Availability, fixation, and liberation of potassium in high-lime soils

Unproductive high-lime soils from nine different fields in north central Iowa were compared with soils from the same fields which supported normal crop growth. The results may be sumrnarized as follows: 1. Within any given field the productive soil contained from 31 to over 700% more exchangeable potassium than the unproductive soil. The latter averaged 151 pounds of exchangeable potassium per acre (2 million pounds), whereas the productive soils averaged 396 pounds. 2. Seven of the 10 unproductive soils contained less than 175 pounds of exchangeable potassium per acre, whereas all 12 of the productive soils contained more than this amount. 3. The unproductive soil was generally higher in calcium carbonate than the productive soil from the same field, the average content of the former being 24.0%, and of the latter 12.6%. 4. Eight of the 12 productive soils contained less than 15% calcium carbonate in the surface 6 inches, whereas 9 of the 10 unproductive soils contained more than this amount. 5. In every field, the unproductive soil showed a greater potassium-fixing power than the productive soil. The average amount of potassium fixed by 10 unproductive calcareous soils was 211 pounds per acre, by 12 calcareous productive soils 105 pounds, and by 10 acid soils only 13 pounds. 6. In four out of five fields studied the productive soils showed a more rapid liberation of non-exchangeable potassium, although the differences were relatively small. From these results it may be concluded that: 1. Differences in the amounts of exchangeable potassium present may account, at least in part, for the differences in plant growth between unproductive high-lime soils and adjacent soils which support normal plant growth. 2. The high-lime soils of Iowa apparently require more exchangeable potassium in order to support good crop growth than do the normally acid soils of central United States. 3. Excessively high concentrations of calcium carbonate and bicarbonate may contribute, either directly or indirectly, to the low productivity of these soils. 4. On the basis of the relatively few soils studied in this investigation, it would appear that soils of north central Iowa which contain more than 15% calcium carbonate and less than 75 pounds of exchangeable potassium per acre will likely show signs of extreme potassium deficiency and respond markedly to potassium fertilization. Some high-lime soils containing more than this amount of exchangeable potassium may also give good response to potassium fertilization. 5. The high potassium-fixing power of unproductive high-lime soils may be responsible for the relatively low amounts of exchangeable potassium in these soils no doubt explains the small residual effects obtained from applications of potassium fertilizers.