Sources of potassium taken up by ryegrass from some soils of the North Island

Abstract

The contribution of exchangeable K to the K uptake by ryegrass in the field was studied on soils of the North Island whose dominant clay minerals were considered not to contain non-exchangeable but plant available K. Six were derived from basalt or andesitic volcanic ash and 4 from sedimentary rock. On 8 sites a large proportion of K was taken up from sources other than exchangeable K on the top 250 mm (6 sites) or the top 450 or 610 mm (2 sites). On soils derived from volcanic parent materials the balance (Kb) was thought to come mainly from exchangeable sources below the depth sampled and on those derived from sedimentary materials mainly from non-exchangeable sources. The rate of plant uptake from the latter was found to be fast (500–600 kg K/ha/year) on at least one site. Illites and vermiculites were present in very small amounts in some soils. A K fixation test indicated plant uptake from non-exchangeable K better than did clay mineralogy. The results suggest that in the northern North Island, a relatively rapid test for non-exchangeable but plant available K would be useful for diagnostic purposes. The limitations imposed by sampling to a relatively shallow depth are also shown.

Keywords:

• potassium (nutrient)
• plant nutrient absorption
• soil chemistry
• soil analysis
• leaching
• Lolium