Determination of the Critical Limit of Phosphorus for Shallot Growth in Brebes Regency, Central Java

Abstract

Intensification of balanced fertilization is one way to improve shallot quality and production to meet market demand. In order to achieve optimal crop yields, soil phosphorus (P) availability must be above a critical level. This study aimed to determine the critical limit of the P nutrient and determine the best extractor between Bray 1, Olsen, Mehlich I, and 25% HCl and evaluate the effect of P fertilization on the growth of shallot plants in Brebes Regency, Central Java. Determination of the critical level of P in the soil was determined by extracting soil P from 16 locations spread across Brebes Regency. P fertilization was carried out by applying KH₂PO₄ solution as much as 0, ½X, X, and 2X where X is the recommended dose of P fertilization in the form of P₂O₅ of 120 kg ha^-1. The plants were harvested six weeks after planting. The critical level of soil P is determined using the Cate and Nelson method. The results showed that each P extraction had different correlation coefficient values influenced by the type of extractants. The correlation coefficients between soil P nutrient content extracted by Bray 1, Olsen, Mehlich 1, and HCl 25% methods and plant dry weight were 0.630, 0.341, 0.821, and -0.089, respectively. The correlation values for Bray 1 and Mehlich 1 methods with plant dry weight were statistically significant and highly significant, respectively. The critical limit of soil P content to achieve 90% of relative yield of shallots in Brebes district based on Bray I, and Mehlich I methods were 40 ppm P. The best soil P extractant in Brebes was Mehlich 1. Fertilization was recommended on soils with soil P content less or equal to 40 ppm P.