Introduction: The primary nutrients supplied by fertilizers are nitrogen, phosphorus and potassium. Their concentration in a fertilizer is expressed as percentage of N, P₂O₅ and K₂O.

According to nutrient content of fertilizers, it can be classified as straight and complex fertilizer. Straight fertilizer contain only one plant nutrient where as complex fertilizer contain more than one primary or major nutrient element.

1. Primary nutrients

2. Secondary Nutrients

3. Micronutrients

Primary nutrients

Primary nutrients are normally supplied through chemical fertilisers. They are chemical compounds containing one or more of the primary nutrients and are generally produced by chemical reactions. Whatever may be the chemical compounds, its most important ingredient for plant growth is the nutrient content.The primary nutrients are nitrogen, phosphorus and potassium; however, their concentration in a chemical fertiliser is expressed as a percentage of total nitrogen (N), available phosphate (P ²O⁵) and soluble (K₂ O).

Straight fertilizer

Nitrogen: Nitrogen is the first fertilizer element of the macronutrients usually applied in commercial fertilizers. Nitrogen is very important nutrient for plants and it seems to have the quickest and most pronounced effect. In the case of nitrogenous fertilizers, nitrogen may be in the ammoniacal, nitrate (or a combination thereof) or amide form.

Nitrogenous fertilizer - Source of Nitrogen

Phosphorus: Phosphorus is the second fertilizer element and it is an essential constituent of every living cells and for the nutrition of plant and animal. It takes active part in all types of metabolism of plant. Phosphate present in phosphatic fertilizers may be in the water soluble form or citrate soluble form. That portion of phosphate which is soluble in water is called water soluble phosphate and that which is not soluble in water but in 2 per cent neutral ammonium citrate solution is called citrate soluble phosphate. The sum of water soluble and citrate soluble values is termed as available phosphates.

 Phosphatic fertilizer – Source of phosphorous

Potassium: Potassium is the third fertilizer element. Potassium acts as a chemical traffic policeman, root booster, stalk strengthener, food former, sugar and starch transporter, protein builder, breathing regulator, water stretcher and as a disease retarder but it is not effective without its co-nutrients such as nitrogen and phosphorus.

Potassic fertilizer – Source of potassium

Complex fertilizers

Complex NPK fertilizers have the advantage of having each nutrient in each granule. They are more expensive than the equivalent quantity of nutrients achieved by applying the equivalent quantities of straight fertilizers, but the product quality is guaranteed. Sulphur and/or other nutrients can easily be incorporated if required. By using the appropriate complex fertilizer, farmers are applying at least an approximation of each nutrient required. Absolute precision is not necessary in view of soil variability even within the same field.

Complex fertilizers are, therefore, efficient products but trends during the past thirty years in the types of fertilizers used have been determined by their economics, not by considerations of agricultural efficiency or sustainability.

Complex fertilizers

Secondary Nutrients

Calcium, magnesium and sulphur are called the secondary nutrients. It is required for plant growth in relatively smaller quantities than primary nutrients. Any deficiency of the secondary nutrients and other essential elements reduce the efficiency of primary nutrient by restricting the yield to low levels. Therefore, to obtain optimum results, crops have to be supplied with secondary nutrient in addition to primary nutrients.

Calcium and magnesium are the elements that are largely responsible for the acidity or alkalinity of soil. They are moderately soluble, so in regions of high rainfall they are washed from the soil and carried away. Soils in these regions are usually acid, and lime (calcium carbonate) is often added every year to make the soil less acid. In areas of low rainfall, and where the soil is derived from limestone, the soil is high in calcium and magnesium and is usually alkaline.

Sulfur is present in many commercial fertilizers, even though it is not listed on the label. As sulfate, it is highly acid and is often used to make alkaline soils acid.

Fertilizer sources containing secondary nutrients:-

Calcium sources

Calcium is mostly applied in liming materials to correct soil acidity. In the event of a deficiency, calcium may be applied as calcium chloride or as a component of foliar sprays

Common calcium sources

*Based on pure calcium carbonate at 100%

Magnesium sources

Crops' requirements of magnesium are substantial. Deficiencies are fairly common especially on light, sandy soils. The most common source of mg is dolomitic limestone, which provides both Ca and Mg while neutralizing soil acidity. Other sources include potassium-magnesium sulphate, magnesium oxide and basic slag.

Common Magnesium sources

The common Mg sources along with their percent Mg content are given in the above table. The Mg sulphate forms are more soluble than dolomitic lime and may be the preferred Mg source on those soils where a quick crop response is required.

Sulphur sources

Soil organic matter is the primary soil S source. More than 95 percent of the S found in the soil is tied up in organic matter. Other natural sources include animal manures, irrigation water and the atmosphere. Animal manures contain S levels ranging from less than 0.02 to about 0.3 percent. Obviously, the content varies considerably, depending on species, method of storing and application, etc. sulfur dioxide and other atmospheric gases dissolved in rain and snow can contribute up to 22 kg of S per hectare per year even more in some industrialized areas. Irrigation water can contain fairly high levels of S. When the sulphate-S content of irrigation water exceeds 5ppm, a S deficiency is highly unlikely. Still, starter fertilizer applications of S for new crops may be beneficial because of sulphate mobility during winter precipitation. Common sulphur sources are given below.

Common sulphur sources

Most fertilizer S sources are sulphates and are moderately to highly water soluble. Soluble forms also include bisulphites, thiosulphates and polysulphites. The most important water insoluble S fertilizer is elemental S, which must be oxidized to the sulphate-S form before plants can use it. Bacterial oxidation of S in the soil is favoured by:

1. Warm soil temperature

2. Adequate soil moisture

3. Soil aeration

4. Fine particle sizes

The water soluble sulphates are immediately available to plants and should be used when S is needed quickly. They are commonly used in dry fertilizers, although ammonium sulphate solutions are also common. Ammonium thiosulphate (ATS; 12-0-026) is a clear liquid well-suited for use in fluid fertilizers or addition to irrigation water. It should not be seed placed and, if band applied, should be at least 2.5 cm from the seed. Ammonium polysulphide (APS) is a red fluid S source with strong odor of ammonia, commonly applied in irrigation water. Sulfur in APS must be oxidized to the sulphate form to be available to plants.

Although gypsum (calcium sulphate) is less water-soluble than the other sulphates, it is an effective and inexpensive S source.

Micronutrients

Micronutrients are a group of nutrients which are essential for plant growth but are required by plants in small quantities. Intensive cropping depletes all nutrients including micronutrients from the soil at a fast rate. Therefore selective use of micronutrients is necessary for increasing agricultural production. Micronutrients are often applied incorporated in NPK fertilizers. They may also applied in solution, directly or sprayed on bulk blends. Where quick action is required, salts of micronutrients dissolved in water are sprayed onto crop foliage.

Iron, zinc, manganese, copper, boron, molybdenum and chlorine fall under this category.

Boron sources

Boron can be applied to the soil as a broadcast or band application or applied foliar as a spray or dust. Soil application rates for responsive crops may be high as 3 kg B/ha and for low and medium responsive crops, 0.5 to 1.0 kg/ha. Common B fertilizer sources, their B contents and water solubility are given below:

Boron fertilizer sources, boron contents and water solubility

Copper Sources

The following table lists common Cu sources, their Cu contents, water solubility and application methods.

Iron Sources

Common Fe fertilizer sources and their Fe contents are given in the table below. Soil application or foliar spray can correct crop deficiencies to some degree. Applying soluble materials (such as ferrous sulphate) to the soil is not very efficient, because the Fe converts rapidly to unavailable forms. When these materials are applied as foliar sprays, they are much more effective. Injections of dry Fe salts directly into trunks and limbs have controlled Fe chlorosis in fruit trees. Most Fe fertilizer sources are best applied as foliar spray. This method involves lower rates than soil application.

Altering soil pH in a narrow band in the root zone can correct Fe deficiencies. Oxidisable sulphur (S) products will lower soil pH and convert insoluble soil Fe to a form the plant can use.

Manganese Sources

Manganese deficiencies can be corrected by adding soluble Mn salts, such as Mn sulphate, with starter fertilizer and apply in a band. High P starter fertilizer helps mobilize Mn into the plant. A field deficiency symptom can be corrected by foliar application. Spraying with 10 kg/ha MnSO in water is a common treatment for Mn deficient soybeans. In extremely acid soils (pH<5), Mn toxicity may occur. Liming will eliminate this problem. Following table shows common Mn fertilizer sources and their Mn contents.

Molybdenum

The common Mo fertilizer sources, their Mo contents and water solubility are given in the following table.

Zinc Sources

The best procedure for correcting Zn deficiencies in field crops is to apply a Zn fertilizer broadcast or in a starter fertilizer beside the row. Application rates range from 1 kg Zn/ha to as high as 10 kg/ha, depending on soil test levels. Very low rates should be applied in a starter to improve the effectiveness of application. Zinc has excellent residual effects and high application rates may be sufficient for 3 to 4 years. Zinc fertilizer sources are listed in the table below.

When soil tie-up of Zn is expected under pH conditions or where an emergency situation exists on an established crop, Zn may be applied as foliar spray. Foliar sprays usually require about 0.5 to 1.0 kg Zn/ha.