3.3 Available nutrients in soil
Nitrogen and potassium are the most important nutrients required by coffee. The higher N content in Chikmagalur forest (407.7 kg ha-1) may be attributed to the higher organic carbon content (4.23 %) in the soil, when compared to Wayanad and Idukki forest soils. The higher concentration of N in coffee soils is a result of N fertilization. The phosphorus content decreased with depth and this trend was clear irrespective of forest or coffee plantation ecosystem. The higher P in the surface is attributed to the application of phosphatic fertilizers to coffee plantation and the effect of slightly higher pH of coffee soils. Coffee berries contain high concentration of potassium, which will be removed from field through harvesting. When forests are cleared for agricultural use, the efficient nutrient recycling mechanisms of the forests are disrupted and the nutrient availability in the topsoil is increased by fertilization thereby increasing the potential for nutrient leaching (Schroth et al., 1999). Coffee ecosystem soils recorded lesser K than forest soils, may be due to the greater uptake and removal of K by coffee. Continuous cultivation of coffee in the same site will lead to a depletion of soil K (Jessy, 2011). Application of acid forming inorganic fertilizers enhanced depletion of K+ from the soil (Gebrelibanos and Mohammed, 2013). The relatively higher available Ca content in coffee soils (932.5, 1125.0 and 530.5 mg kg-1 of soil) is due to the application of liming materials to the plantation. The decrease in Ca and Mg contents with depth is attributed to leaching caused as a result of heavy rainfall in the areas considered for study. The lower concentration of Ca at the deeper layers of forest soils is due to root uptake of nutrients by the forest trees with roots extending beyond 1 m of depth. This uptake by plants might have removed the nutrient from the system. Coffee plantation ecosystem soils recorded higher available sulphur than that of forest ecosystem soils. This is attributed to the application of sulphur containing fertilizers. Coffee plantation ecosystem soils recorded higher Zn and Cu content in surface horizons than forest ecosystem soils. The relatively higher concentration of copper than forest ecosystem soils is a result of application of Bordeaux mixture to the plantation. High rainfall and high temperature in the granite gneiss landscape and laterization assisted the accumulation of sesqui-oxides under redoxi-morphic conditions favouring higher concentration of iron, aluminium, manganese and sometimes copper also.
The available nutrients viz. N, P, Ca, Mg, S, Zn and Cu were higher in coffee soils than forest soils indicating better quality of soils on conversion from natural forest to coffee plantation, an indication of better level of management as an enterprise with replenishing of nutrients with absorption or removal and better soil water conservation structures and with shade trees for canopy management help preventing vagaries of soil erosion.