Phosphorus (P) was first discovered by Hennig Brandt in Germany in 1669. It is ironic that the discovery was made when he was busy with an experiment with urine, sand and coal. Phosphorus is absorbed by the plant from the nutrient solution in the form of H2PO4-1 or HPO4-2 at pH values between 4.5 and 8.5. When the pH increases above 9, phosphorus is absorbed as H3PO4-3. The phosphorus concentration varies between 0.3 and 3.0% depending on phosphrous levels in nutrient solution and plant species. It is an important constituent of proteins and influences hydrolysis and the synthesis of starch. It is for this reason that flowering and fruiting plants require substantial more phosphorus that green plants such as spinach and lettuce.
Phosphate and nitrogen form important interactions. Low phosphorus values in the solution result in nitrogen accumulation in the plant. Conversely, if phosphorus values are high, nitrogen compounds are depressed. It is also true that excess nitrogen in the solution will depress absorption of phosphorus.
During low phosphorus concentration in the nutrient solution a characteristic purple to red colouration occurs in the vegetative portions of plants, especially the leaf stems. This colouration is caused by the accumulation of sugars which cause the production of anthocyanins. Be careful, some plants, especially spinach have natural discolouration at the base of the leaf and should not be confused with P deficiency. Any deficiency of phosphorus will result in stunted plant growth. Induced phosphorus deficiency symptoms may occur when Zinc and Calcium levels and pH values are very high. Adding more phosphorus will not solve the problem. The best is to dilute the solution by adding additional water in the nutrient tanks and adapt the concentration of the other nutrients accordingly. During cold periods in winter and even in summer sugar accumulation may start in leaves indicating the same symptoms as phosphorus deficiency.