Assessment of old cocoa plantations of different ages at Ibadan on soil management and nutrient needs for optimal production

Abstract

A study was conducted in Ibadan; southwest Nigeria to assess two adjacent forests and five old cocoa plantations for their soils physico- chemical properties as well as the cocoa plants nutrient contents to know their status for proper rehabilitation and management needs  to maintain sustainable optimal cocoa yields and quality produce from the cocoa plantations. One hectare (1ha) plot was marked out from  each study site that comprised two adjacent forests and 5 cocoa plantations for a total of 7 plots. The cocoa plots were established  at CRIN Headquarters in 2005, 2004, 2001, 1999 and 1949, with age range of 17-73 years. Ten soil samples were collected at 0-30cm using  soil auger from each plot in a stratified sampling method. Soil samples per plot were air-dried, sieved with 2mm sieve and mixed to form  composite soil samples. Leaf samples from the cocoa trees close to each soil sampling point were collected, oven dried at 70^oC to  constant weight and milled for a total of 5 composite leaf samples. The soils were analysed for the textural separates, pH, OC, ECEC, BS,  EA, N, P, K, Ca, Na, Mg, Mn, Zn, Cu and Fe. The cocoa leaf samples were analysed for N, P, K, Ca, Na, Mg, Mn, Zn, Cu and Fe contents using  standard analytical procedures. The results were compared with their critical values for soil and foliar levels. Results indicated that the  soils were generally sandy loam textured with the silt + clay contents less than 32%, while the OC was below critical (30g/kg  soil),indicating low water and nutrient retention capacity. The soil macro and micro nutrient contents were generally below their critical  levels with the problem of insufficient nutrient supply from the soils to sustain crop nutrient need for optimal yield and quality produce.  There was the problem of nutrient imbalance, more importantly on Mg and K supply, with non-proportional Mg/K ratios. The plantations  thus need appropriate supply of these nutrients through rational fertilizer application as well as to build up the soil organic matter in  order to improve the soil water and nutrient holding capacity and to maintain the soil buffer capacity.