Long-term K dynamics in organic and conventional mixed cropping systems as related to management and soil properties

The effects of management and soil properties on potassium (K) dynamics and delivery capacity have been investigated in mixed crop rotations that included a period of ley. The study was conducted using three long-term field experiments on sandy loam soils each including a conventional and organic cropping system. Results are based on data collected since the start of the experiment in 1987 up to 2004 which includes three full crop rotations. The mean annual K field balance for the 18 years varied from −21 to −60 kg K ha−1 year−1 in the conventional system and from −22 to −75 kg K ha−1 year−1 in the organic one, depending on site, indicating that present management systems rely to some extent upon K from soil reserves. The K concentration in the ley herbage decreased significantly over time at all sites and both systems, as demonstrated in long-term annual data as well as within individual growing seasons. Exchangeable K (Kex) concentration in the surface soil (0–20 cm) decreased significantly at all sites and systems during the study period, and there was also a decrease in acid-extractable K (KHCl) at two sites. The decrease in the Kex pool (kg ha−1) in the surface soil was smaller than the calculated total deficit of K from the cropping systems based on the accumulated field balances. Thus the crop must have been supplied with K from either deeper soil layers and/or from components of the non-exchangeable K pool. The amounts and distribution of K between different chemically (Kex and KHCl) and mineralogically (phyllosilicates and K-feldspars) defined K pools as well as the root distribution varied considerably between sites, indicating likely site differences in K delivery capacities. The size of the annual K deficit represents between 15 and 40% of the soil Kex pool (0–25 cm) compared to ≤1% of the K present in the phyllosilicate pool. The observed long-term decrease in herbage and soil K concentrations suggests that the current nutrient management plans are not adequate in terms of K. In addition, there is a risk for late season herbage K deficiency through short-term exhaustion of soil Kex. In order to maximise sustainability over the long-term, regionally summarised soil geochemical and mineralogical information could possibly help to improve the predicted long-term K supply from internal soil sources and be a useful compliment to standard soil tests, fertiliser recommendations and nutrient balances.