THE EXTERNAL PHOSPHATE REQUIREMENT OF-COWPEA ON FIVE DISSIMILAR SOILS

We established a greenhouse study on five different Venezuelan soils to evaluate the external phosphorus (P) requirement of cowpea (Vigna unguiculata var. Tuy). The external requirement for the various soils was almost identical (approximately 1.00 μg/ml) and independent of edaphic factors, such as texture and mineralogy. The values differ greatly from those previously reported for the same species growing in a high P-fixing Hawaiian soil. We propose that pot effect, variety differences, and the presence (or absence) of my-corrhizal associations influence the differences between these external P requirements. The external P requirement of crops, in conjunction with P sorption curves, provides a rational basis for P fertilizer requirements. However, because the value is not a single-valued constant, affected by many biotic and abiotic factors, much more information should be accumulated before it can be widely recommended as a common fertilizing technique.