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.