Apparent soil electrical conductivity in the delineation of management zones for cocoa cultivation

Abstract

Apparent electrical conductivity is an important parameter for describing the spatial variability of physical and chemical attributes of the soil and for the delineation of management zones. The objective of this work is to outline management zones for the cocoa cultivation based on the spatial variability of the productivity and the apparent electrical conductivity (ECa) of the soil. Data collection was performed in a regular sample grid containing 120 points in an area cultivated with cocoa trees, located in the municipality of Ilhéus, state of Bahia, Brazil. At each sampling point (cocoa tree), soil samples were collected to determine chemical attributes. Productivity was measured for one year, counting, monthly, the number of fruits, which were classified into off-season cocoa, harvest and annual production. Measurements of the apparent electrical conductivity of the soil were performed at different times of the year using a portable conductivity meter. The data were analyzed using classical statistics and geostatistics. The management zones were delineated using the fuzzy k-means algorithm. The ideal number of class was defined using the fuzziness performance index (FPI) and the entropy of the modified partition (MPE) indexes. The Kappa coefficient was used to validate the management zones, assessing their agreement with the chemical attributes of the soil. The ECa of the soil values presented moderate temporal variation, with maximum amplitude of 19.37mSm−1 and minimum of 0.82mSm−1 between measurement periods; higher averages of the ECa coincided with the highest levels of water in the soil. The measurements of the ECa of the soil carried out in April and October showed greater correlation with the chemical attributes of the soil, with significant values for 11 and 8 of the 17 attributes evaluated, respectively. The management zones from the ECa measured in April showed: a) reduced number of classes; b) spatial continuity between classes, and; c) agreement from reasonable (kappa between 0.20 and 0.40) to good (kappa > 0.41) with most of the chemical attributes of the soil. The ECa of the soil measured in April is, individually, the variable recommended for the management of soil fertility in tropical areas cultivated with cocoa trees.