Soil-Solution Equilibration Parameters and their Relationship to the Accumulation of Potassium by Ryegrass

Three Tennessee soils (Hartsells, Maury and Grenada) were investigated with respect to the soil-solution equilibration parameters and the content of potassium in ryegrass grown in a greenhouse. The soils were fertilized with N, P and K which was varied from 0-100 ppm added to the soil. Two harvestings of ryegrass were made. The content of K, Ca and Mg in the ryegrass was measured. Soil samples were taken immediately following the first harvesting and subjected to the soil-solution equilibration technique. Two forms of equilibration curves resulted. Seven of the 33 equilibration curves were curvilinear in form, whereas the remaining 26 curves were linear. The form of the curves indicated that no release of fixed potassium occurred during the equilibration period. Liming had a significant effect on the ΔK values except for the Maury soil. The potassium buffering capacity was not affected by the K treatments for each soil; however, the buffering capacities were different for each soil. The K labile pools were significantly different due to K treatments for all soils except for the Grenada. The Maury soil had the largest K labile pool and the Grenada had the smallest.

The content of K in the first cutting of ryegrass ranged from 2.8 percent to 7.8 percent while the K content of the second cutting was considerably lower. Much greater variability in the K content was found in the first cutting of ryegrass. Various regressing models were used to test the effects of soil parameters on the plant parameters. The results indicated that when any two of the three soil-solution equilibration parameters were used together in a regression model, they accounted for the variability of plant K content as well as but no better than the single parameters of soil exchangeable K or the equilibrium solution K activity.