Potassium Reserves in a ‘Harwell’ Series Soil

Summary A sample of Harwell soil containing 36 percent fine clay (< 0.3 μm ) and 14 per cent coarse clay plus fine silt (0.3–5μm) was separated into fractions, and the K‐supplying power of soil and fractions measured by cropping with ryegrass, exchange with Ca resin and double‐label isotopic exchange with 42 K and 45 Ca ions. Mineralogical examination of the fractions coupled with the cropping experiments showed that the K‐supplying power of the soil to ryegrass can be explained by the presence of a zeolite, clinoptilolite‐heulandite, in addition to the clay minerals, mica, and interstratified illitic smectite, commonly found in a glauconitic clay‐rich soil. The 0.3–5 μm fraction, containing much zeolite, has an exchange diffusion coefficient for K ions to Ca resin of 1.8 × 10 −16 cm 2 sec −1 compared with a value of 5.7 × 10 −20 for the < 0.3μm fractions in which interstratified illitic smectite is the dominant mineral. Isotopic exchange shows that all Ca ions in fractions < 50μm are isotopically exchangeable. In fractions coarser than 20μm, some of the K ions in felspar and mica were not exchangeable within the duration of the experiments.