Nutrient balance and chemical fertilizer control in Iran

Adequate nutrition of plants is one of the most important factors in the improvement of the quantity and quality of crops. For appropriate plant nutrition, it is not only essential to provide the required amount of each element to the plant, but to establishment of the correct balance between nutrients is a top priority, because under conditions of imbalance, yield and efficiency of fertilizer will be reduced. Unfortunately in Iran, the ratio of N, P, and K used is often ill-founded, and it may congruent with the requirements of the crop. In addition the time when these nutrients are most required has not been included in the fertilizer recommendation. Thus N fertilizer is generally applied to the crop in one application and, in most cases, in amounts in excess of the plant's need, whereas split applications would be more effective and allow the crop to use the fertilizer gradually. Persistence of this trend, in addition to causing loss of time and money by farmers, results in leaching of nitrate and pollution of groundwater. It may also result in accumulation of nitrate in hazardous amounts (more than 0.25 percent on dry weight basis) in the edible parts of vegetables such as spinach and lettuce. Although the needs of plants for P in comparison with N and K are low, our farmers are still extravagant in the use of ammonium phosphate resulting in over use of P with possible deleterious effects on the uptake of other nutrients, especially zinc. Due to excessive application of N and P fertilizer, use of K fertilizers, in terms of crop yield and quality is neglected. In order to safeguard the equilibrium between soil nutrients, and yield improvement, with due attention to sustainable agriculture, it is proposed that: (1) The nitrate concentration in the 0-60 cm soil layer in the first month of the crop growth period should not exceed 20 ppm and PSNT (pre-side Nitrate Test) method should be applied for cash crops such as corn and potato, (2) The ratio of N, P, and K fertilizer used should be adequate and must not exceed 1:2:1 times that removed by the crops, (3) In fields where large quantities of P fertilizers have been used, applications of this fertilizer should be suspended for a few years especially when available P is around 15-20 ppm, (4) Potassium-fertilizer should be applied to crops which are heavy consumers, and to soils where K has been depleted