High Yields of Non-fertilized Protein-rich Pigeon Peas on Tropical Soils of Low Inherent Fertility of Puerto Rico: An Explanation of a Paradox

Abstract

Pigeon peas do not respond to the application of fertilizers on soils of low inherent fertility under conditions in Puerto Rico, although sizeable yields, sometimes over 7800 kg/ha of green pods and seeds, with over 20% protein, are obtained with high-yielding cultivars, sound crop protection practices and adequate soil and crop management techniques. It is estimated that in terms of kg/ha a pigeon pea crop of some 7800 kg/ha removes nutrients at approximately the rates of N, 60; P, 15; K, 20; Ca, 20; and Mg, 10. A plausible explanation for the lack of response to N can be based on the known fact that pigeon peas can fix atomospheric N through rhizobial action at a rate of at least 14.5 mg/day/plant to supplement the N provided from the reserves of soil organic matter and root residues. For a plant population of 17,284/ha this fixation would amount to some 45 to 50 kg for the pigeon pea growing period. Large P applications over the years coupled with P fixation on the soil and gradual release afterwards might explain the observed lack of response to fertilizer P. On the other hand, the relatively high K-supplying power of soils where pigeon peas are grown, together with the small demand of the crop, explains the lack of response to fertilizer K. Many acid soils of the tropics can supply enough Ca as a nutrient to meet the demands of pigeon pea crops. Pigeon peas use relatively small amounts of Mg, which appears to be available in many soils of Puerto Rico.