Macro-, secondary and micro-nutrients, slow-release fertilizers and fertigation products

1. Macronutrients

1.1 Nitrogen

Since 1973/74, most of the increase in world nitrogen (N) consumption is accounted for by urea, most of the phosphate (P205) by diammonium phosphate and the potash (K2O) market is dominated by potassium chloride. The main factor influencing the share of the different products is the nutrient concentration of the product - the higher the concentration the lower the distribution, storage and handling costs per unit of nutrient; these are major items in the total cost structure. It is no coincidence that each of the most important products, urea, diammonium phosphate and potassium chloride has a high nutrient content.

As can be seen from the diagram, the increase has been particularly strong in the case of urea. Between 1973/74 and 1997/98, urea consumption increased from 8.3 million tonnes N to 37.6 million tonnes N, from 22% to 46 % of total world nitrogen consumption.

1.2. Phosphates

More than 99% of all phosphate fertilizers are derived from phosphate rock; a relatively small quantity being supplied in the form of basic slag, a by-product of the steel industry. About 2% of the rock is applied directly, without processing, most of the remainder being solubilized with mineral acids, essentially phosphoric and sulphuric acids. Two thirds of phosphate fertilizers are derived from rock treated with phosphoric acid; during the past 30 years a large proportion of the net addition to phosphate fertilizer production has been in the form of phosphoric acid based fertilizers.

The ammonium phosphates, diammonium phosphate, (DAP) and monoammonium phosphate (MAP), account for all the increase in world phosphate consumption since 1973/74.

1.3. Potash

Virtually all potash fertilizers are derived from mined potash salts. 95% of world potash production is in the form of potassium chloride and is used for fertilizers. Over 60 % of fertilizer potash is used without further processing, whilst the remainder goes to produce multi-nutrient fertilizers (including potassium sulphate).

Complex fertilizers, secondary nutrients, micronutrients, slow release fertilizers

Compound fertilizers are those containing more than one of the fertilizer nutrients, either in chemical combination and/or as a mixture of different fertilizers. A complex fertilizer is a compound in which all the nutrients are present in each granule.

1.4. Complex Fertilizers

Up to the mid 1970s the global trend in fertilizer consumption was towards the use of NPK complex fertilizers supplemented by straight nitrogen fertilizers according to crops' requirements. For example, from 1960/61 to 1973/74, 64% of the increase in phosphate consumption and 52% of the increase in potash consumption was in the form of NPK complex and PK compound fertilizers. Between 1973/74 and 1997/98, on the other hand, 140% of the phosphate increase was in the form of ammonium phosphate and 193% of the potash was in the form of muriate of potash. The global share of NPK-N declined from 15% in 1973/74 to 8% in 1997/98. Urea consumption has increased during the latter period from 8.3 Mt N to 37.6 Mt N or from 22% to 46% of total N.

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.

2. Secondary Nutrients

Calcium, sulphur and magnesium also are required in large quantities by plants. They are often called 'secondary nutrients'.

2.1. Calcium

Most soils contain sufficient calcium. Calcium is applied incidentally in liming materials used to correct soil acidity. In the event of a deficiency, calcium may be applied as calcium chloride or as a component of foliar sprays.

2.2. Sulphur

Sulphur deficiencies occur with increasing frequency, due to the increasing share of fertilizer such as urea, ammonium phosphate and potassium chloride, that do not contain sulphur, and to obligatory reductions in emissions of sulphur dioxide from the burning of fossil fuels. The content of sulphur applied annually in various fertilizer products, such as single superphosphate, ammonium sulphate, potassium sulphate and incorporated in NPK compound fertilizers is estimated at ten million tonnes S.

2.3. Magnesium

Crops' requirements of magnesium are substantial. Deficiencies are fairly common especially on light, sandy soils. Soil magnesium can be maintained through the application of liming materials such as dolomitic limestone, kieserite (magnesium sulphate) or other fertilizers containing magnesium.

3. Micronutrients

Although needed in very small amounts (g/ha) compared with macronutrients (kg/ha), if there is a deficiency crop yield and quality may be affected in the same way. Micronutrients whose content in plants is critical are the metallic elements iron, manganese, zinc, copper and molybdenum and the non-metallic elements chlorine and boron. In addition certain other nutrients may be beneficial for certain plants, such as sodium, silicon, cobalt, chlorine and aluminium.

Micronutrients are often applied incorporated in NPK fertilizers. They may also by 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.

4. Slow Release Fertilizers

Slow release fertilizers can be a more efficient source of nutrients than conventional fertilizers but, because of their higher cost, they are not normally applied to field crops. Japan is one of the few countries where the use slow-release fertilizers is developing on field crops, especially paddy rice. They are used on higher value crops such as vegetables and ornamentals. Further information on slow release fertilizers can be obtained from the IFA publication 'Controlled-release and Stabilized Fertilizers in Agriculture' (1997).

5. Fertigation

Fertigation is the application of fertilizers through special micro irrigation equipment. Fertigation is potentially the most efficient and environmentally sound system of irrigation. However, it is a demanding system and is efficient only if managed properly. To avoid blockages, fertigation requires the use of high quality, fully water-soluble fertilizers. It is estimated that fertigation is applied on about a million ha of land, and it is increasing.