Calcium, Magnesium, and Potassium Uptake by Crested Wheatgrass Grown on Calcareous Soils

Forage intake with potassium/(calcium + magnesium) [K/(Mg + Ca)] values in excess of 2.2 are associated with grass tetany and Mg deficiencies in ruminants. This study was conducted to determine the degree to which forage K and Mg concentrations and K/(Ca + Mg) ratios could be predicted from soil bicarbonate (HCO3) extractable phosphate-phosphorus (PO4-P), and saturation extract Ca, Mg, K, sodium (Na), and nitrate-nitrogen (NO3-N) concentrations. Crested wheatgrass (Agropyron spp) strains and cultivars representing four ploidy levels were grown in the greenhouse on eight calcareous soils with different saturation extract Ca, Mg, K and K/Mg ratios. The plants were harvested three times. Soil solution 1C/(Ca + Mg) and K/Mg ratios were the only measured soil parameters that showed a consistent correlation with plant K/(Ca + Mg) ratios. Bicarbonate extractable soil P was positively related to plant P and K uptake in the first harvest, but was not related in the second and third harvests nor was soil P related to plant Ca or Mg content. There was a tendency for the higher ploidy level entries to have higher plant K/(Ca + Mg) ratios. It was concluded that soil K/(Ca + Mg) ratios can be used to predict relative forage K/(Ca + Mg) ratios for grasses grown under similar conditions. INTRODUCTION Forage plants that contain very low Mg concentrations and/or have high K concentrations tend to promote Mg deficiencies (grass tetany) in ruminants. Grass tetany losses in the United States alone are currently estimated at $150 to $400 million annually (Grunes and Mayland, unpublished). This problem could be 915 Copyright © 1993 by Marcel Dekker, Inc. 916 ROBBINS AND MAYLAND It should be remembered that while K/Mg or K/(Ca + Mg) values may provide a measure of the grass tetany risk, there are other factors that also influence grass tetany potential in ruminants. In southern Idaho, soils formed from mafic parent materials (high in Ca and Mg relative to K) exist adjacent to soils derived from felsic parent materials (high in K relative to Ca and Mg). It is not uncommon to find rangeland soils in this area where K accounts for 30 to 60 percent of the soluble cations at concentrations up to 30 mmol/L in saturation extracts (Robbins and Carter, 1983). The purpose of this study was to determine the effect of native soil Ca, Mg, Na, and K concentrations and ratios on cation accumulation in crested wheatgrass (Agropyron spp.) and to determine the variation in cation composition among four crested wheatgrass strains and cultivars with different ploidy levels grown on eight calcareous soils. MATERIALS AND METHODS Soils The eight soils used in this study (Table 1) were selected to provide a range and mix of Ca, Mg, and K concentrations, and K/(Ca + Mg) and K/Mg ratios in the saturation extracts (Table 2). Portneuf silt loam was derived primarily from mafic parent material loess and tends to be relatively higher in Mg than K when compared to the other soils used in this study. Marshdale loam was derived from a mix of mafic and felsic parent materials and formed in low lying flat areas that are cool and wet most of the year. The other four soils were derived primarily from felsic materials and tend to have higher K contents relative to Mg than the first two soils. The Portneuf, Marshdale, Starhope, Bedke, and Freedom #1 soils had not previously been irrigated nor cultivated. The Declo soil had been irrigated for over 100 years with low EC (<0.3 dS/m) water. The Freedom #2 soil had been irrigated with 0.45 dS/m water for one barley (Hordeum vulgare L.) crop and the Freedom #3 soil had been irrigated for three alfalfa (Medicago saliva L.) cuttings with 1.3 dS/m water. The initial soil ion concentrations and EC data reported in Table 2 were used to select the soils. The soils were air dried, sieved through a 2-mm screen, and thoroughly mixed. Saturation soil pastes were made and allowed to stand overnight. The paste pH was measured, and the saturation paste water content was Ca, Mg, AND K UPTAKE 917 TABLE 1. Taxonomic Class and Predominate Parent Materials of the Soils Used. Soil Series Taxonomic class Original parent material