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Phosphorus deficiency symptoms can appear at all stages but are more pronounced in young plants. Contrarily to other nutrients, the symptoms of this deficiency are generally not very striking and can be difficult to identify. In mild cases, a possible indication for this disorder is that plants are dwarfed or stunted. However, no obvious symptoms are observed on leaves. In severe deficiencies, stem and petioles show a dark green to purple discoloration. The lower sides of older leaves also show a purple pigmentation, starting on the tips and margins and later expanding to the rest of the lamina. These leaves may become leathery and veins may form a brown netting. In some cases, phosphorus deficiency is characterized by burnt tips and the development of chlorosis as well as necrotic patches on the leaf margins. Flowers and fruits are produced, but fruit yields are low.
There are differences in susceptibility to phosphorus deficiency between different crops. Roots absorb phosphate ions when they are dissolved in the soil water. Calcareous soils with high calcium concentrations can be poor in phosphorus. Most commonly, however, it is the availability of this nutrient that is limited because phosphorus is adhered to soil particles and cannot be taken up by the plant. Both alkaline soils and acidic soils can show low availability. Soils with low organic matter or iron rich soils can also be problematic. Cold weather that hinders proper development and function of the roots can also lead to this disorder. Drought conditions or diseases that limit the absorption of water and nutrients by the roots can trigger deficiency symptoms. Soil moisture, in turn, increases the uptake of this nutrient and results in significantly higher yields.
Phosphorus levels in soils can be replenished by applying farmyard manure, or other materials (organic mulch, compost and guano) or a combination thereof. The incorporation of residues to the soil after harvest can also contribute to maintaining a positive phosphorus balance in the long term and improve soil structure. The decomposition of organic matter provides a steady supply of plant-available phosphorus.
Amendments are inexpensive and varied, and they can be easily applied to alleviate deficiency problems. Phosphorus is applied to plants in the form of phosphate together with nitrogen and potassium (the N-P-K trio in fertilizer products). Soil testing is recommended to determine the ratio of each of these nutrients. It varies depending on the type of soil, the plant variety in question and the concentration of nitrogen in soil. Usually, it is recommend to apply phosphorus and potassium a few weeks before sowing or planting. Nitrogen, in turn, should be applied in split applications later during vegetative growth.