Phosphorus Deficiency in Tomato
Contrarily to other nutrient deficiencies, the symptoms of phosphorus 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. 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 varieties of tomatoes, phosphorus deficiency is characterized by burnt tips and the development of chlorosis as well as necrotic patches on the leaf margins. Flowers are produced, but fruit yields are low.
Some soils are inherently low in phosphorus. This is particularly the case in calcareous, alkaline soils in which calcium is present in high concentrations, or in acidic soils. Optimal pH for the growth of tomato plants is 6.5 to 6.8. Soils with low organic matter or iron rich soils can also be problematic. Cold or wet weather that hinder a proper development of the roots can also lead to this disorder. However, most of the time the deficiency occurs due to the lack of proper application of the nutrient. There are differences in susceptibility to phosphorus deficiency between different varieties. If the field was managed properly, residual fertilizer from former crops might provide the following crop with enough phosphorus so that no additional phosphorus is needed.
Phosphorus levels in soils can be replenished by applying farmyard manure, or other materials (organic mulch, compost) or a combination thereof. The incorporation of residues into the soil after harvest can also contribute to maintaining a positive phosphorus balance in the long term and improve the soil structure.
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 before transplanting. Nitrogen however, should be applied later during vegetative growth.