Potassium leaching potential and fertilizer recommendations for smallholder coffee gardens of Papua New Guinea

Abstract

The need to assess, as part of a fertilizer recommendation scheme, a soil's tendency to lose potassium (K) by leaching has been examined by determining the mineralogy, K–calcium (Ca) exchange isotherms and K status of eight surface soils which fall into two groups, (a) four with curvilinear isotherms and (b) four with straight line isotherms. Group (b) soils have smaller buffer powers, smaller equilibrium soil solution concentrations and generally lower exchangeable K. All the soils contain kaolinite, sesquioxides and high organic matter contents. Group (a) soils contain mica in the whole soil and 2:1 clay minerals in the clay fraction except for one containing cristobalite. Group (b) soils have larger amounts of sesquioxides and any 2:1 minerals are in intimate association with the oxides. These mineralogical differences appear to be the reason for the differences in the isotherms. The lower K status of the Group (b) soils may be the result of greater leaching losses due to (i) their lower buffer powers especially at low potassium concentrations, and (ii) dilution of soil solution causing decrease in the amount of variable charge. Site-specific K fertilizer recommendations should take account of amounts of exchangeable K and of the tendency of soils to lose K by leaching, based on either mineralogy, exchange isotherms or related soil-mapping units.