Potassium content in soil, uptake in plants and the potassium balance in three European long-term field experiments

This study quantitatively assesses the fate of K derived from mineral ferti lizers and organic manures and the effective K balance in three long-term f ield experiments at Rothamsted (UK), Bad Lauchstaedt (Germany) and Skiernie wice (Poland). Plant availability, uptake and the overall utilization of K over the last 30 years (1965-1996) are discussed and related to soil K Avai lability Indices determined by the standard methods used in each of the thr ee countries. In addition, to provide a standard comparison of the three si tes, Exchangeable K (1 M NH4OAc) and Non-exchangeable K (K extracted by boi ling with 1 M HNO3) were measured on one recent (1995) set of soil samples. Plant availability and utilization of K was partly related to clay content , but more closely to the cation exchange surfaces associated with both min eral and organic constituents and also, at Rothamsted, to the capacity of c lay minerals to fix K. The recovery rate of K from mineral fertilizer by cr ops did not exceed 62%. Fertilizers were least effective in the most strong ly K fixing soil at Rothamsted (44% maximum) and most effective in the soil with the highest cation exchange capacity (CEC) at Bad Lauchstaedt (62%), where the greater quantity of exchange sites appear to be associated with h umic material. Recoveries of K from farmyard manure (FYM) varied from 22-11 7% (values of >100% indicating subsoil uptake or the release of reserves). Deficiencies of N, P and Mg in some treatments decreased the effectiveness of applied K and may have caused increased leaching of K from the plough la yer. FYM was generally more effective than mineral fertilizer where mineral N and P were not applied because these nutrients were effectively supplied in the manure. But the effectiveness of mineral K fertilizer decreased whe n applied in combination with FYM because FYM was the preferred source of K . Where FYM application increased the CEC of soils, this also improved K ut ilization but only where K was not extensively leached or fixed.