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Soil fertility in relation to the dynamics of soil potassium in a coconut garden was assessed to study variations due to differential nutrition of coconut. Soil samples from the root zone (0-60cm) of the 25 year old coconut palms from a 33 confounded factorial experiment on NPK were analysed for various soil chemical constituents including fractions of potassium and Q/1 parameters of soil potassium. Treatment effects on the yield at 25th to 28th years (1989 to 1992) as well as above parameters were analysed statistically.
Acidifying effects of NH+4 ion was revealed in the plots which received continuous addition of (NH4)2SO4 as the source of N for coconut. The soils recorded low organic carbon content and cation exchange capacity, indicating predominance of low activity clays. Kaolinite was the dominant clay mineral observed in X – ray diffraction studies.
Exchangeable calcium increased with increasing rates of super phosphate addition. Exchangeable Mg, Fe, Mn and Zn were not affected by the treatments while sodium contents were influenced by nitrogen doses and the Cu content by N and K. Correlation studies revealed the influence of exchangeable AI on Zn, Mn and Mg as well as soil reaction.
Potassium application resulted in higher concentrations of water soluble K, available K, exchangeable K, HNO3 – K and total K, while non exchangeable K was reduced.
Potassium application increased the ARke values and decreased the PBCk. The AG values, indicating the enrgy required by the crop to extract soil potassium, was found to be within the range of potassium deficiency, suggesting the need for potassium nutrition. Significant correlations were obtained between the difficulty extractable forms of potassium and two principal soil characteristics – CEC and soil reaction. Otherwise under low potassium status, potash nutrition and the resultant available forms of potassium were not influenced by inherent soil properties.
The potassium availability indices suggested for the coconut growing environment, based on correlation and regression analysis are NH4 – OAC K as well as the Q/1 parameters namely ARke, PBCk and ΔG.
Levels of potassium significantly influenced the coconut yield. More than 200 % increase in nut yield was observed with potash addition, over the no potassium treatment. Nitrogen application without potassium decreased the yields.
Yield of coconut was significantly and positively correlated with available K, exchangeable K and water soluble K. Yield estimates are prepared through simple linear and multiple regression equations, considering the treatment effects and the correlation between various parameters.