloride to it. As the temperature increase, more and more sylvinite is gradually added until a hot saturated solution of potassium chloride is formed. It is suddenly cooled to obtain crystals of potassium chloride. It is marketed in granular form guaranteed to contain 60 per cent potash.
2. Potassium Sulphate (K2SO4):
It is manufactured in two ways:
Langbeinite and
Mannheim process.
Production from Langbeinite proceeds according to the equation:
K2SO4.2MgSO4 + 4KCl à3K2SO4 + 2MgCl2
The overall reaction in Mannheim process is:
2KCl + H2SO4→K2SO4 + 2HCl
Potassium sulphate (sulphate of potash) contains 52 per cent K2O. From the point of view of plant nutrition, both the above fertilisers are equally effective. Since the quality of potato and tobacco is affected by the amount of chlorine in the soil, sulphate of potash is recommended for these two crops.
Fertiliser Calculations:
A. Nutrient Content in Fertilisers:
Nutrient content in fertilisers (per cent) can be calculated as given below:
1. Urea:
Chemical formula: CO (NH2)2
where C = 12, O = 16, N = 14, H = 1.
Therefore, molecular weight of urea = 12 + 16 + 2 (14 + 2) = 12 + 16 + 32 = 60
For a molecular weight of 60, N = 28
For a molecular weight of 100, N =28 x 100/60 = 46.6 or 46
2. Muriate of Potash:
Chemical formula: KCl
where KCl = 39.1, CI = 35.5
Molecular weight of KCl = 39.1 + 35.5 = 74.6
For a molecular weight of 74.6. K = 39.1
For a molecular weight of 100. K =39.1 x 100/74.6 = 52.4 or 52
B. Fertilisers and Plant Nutrient Supplies:
1 kg nitrogen = 5 kg ammonium sulphate
= 4 N kg CAN = 2.22 kg urea
1 kg phosphate = 6.25 kg superphosphate
1 kg potassium = 1.666 kg MOP 60% or 2 kg MOP 50%
1 kg nitrogen + 1 kg phosphate = 5 kg suphala 20:20: 0
1 kg nitrogen + 1 kg phosphate = 3.5 kg urea ammonium phosphate
1 kg nitrogen + 1 kg phosphate + 1 kg potash = 6.660 kg suphala 15: 15 : 15
C. Required Quantity of Fertilises for Application:
Recommended rate of nutrient application for rice crop is 120:60: 60 (N: P2O5: K2O). Calculate the quantities of urea, single superphosphate and MOP required for an ha of cropped area.
1. Urea:
1 kg N = 2.22 kg urea
120 kg N = 2.22 x 120 = 266.40 urea
2. Superphosphate:
1 kg P2O5 = 6.25 kg single superphosphate
60 kg P2O5 = 6.25 x 60 = 375 kg single superphosphate
3. MOP:
1 kg K2O = 1.666 MOP
60 kg K2O= 1.666 x 60 = 99.96 kg MOP
Alternately,
Urea required: 45 kg N = 100 kg urea
120 kg N = (120/45) x 100 = 266.66 kg urea
Single superphosphate required:
16 kg P2Os = 100 kg SSP
60 kg P2O5 = (60/16) x 100 = 375 kg SSP
MOP required: 60 kg K2O = 100 kg MOP
60 kg K2O = (60/60) x 100 = 100 kg MOP
Essay # 4. Calcium, Magnesium and Sulphur:
Calcium, magnesium and sulphur are called secondary nutrients as these nutrients are of secondary importance in the manufacture of commercial fertilisers. These are added to the soil through some of the commercial fertilisers.
Calcium:
Calcium concentration of the earth’s crust is about 3.64 per cent. Soils vary widely in calcium content with sandy soils of humid regions containing very low amounts. Main source of soil calcium is rocks and minerals from which the soils are formed.
Mineral anorthite (CaAl2Si2O6) is the most important source of calcium. Calcite (CaCO3) is the dominant source of calcium in soils of semi-arid and arid regions. In some arid soils, gypsum (CaSO4, 2H2O) may be present. Approximate percentage of calcium in commonly used fertilisers is given in Table 6.15.
Magnesium:
Magnesium constitutes 1.93 per cent of the earth’s crust. It ranges from 0.1 per cent in coarse sandy soils of humid regions to about 4.0 per cent in fine textured arid or semiarid soils. Primary minerals such as biotite, dolomite, olivine and serpentine are the major source of soil magnesium. Secondary minerals like chlorite, illite, montmorillonite and vermiculite also contribute to the soil magnesium.
Magnesium deficiency usually occurs in soils with wide ratios of exchangeable Ca/Mg. The ratio should not be greater than 7:1. High levels of exchangeable potassium also interfere with magnesium uptake. On a weight basis, general recommended K/Mg rations are less than 5: 1 for field crops, 3: 1 for vegetables and 2: 1 for fruit crops.
Magnesium is indirectly supplied to soils (Table 6.16) through commercial fertilisers and soil amendments.
Sulphur:
The earth’s crust contains about 0.6 per cent sulphur, mostly present as sulphides, sulphates and organic combinations with C and N. Initial source of sulphur is sulphides contained in plutonic rocks. The main form of sulphur in humid zones is organic sulphur. In arid zones, sulphates of Ca, Mg, Na and even K predominate. Another source of sulphur is the atmosphere. The third source of soil sulphur is added fertilisers (Table 6.17).
The C:N:S ratio in the soil is, generally, in the order of 130:10: 1.3. Generally, crops need one-tenth to one-fifteenth as much S and N. Legumes, turnips, onion, garlic and mustard need more sulphur than other crops.
N: S Ratio:
Sulphur metabolism in plants is linked with N, as both are constituents of proteins. A deficiency of S in plant retards its N metabolism leading to increased concentration of N in amide and NO3 forms. Optimum N: S ratio in plants has been found to be 15-16: 1 in legumes and 11-12: 1 in cereals.
The ratio varies among soils due to differences in available soil N and S levels. However, one estimate of suitable available N: S ratio (soil NO3–N + fertiliser N)/(soil SO4–S + fertiliser S) is approximately 7 under upland conditions. A 10: 1 N: S ratio in fertiliser can adequately meet crop requirements. Shortage of S in relation to N leads to poor N use efficiency.