Crop, Soil, and Management Effects on Phosphorus Soil Test Levels: A Review

There is renewed interest in reducing surface water quality problems associated with excessive P concentrations. A P site index has been proposed 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 management factors can influence soil test P in a variety of ways. It is quite probable that anticipated crop P removal at a particular site would 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 efficiency (crop P uptake divided by P application rate) that is typical for P. Vegetable and specialty crops in the cropping sequence and high subsoil P levels generally lead to higher surface soil P levels. Modeling efforts, using published P fertilizer recommendations and crop P removal rates, predicted the effects of two crop rotations on relative 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 influence the probability that a particular site may have a high soil P test. The maintenance approach will generally lead to higher soil P test levels than the sufficiency level approach. Reduced tillage may require greater P fertilization and lead to higher soil P levels. These crop, soil, and management factors may be used to differentiate areas that may vary in P soil test levels and, therefore, probably vary in their potential for P movement off‐site. This would be particularly useful for large‐scale applications of the P site‐index where P soil test levels cannot be determined on every tract of land. Research Question There is increasing interest in reducing nonpoint source P loadings to surface waters. The proposed Lemunyon‐Gilbet P site index, designed to determine the relative potential for off‐site movement of P from agricultural land, has soil test P as a characteristic that contributes to overall site vulnerability. Crop rotations, P fertilizer application rate, tillage, subsoil P, crop removal of P, and soil testing philosophies are factors that can influence P soil test levels in the surface horizon. The purpose of this paper is to review the literature to determine the extent to which these factors can be used to predict surface horizon P soil test levels as an aid in using the Lemunyon‐Gilbert P site index. Literature Summary Several studies have shown that total P losses in surface runoff from a field increase as the P application rate increases. If one assumes that the P inputs are balanced with P outputs, anticipated removal of P by various crops can be used to rank the relative risk of P losses in runoff. A table of average yearly P removal by various crops is provided. The data also has been normalized to pounds P removed per common harvest unit to allow calculation of P removal at various yields. Examples are shown in Table . Modeling efforts have some merit in predicting the effects of crop rotations on changes in available P in soils. Available P levels in a soil were predicted to reach a higher level with continuous corn than with a cornsoybean rotation, based on Kansas recommendations. One long‐term field study found a linear relationship between the calculated net change in soil P (P inputs minus P removed in the harvest portion of the crop) and soil test P after 25 yr. These data were used to support the model predictions in that a continuous corn rotation would have had a higher P soil test than a corn‐soybean rotation after 25 yr. Many studies have shown a stratification of P in no‐till systems with P accumulating at the soil surface. This stratification may lead to slightly higher P soil test recommendations for crops grown with no‐till than with conventional tillage. Soil test philosophies will also influence P soil test levels. The maintenance approach, which advocates a build‐up of soil P to some optimum level followed by maintenance applications of P, generally leads to higher soil test levels than the sufficiency level approach, which recommends P only when there is some probability of response. When subsoil P levels are high there is an increased probability that the surface soil will also be high. Optimum P soil test levels for vegetable and specialty crops are generally higher than those for field crops (Table ). Applied Question How can crop, soil, and management information be used in conjunction with the P site index? The purpose of the P site index is to provide a relative ranking of land management units (ideally, this would be individual fields) in terms of their risk of delivering P off‐site. Resources aimed at reducing P loadings to a surface body of water could then be used where they would have the greatest benefit. A complete application of the P index within a watershed would require that P soil test levels be determined on every tract of land. When this is possible, crop, soil, and management information may only be useful for estimating P fertilizer application rates based on the assumptions that the next crop is known and that the P application rate will be approximately what the crop is expected to remove. More likely, however, is the situation in which P soil test levels cannot be determined on every tract of land. A mechanism by which fields could be grouped according to probable relative differences in soil test P levels would be helpful in a preliminary assessment of the study area. Less intensive sampling could then be directed at verifying the grouping. We have identified several factors that can contribute to increases in soil test P levels. These include greater total P fertilizer inputs in the cropping sequence, specialty crops in the cropping sequence, the use of the maintenance approach as opposed to the sufficiency level approach in making P soil test recommendations, and high subsoil P levels. The factors that are not uniform across the study area may be used to differentiate areas that probably vary in P soil test levels; thus would vary in the loss of P to surface waters. Phosphorus removal for selected crops. P removal/ P P common Crop concentration Yield removal harvest unit % per acre lb P/acre Corn 0.28 150 bu 24 0.16 lb P/bu Potatoes 0.38 400 cwt 15 0.038 lb P/cwt Snap beans 0.55 12 tons 13 1.1 lb P/ton Soybeans 0.58 60 bu 11 0.18 lb P/bu Tomatoes 0.04 30 tons 24 0.80 lb Phon Wheat 0.45 40 bu 11 0.28 lb P/bu Optimum P sol test levels and P fertilizer recommendations for vegetable and field crops in Wisconsin. Range of optimum P Range of recommended Crop soil test levels P application rates ppm lb P/acre Corn 11–20 13–33 High demand vegetables 31–45 4.4–44 Potatoes 81–110 16–28 Soybeans 6–13 8.7–31 Kelling et al, 1991