|Mineral Fertilizer Use and the Environment (IFA - UNEP, 1998, 52 p.)|
Fertilizer use at excessive rates has deleterious effects on crop growth. Examples are the lodging of cereals and the low sugar content of sugar beet resulting from excess quantities of nitrogen, nutritional disorders involving trace elements such as zinc due to excessive phosphate fertilizer and lime, impeded seed germination and seedling injury from too much soluble fertilizer salt adjacent to the seed row, the acidifying action of nitrogen fertilizer on soil, and increased incidence of plant and pest attacks with excessive nitrogen fertilizer. If the nitrogen application leads to acidification of the soil, it can induce aluminum and manganese toxicity if compensating lime is not applied.
As regards crop diseases, the most important impact of nitrogen is on vigour and plant growth. These two factors have an important impact on plant susceptibility to many diseases. Vigorous plants with rapid growth are generally more sensitive to obligate parasites and some pathogens are specifically more aggressive towards vigorous plants. However, most of the necrotic pathogens attack less vigorous plants with nitrogen deficiency. Balanced fertilization provides excellent protection. The time of application of fertilizers is important. A wrong timing may induce substantial growth of foliar parts of plants and maintain high humidity conditions in the crop canopy which are favorable to disease development.
Phosphorus application seems to favour plant protection against diseases, either by correcting a deficiency in soil phosphorus, and thereby inducing a better growth of the plant, or by speeding up the maturation process, disfavoring some pathogens like downy mildew that effects the young tissues.
Potash can increase the efficiency of use of other nutrients by plants, particularly of N. Potash has a beneficial effect on the quality of a wide range of crops, especially in terms of improved protein quantity and quality. Potash can decrease the incidence of plant diseases and reduce abiotic stresses, particularly cold stress. The element may have a direct action on pathogen penetration, lesion size and on inoculum density. An indirect effect of potassium on disease development is to stimulate the healing process (interaction with the scar parasites), to increase the resistance to cold, and also to delay the maturity and senescence of fruits. There is no known pollution or health hazard due to the use of potash fertilizers in agriculture. However, the application of potassium chloride to chloride-sensitive crops should be avoided, as should its use on certain saline soils.
Calcium may have an effect on the cell wall of plants by making them more resistant to pathogen penetration. A deficiency in calcium increases the sensitivity of plants to many fungi.