Release of non-exchangeable potassium from soils and micaceous minerals during short periods of cropping in the greenhouse

An evaluation of the availability of potassium in crop residues

“…Following 72 days of incubation, under moist conditions, after the greenhouse cropping, the exchangeable K level of the soil receiving no residue additions remained essentially at the level of maximum depletion (Table 3). This finding is in agreement with those reported by Verma (1963) for the moist incubation of three Iowa surface soils after reduc ing the exchangeable K level by cropping. This result indicates that the level of soluble K present during the moist storage period was sufficiently high to block any shift in an equilibrium level from the nonexchangeable to the exchangeable form.…”

Potassium availability to plants in aggregates from different Iowa soils

“…Therefore, these two sized aggregates had different exchangeable K contents. However, Verma (1963) showed that crushing each of these air-dried rewetted aggre gate sizes to less than 0.5 mm in diameter before cropping caused an increase in K uptake by the plants as compared to the corresponding uncrushed aggregates. Step K" was found, but no change was observed in the "con stant rate K".…”

“…And, from the plants analyses for N and P, it was found that tomato plants, rye,sunflower and wheat from the fine aggregates had higher percentages of P than they did from the coarse ones, therefore the % N/% P ratio was lower in plants grown on the fine aggregates as compared to that from those grown on the coarse aggregates. According to PettiJohn (1965) the two aggregate sizes which were used by Verma (1963) were obtained from two differ ent sources. Apparently the aggregates 5-3 mm in diameter were obtained from a fertilized plot of Nicollet soil, while the 1-0.5 mm aggregates were obtained from non-fertilized plot of Nicollet soil.…”