Soil Properties Affecting the Availability of Potassium in Highly Weathered Soils

Abstract

AbstractThe interpretation of soil tests for K in highly weathered soils may be improved if the K buffering power is considered in addition to extractable K. This study was conducted to determine if cation exchange capacity (CEC) and humic matter (HM) should be included in the development of more meaningful K availability indices. Seventeen soils from the Coastal Plain, Piedmont, and Mountain regions of North Carolina and two soils from the Amazon Basin of Peru were studied. The buffering power tended to correlate positively with CEC, but that of soils high in HM was lower and less affected by CEC than that of the low‐HM soils, indicating a lower capacity to replenish K removed from the soil solution upon depletion of K. The K concentration in the equilibrium solution increased linearly with percent K saturation of the CEC, and high HM soils had a higher concentration of K in the equilibrium solution at any given percent K saturation than low HM ones. A larger portion of extractable K was found in solution as HM increased and CEC decreased. It was noted that high HM soils were behaving similarly to low CEC and low HM ones so the soils were grouped into two categories: (i) soils of low K buffering power, which are those with HM ⩾ 1.9% and a CEC ⩽ 4.6, and (ii) soils of higher buffering power, which are those with HM < 1.9% and CEC ⩾ 4.6. The use of this grouping should improve K fertilizer recommendations in highly weathered soils.