Potassium Release Characteristics of Selected Peruvian Soils

To evaluate K release, 16 Peruvian soils were intensively cropped in growth chambers using the Stanford‐DeMent technique, and K was extracted with sodium tetraphenylboron (NaTPB) for varying periods. When 1‐g soil samples were equilibrated with solutions containing NaTPB for 0.25, 1, 10, 100, 1,000, and 2,000 hours, K extracted with NaTPB was linearly correlated with the logarithm of time from 10 to 1,000 hours. Rates of K release varied from −0.03 to 7.43 meq K/100 g soil per week. Large differences in the total amount of K removed in 2,000 hours (1.9 to 66.9 meq K/100 g soil) were related to the presence and apparent degree of crystallinity of illite in the clay fraction of the soils. Amounts of plant‐available K, as determined from cumulative K uptake by five successive 2‐week‐old oat ( Avena sativa ) crops, varied from 0.35 to 3.24 meq K/100 g soil, and rates of K uptake varied from 0.016 to 0.215 meq K/100 g soil per week. Omitting three soils which released relatively large amounts of K (two at low and one at high levels of exchangeable soil K), the uptake of K by the first oat crop was highly correlated with initial exchangeable soil K.