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Soil Test Phosphorus Status and Trends in Delaware

Soil Testing: Traditional and Contemporary Roles

The value of soil testing for determining crop fertilizer needs and maintaining soil fertility was recognized in the 1930’s, and organized soil testing efforts in the U.S. began in the 1940’s. Traditional agronomic soil analysis designed to predict the likelihood of profitable response to fertilizer applications remains a primary goal of soil testing programs. However, current environmental issues, such as the contamination of ground and surface water by nutrients, land application of agricultural, municipal, and industrial wastes, and reclamation of severely disturbed soils, have prompted many soil testing programs to re-evaluate their role in soil management and land-use policy formulation. Advances in soil science research have enabled soil testing programs to transcend traditional agronomic services and assume new roles that may help solve environmental problems.

University of Delaware Soil Testing Program

The University of Delaware Soil Testing Program (UDSTP) was established in 1947 to provide a soil analysis service for Delaware farmers. The goal then was to promote the proper use of lime and fertilizers to maximize soil fertility and crop yields. Today, the UDSTP has two goals. The first is to provide accurate, rapid soil analyses and lime and fertilizer recommendations for all who manage Delaware’s soil resources, including farmers, homeowners, environmental consultants, crop consultants, fertilizer dealers, research scientists, extension specialists, and conservation planners. The analytical methods and recommendations of the UDSTP are always based on research conducted at the University of Delaware and in other states with similar soils, crops, and climates. The second goal is to provide soil analyses and interpretive guidelines to help minimize the environmental impacts of all land management practices. This goal is accomplished by the use of innovative soil testing procedures and comprehensive educational programs on nutrient management.

The Use and Value of Soil Testing Summaries

Since 1992, the UDSTP has been able to prepare computerized summaries of all soil test results (Sims and Gartley, 1993). Such summaries are valuable because they show the current status of and the long-term changes in soil properties for a field, farm, county, state, or region. These properties include concentrations of nutrients and non-essential trace elements, pH, and organic matter content. Compiled over years or even decades, soil test summaries can be used to determine the short and long-term impacts of soil management practices and to identify emerging soil management problems. Periodic assessment of soil test summaries enables those responsible for soil management recommendations to stay abreast of changing soil properties and make necessary adjustments to their recommendations. Soil test summaries can also be used to provide direction to soil management research by identifying links between management practices and soil properties, to educate the public about proper soil management, and to develop land-use plans based on soil characteristics. This fact sheet illustrates the use of soil test summaries to identify and help resolve a contemporary soil management concern in Delaware: the increasing number of soils now rated as "excessive" in phosphorus (P), relative to crop requirements, from long-term applications of fertilizers and animal manure.

Soil Test Phosphorus Status and Trends in Delaware Soils

Phosphorus in fertilizer and manure is applied to soils to promote plant growth, achieve economically optimum crop yields, or establish vegetation to reduce soil erosion. Because P is well known to promote eutrophication in many surface waters, the loss of P from soils and its subsequent transfer to surface waters has emerged as an international nonpoint source pollution concern. Eutrophic waters are more likely to experience algal blooms, fish kills, low biological diversity, scums, odors, turbidity, and sedimentation, all of which are undesirable and difficult to correct. To protect the quality of Delaware’s surface waters, it is necessary to minimize the accumulation of P to excessive levels in soils and the transport of P from soil to surface water bodies.

Statewide UDSTP soil test summaries for 1992-1996 show that of the 12,000 Delaware soils analyzed, 55% were rated as excessive (>50 ppm) in soil test P (STP), 28% optimum (25-50 ppm), 12% medium (13-25 ppm), and only 5% low (<13 ppm) (Figure 1). In most cases, the UDSTP recommends that P be applied to only those soils rated "low" or "medium".

Figure 1. Number of soil samples in each fertility category for 12,000 soil samples from Delaware analyzed by the UDSTP from 1992-1996.

The STP status for Delaware soils observed in the UDSTP summaries is consistent with data from a 1996 soil test summary of Delaware and Maryland soils prepared by a private soil testing laboratory cooperating with the UDSTP (Figure 2). Because the private laboratory uses the Mehlich 3 soil test method and a different rating system than the UDSTP, only the trends of its and the UDSTP’s soil test summaries can be compared. The private laboratory’s summary shows that of the 5000 soils analyzed, 84% were rated as high or very high, 12% optimum, 2% medium, and 2% low or very low in STP.

Figure 2. Number of soil samples in each fertility category for 5000 soil samples from Delaware and Maryland analyzed by a private laboratory in 1996.

The UDSTP summaries also show that the distribution of soils rated as "excessive" in STP varies geographically within Delaware (Figure 3). More than 64% of the 6200 samples analyzed by the UDSTP from Sussex County were excessive in STP, while only 2% were low in STP (Figure 4). The large number of Sussex County soils rated as excessive in STP reflects the intensive animal-based agriculture and the practice of applying animal waste to cropland in the county.


Figure 3. Percent of soil samples in each fertility category for all soil samples from each of the three counties in Delaware analyzed by the UDSTP from 1992-1996.

Land application of animal waste can add more P to soils than is removed in harvested crops, resulting in a long-term accumulation of soil P. In Kent County, where cash-grain, vegetable, and animal-based agriculture are well mixed, 49% of soils were excessive in STP; and in New Castle County, where cash-grain agriculture predominates, only 20% of soils were excessive in STP. The UDSTP summaries also show that the median STP value (i.e., 50% of soils were greater and 50% of soils were less than the median value) varied among the three counties (Figure 4). The median STP value was 66 ppm in Sussex County, 50 ppm in Kent County, and 24 ppm in New Castle County. Comparatively, the critical value for STP (the value when crops no longer respond to P fertilization) commonly used in the Mid-Atlantic region ranges from 20 to 25 ppm.

Figure 4. Median Soil Test P for soil samples from the three counties in Delaware analyzed by the UDSTP from 1992-1996.

Implications for Soil Phosphorus Management

The UDSTP summaries show that most agricultural soils in Delaware are "optimum" or "excessive" in STP. These soils should require no P fertilizer in any form, in some cases for many years, to produce economically optimum yields. This "residual" fertility should enable farmers to reduce fertilizer costs. However, "excessive" STP values raise serious questions concerning the potential for agriculture to contribute to the eutrophication of Delaware’s surface waters as P moves from soils to water bodies. Research shows that P loss from agricultural soils in erosion, runoff, or drainage increases as STP values increase. Therefore, farmers whose soils are rated "excessive" in STP should consider implementing nutrient management plans and soil conservation practices that are designed to prevent P loss from soil in erosion, runoff, and drainage and to avoid further increases in STP. Examples of such plans and practices are detailed in the references cited below.


Sims, J.T. 1996. The phosphorus index: A phosphorus management strategy for Delaware’s agricultural soils. University of Delaware. Newark, DE. Cooperative Bulletin ST-05.

Sims, J.T. and K.L. Gartley. 1996 Nutrient management handbook for Delaware. University of Delaware. Newark, DE. Cooperative Bulletin #59.

Sims, J.T. and K.L. Gartley. 1993 The University of Delaware soil testing program: History, philosophy, and value, A thirty seven year summary (1957-1993). University of Delaware. Newark, DE. Cooperative Bulletin #45.

Sims, J.T. and P.A. Vadas. 1997. Nutrient management strategies for the profitable, environmentally sound use of phosphorus. University of Delaware. Newark, DE. Cooperative Bulletin ST-08.

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