Production of Alfalfa in Pennsylvania Soils of Differing Wetness 1

Plant nutrient applications and plant disease resistance are management factors that may contribute to successful alfalfa ( Medicago sativa L.) production on soils that are not inherently well suited to growing alfalfa. Applications of K and P, and cultivar resistance to Phytophthora root rot ( Phytophtora megasperma f. sp. medicaginis ; PRR) were evaluated on six well to poorly drained field soils to determine the contributions of these factors to alfalfa production under less than ideal conditions. The six soils were representative of two extensive soil associations in northwestern Pennsylvania. Factorial combinations of 0,175,350, and 525 kg ha− 1 yr− 1 K, and 45 and 90 kg ha −1 yr −1 P were applied annually for 3 yr. Significant yield response to only applied K was measured on two of the three soils of each association. Economic optimum rates for the 3 yr ranged from 137 to 263 kg ha −1 yr −1 K. Herbage K concentrations of 16.0 to 19.5 g kg −1 and exchangeable soil K of 275 kg ha −1 K were the minimum levels necessary to produce 90% of the maximum yield for the first harvest year. When the soil water table was within 50 or 25 cm of the soil surface at some time during the growing season, total yields for the 3‐yr period were reduced by 20 to 25% or 80 to 85%, respectively. Due to the lack of PRR development, cultivar resistance to PRR did not influence the total alfalfa yields. Neither management factor of nutrient application nor cultivar resistance to PRR could compensate for the adverse effects of excess soil wetness on alfalfa. The extent of the area in each association in which excess soil water would adversely alfalfa production ranges from 10 to 75%