Relationship Between Soil Test and Small Grain Response to P Fertilization in Field Experiments 1

Yield data from 74 small grain field experiments over a l3‐year period were used to evaluate the relationship between various soil tests and P response. Past field studies have resulted in low correlations between P soil test value and yield response, indicating that either alternative soil tests should be evaluated or additional soil and climatic factors should be considered. The soil tests examined in this study were the Bray 1 (soil to solution ratio of 1:7, 1:10, 1:20, and 1:50), Olsen P, and P sorption index of Bache and Williams. The crop species included were Triticum aestivum L., Hordeum vulgare L., and Avena sativa L. At each location soil samples were taken and stored and yield response to P fertilizer, applied with the seed, was recorded. The highest correlation between soil test value and yield response to P fertilization was found with the Bray 1, 1:50 (r 2 = 0.41). An additional 23% of the yield response variation could be explained by including six additional variables in a multiple regression analysis employing dummy variables. Borolls were generally less predictable in their response to P than were Ustolls. Ustolls responded less to P fertilization than Borols while soils of the vertic subgroup responded more to P fertilizer according to the Bray 1, 1:50 soil test. The r value between P sorption index and yield response on alkaline soils was −0.70 (the highest of the soil tests examined) but was near zero for acid soils. Differences were also noted in degree of response between parent materials and crop species. Organic matter content appeared to influence response on some soils.