Birdsfoot Trefoil Seedling Response to Soil Phosphorus and Potassium Availability Indexes

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Abstract
Birdsfoot trefoil (Lotus corniculatus L.) is a promising forage legume, yet little is known about its nutrient requirements during early growth. Several soil P and K indexes were evaluated for their ability to predict dry mass (DM) and P and K concentration and uptake by ‘Fergus’ and ‘Maitland’ birdsfoot trefoil seedlings grown for 6 wk in the greenhouse in four diverse soils amended to attain a broad range of plant‐available P and K. Soils were extracted with 0.01 M CaCl2, neutral 1 M ammonium acetate, or H2SO4 to estimate plant‐available K, and 1 mM SrCl2, Bray and Kurtz no. 1 (Bray‐P), or anion exchange resin (AER) to estimate available P. Shoot DM was not influenced by soil K and increased with increasing soil P, being best predicted across soils by AER‐P. Maitland DM exceeded that of Fergus at all fertility levels. Tissue concentration of P and K were best related to soil solution concentration (extracted with weak salt solutions), but only one‐half the variation in tissue P concentration was explained by soil solution‐P concentration. Across soils, P and K uptake were best related to Bray‐P and to solution‐K concentration, respectively, but exchangeable‐K exhibited less sensitivity to cultivar differences than solution‐K. Anion exchange resin appeared to extract a constant fraction of nonoccluded Fe‐ and Al‐bound P (47%), regardless of the oxide content of the soil, whereas the Bray‐1 reagent appeared to extract a constant proportion of P per unit mass Fe oxide present. These differences may account for the relatively poor performance of AER in predicting P uptake across soils differing in parent material.