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Model Farming


Ftilizer selection for sustainable agriculture  
Compiled by: Mohammad Ali Khaskheli
Agriculture Officer Shahdadpur

Crops require 16 essential elements to grow properly. The elements include carbon (C), hydrogen (H) and oxygen (02); those are derived from air and water. All the remaining nutrients used by plants come from soil in the form of inorganic salts.

The macronutrients (plant nutrients required in larger amounts i.e. > 500 mg/kg) obtained from the soil include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulphur (S). The remaining essential elements needed by plants are known as micronutrients because plants use them in relatively small amounts (i-e. <100 mg/kg in plants). They include: boron (B), chlorine (Cl), copper (Cu), iron (Fe) manganese (Mn), molybdenum (Mo) and zinc (Zn). Carbon, hydrogen and oxygen comprise from 94.0 to 99.5 per cent of fresh plant tissue. The remaining nutrients, which come from the soil, make up the balance of the tissue.

Fertilizer has played a key role in helping farmers to achieve their high level of production. Fertilizers provide essential plant nutrients which are indispensable for producing sufficient and healthy food for the world’s expanding population. Plant nutrients are therefore a vital component of any system of sustainable agriculture. Nitrogen (N), Phosphorus (P), and in recent years, zinc, boron and sulfur are the nutrients of most concern in the grain-production regions. Unfortunately the soils of Pakistan are deficient in Nitrogen (100 %), Phosphorus (90%), Zinc (70%) and Boron (55%). Potassium (K) is generally adequate but its deficiency is emerging rapidly. Deficiencies and responses to other nutrients such as iron (Fe), magnesium (Mg), and other micronutrients are reported for specific crops and areas. When the soil cannot supply the level of nutrient required for adequate growth, supplemental fertilizer applications become necessary.

Prior to the introduction of fertilizes in Pakistan in the early1950s, the use of fertilizers have increased significantly. Total consumption of nutrients in Pakistan has increased from 5 kg/ha in 1966-67 to 133kg/ha in 2001-2002. However, it is still much lower when compared with other countries of the world and highly unbalanced to produce enough and quality food to meet the country demand. The crop yields in countries using higher fertilizer rates (e.g. Korea, Japan China, Egypt etc) are two to three times more than Pakistan.

In Pakistan various types of fertilizers are used, some are locally manufactured and others are imported. In our country, most of the fertilizer is used on irrigated wheat, cotton, sugarcane and rice crops. On these crops the nitrogen application rate is close to 75-80 percent of the recommendations, compared with about 20-40 percent, depending on the crop, in the case of phosphate. Hardly 1-2 percent of farmers apply potash; that is usually applied to fruit, vegetable, and sugarcane crops only. Micronutrient deficiencies are common but less than five percent of the farmers apply micronutrient fertilizers.

Improper use of fertilizer is a common problem in our country. Farmers want to ensure good yields, and apply unsuitable and so much fertilizer that much of it is wasted. The problem is particularly acute with cash and other horticultural crops.

Declining land productivity with reduced crop yields has been also a major problem facing our farmers. The major factors contributing to the reduced land productivity is soil impoverishment caused by continuous cropping without addition of adequate mineral fertilizers and manures. Moreover, negative soil nutrient balances (nutrient removal exceeding nutrient application) during our cropping history have resulted in general deterioration of fertility levels. Sustained, high yield agricultural production can be assured once these negative balances are addressed. The selection of suitable fertilizer according to the crop demand and soil type is one of the main tools available for the solution of above said constraints.

Selection of Fertilizers
What type of fertilizer should buy and apply? The answer to that question is not an easy one. Economics is important, but quality, rather than cost per unit of nutrient, should be the deciding factor in fertilizer selection. The cost of fertilizer usually is small in relation to the total cost of producing a crop.

Fertilizer quality cannot be defined in rigid terms. If you know the composition and properties of fertilizers and their behavior in the soil, you have a guide for choosing them for specific purposes.

Characteristics that should be appraised before a fertilizer is selected are: solubility, effect upon soil pH, form of nitrogen, the salt index, and cost per unit of available nutrient.

Solubility. Fertilizer compounds differ greatly in their solubility in water. These differences are usually unimportant for application in the solid form. Recently attention has been given to the production of fertilizers that have low initial solubility and release nutrients to the soil and plant gradually over an extended period of time. High solubility, however, is one of the major considerations for the grower who purchases solid fertilizer for dissolving in irrigation water, or for application in foliar sprays. Let's compare the solubility of some fertilizers.

Solubility of Some Fertilizers




Solubility (g/l)


*Acidifying effect


Ammonium nitrate


Ammonium sulfate


Muriate of potash




Sulfate of potash


Diammonium phosphate


Monoammonium phosphate


Single Super phosphate


Triple Super phosphate








































*The acidifying effect or value indicates the pounds of 100-score lime needed to neutralize the acidity produced by fertilizer application of 100 pounds per acre.

Effect upon soil pH. Fertilizer nutrients may be divided into three classes according to their influences on the soil reaction (pH): (1) acidic -- pH below 7.0 (2) neutral -- pH of 7.0 and (3) basic -- pH above 7.0.

Use of ammonia compounds will eventually have an acidifying effect on the soil. Due to this soil acidifying action some minor elements such as iron, manganese, copper, zinc, and molybdenum become available. This does not mean that the entire soil becomes acidic, just the area around each fertilizer particle or concentration of particles. This acidifying effect is usually of short duration.

Phosphates are generally neutral in their effect on soil pH, and potassium carriers are basic.

Form of nitrogen. The form of nitrogen applied influences directly the nutrition and growth of the plant. In general, plant roots assimilate two common forms of nitrogen-nitrate (NO3) and ammonium (NH4). Most plants seem to prefer the nitrate form.Nitrate and ammonium are readily absorbed and utilized by plant roots. Under favorable conditions, ammonium nitrogen is converted to the nitrate form by nitrifying bacteria in the soil. Therefore, the ammonium forms are not available as rapidly as the nitrate forms. Under adverse soil conditions (temperature below 500 degrees F, too wet or too dry, high salt concentrations, etc.), the conversion of ammonium to nitrate is retarded or halted, and the soil accumulates high levels of ammonium. These high levels may become toxic to plants. Urea is quite soluble and is rapidly broken down to a usable form of nitrogen.

Besides uptake by plants, nitrogen is also removed from the soil by leaching and volatilization. Leaching means the nitrogen is washed or flushed out of the soil by irrigation or rain water. Volatilization means the nitrogen escapes as gas.

Nitrate nitrogen is sometimes leached from the soil, and ammonium is sometimes volatilized if it is not converted to the nitrate form. Ammonium is rarely leached because it is usually adsorbed to the clay particles of the soil. Nitrate does not become attached.

Stabilized ammonium fertilizer is slowly converted to the nitrate form. Since that nitrate is available to the plants over a longer time, it is less subject to leaching or conversion to nitrogen gas.

Salt index. Seedling injury or "fertilizer burn" occurs when the soil solution in contact with the seed or root contains a high concentration of salts. The plant seedling, because of the high salt concentration, is unable to absorb moisture from the soil solution.

Salt injury may result from a high rate of salt-forming fertilizers, improper placement of fertilizers, irrigation with saline water, or farming on saline soils.

Determination of the salt index of a fertilizer is a means of measuring its tendency to cause seedling injury or plant "burn." The lower the salt index of a fertilizer, the less likely it is to cause damage.

Fertilizers with the highest salt indexes generally supply nitrogen as the primary nutrient, high potash materials have intermediate salt indexes, and phosphate materials have the lowest.


Salt Index of Some Fertilizer Materials



Material and Analysis


Salt Index


Anhydrous ammonia, 82% N

Ammonium nitrate, 34% N

Ammonium sulfate, 21% N, 24% S

Urea, 46% N

Urea-ammonium nitrate solution:

28% N (39% ammonium nitrate, 31% urea)

32% N (44% ammonium nitrate, 35% urea)

Calcium nitrate, 15.5% N

Sodium nitrate, 16.5% N


Single superphosphate, 20% P205

Triple superphosphate, 45% P205

Monoammonium phosphate:

11% N, 52% P205

10% N, 50% P205

Diammonium phosphate, 18% N, 46% P205

Ammonium polyphosphate, 10% N, 34% P205


Potassium chloride, 60% K20

Potassium nitrate, 13% N, 44% K20

Potassium sulfate, 50% K20, 18% S


Ammonium thiosulfate, 12% N, 26% S

Ammonium polysulfide, 20% N, 40% S

Gypsum, 23% Ca, 17% S

Magnesium oxide, 60% Mg

Magnesium sulfate, 10% Mg, 14% S


Calcium carbonate, lime, 35% Ca

Dolomite, 21.5% Ca, 11.5% Mg



































Cost per unit of nutrient. Several factors cause fertilizer nutrients to vary in cost, such as the different manufacturing processes required in their manufacture, shipping and handling costs are greater per unit of nutrient in low analysis fertilizers than in high analysis fertilizers, the physical form of the fertilizer, whether gas, liquid or solid affects handling cost. The size of purchase also affects unit cost. In general a low-analysis fertilizer in a small package is expensive in comparison with high-analysis fertilizers in ton lots in bulk form. The farmer who uses several tons of fertilizer each year has more options in buying fertilizer than the person with a lawn or garden.

Here are some important research facts which should be kept in mind before using or purchasing any fertilizer;

• Ammonia fertilizers, whether dissolved in irrigation water or not, causes a surface "sealing" when applied to a calcareous soil. The symptoms of the seal are similar to a sodium-affected soil-poor drainage, which causes water to pool or stand on the soil surface.

• Phosphates move very little in the soil, and the secondary orthophosphate (HPO4) is the phosphorus form used most by plants in our soils.

• Phosphates are readily and rapidly "fixed" by our alkaline soils. Liquid phosphates are fixed much more rapidly than the dry or granulated forms.

• The polyphosphates seem to be no better or no worse than the usual phosphate forms. There is speculation that the polyphosphates serve as a chelating agent. If so, they would have decided advantages.

Fertilizers are one of the very expensive soil inputs; therefore a soil test should be performed before the purchase or application of any "special purpose" fertilizers. It is not possible to make a blanket statement that one fertilizer is best for all crops every where. It is true that different crops use different nutrients at different rates. The unknown is the reserve of nutrients already in your soil and soils differ widely in their capacity for providing nutrients, depending on the amount of total reserves, on mobilization or fixation dynamics, accessibility of the chemically available nutrients to the roots, etc.

Therefore it remains necessary to assess empirically the nutrient status of soils and plants in order to provide guidelines for effective fertilizer use.

Lack of knowledge is widespread and is usually due to poor coordination between those working in research and those in the field working as extension or field officers. Local research work is required into soil and crop conditions, balanced fertilization, whether macro and micro-nutrients are required, the use of animal manure and compost, the use of improved seed, better cultivation and harvesting techniques, and the economics of fertilizer use. Extension workers must make use of demonstrations, preferably on farmers’ fields and keep up a constant flow of information by farm visits and by the use of radio and television. The availability of proper fertilizers, according to soil type and crop demand should be assured at right time.;

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