CROP, SOIL, AND MANAGEMENT EFFECTS ON PHOSPHORUS SOIL TEST LEVELS

There is renewed interest in reducing surface water quality problems a ssociated with excessive P concentrations. A P site index has been pro posed that qualitatively ranks sites based on their potential for off- site P movement. Soil test P is one of eight site characteristics used in calculating the current P site index. The purpose of this paper is to review the literature on crop, soil, and management factors which may be used as indirect measures of soil test P. Crop, soil, and manag ement factors can influence soil test P in a variety of ways. It is qu ite probable that anticipated crop P removal at a particular site woul d reflect soil P test, and an extensive table was developed outlining P removal for numerous crops. Crops or crop rotations using relatively high P inputs will have higher soil P tests. This is true even when P inputs are balanced with P removal by the crop and reflects the low e fficiency (crop P uptake divided by P application rate) that is typica l for P. Vegetable and specialty crops in the cropping sequence and hi gh subsoil P levels generally lead to higher surface soil P levels. Mo deling efforts, using published P fertilizer recommendations and crop P removal rates, predicted the effects of two crop rotations on relati ve available P over time. Continuous corn (Zea mays L.) was predicted to maintain a higher level of relative available P than a corn-soybean [Glycine max (L.) Merr.] rotation. Soil testing philosophies also inf luence the probability that a particular site may have a high soil P t est. The maintenance approach will generally lead to higher soil P tes t levels than the sufficiency level approach. Reduced tillage may requ ire greater P fertilization and lead to higher soil P levels. These cr op, soil, and management factors may be used to differentiate areas th at may vary in P soil test levels and, therefore, probably vary in the ir potential for P movement off-site. This would be particularly usefu l for large-scale applications of the P site-index where P soil test l evels cannot be determined on every tract of land.