Diagnosing phosphorus deficiency in crop plants

In addition to carbon, hydrogen and oxygen, plants require at least 11 mineral elements to complete their life cycle, with another four mineral elements being potentially beneficial (Marschner 1995). The supply of three of these elements, nitrogen (N), phosphorus (P), and potassium (K) is often such that it limits the growth of plants in agricultural systems. To avoid yield losses and/or poor crop quality, the supply of these elements is ensured by the use of fertilizers. The supply of these elements by fertilizers must be optimized for the crop and its growth conditions. This optimization is necessary to avoid supplying too much of an element, which could have a negative affect on the crop yield or the local environment, or supplying too little of an element, which could prevent the crop from reaching its maximum potential yield. Typically, increasing the supply of a limiting element will increase the maximum yield up to a point (Figure 10.1). After this point, the maximum yield will remain constant (or even decline again) with further increases in the supply of the element. Therefore, supplying elements in excess of the optimum becomes uneconomic, since no extra yield will be realized for any additional input. Understanding the nutritional status of the crop is therefore critical to optimizing the supply of these elements. This chapter will review the potential of current and future techniques for establishing the P requirements of crop plants.