Summer 2003
Crop and Soil Response to Liquid Swine Manure Phosphorus Application
by Antonio P. Mallarino, John E. Sawyer, John Lundvall, and Monica Barbazan, Department of Agronomy
This article summarizes partial results from a project that has been demonstrating crop utilization of liquid swine manure nutrients, mainly nitrogen (N) and phosphorus (P). During the first 3 years of the project (2000–2002), we worked with 16 producer cooperators at 39 production/field sites located in 12 Iowa counties. General goals of the project and details of methods such as manure sampling, analyses, and application rates used were outlined in the Winter 2002 issue of the Odor and Nutrient Management newsletter. Partial results for corn response to manure N and supplemental N fertilization were presented in the Spring 2003 issue. In this article, we present partial results for crop and soil response to applied manure and supplemental P fertilizer.
Swine manure lagoon
Corn and soybean response to manure P application. Effects of direct manure application and supplemental P fertilization on yield of corn or soybean and on soil-test P were demonstrated at 16 locations from 2000 to 2002. Residual effects on second-year crops also were evaluated at several locations, but analyses are not complete and are not discussed in this article. Two manure rates and a nonmanure check were applied to long strips, and small plots were superimposed to the manure strips to apply various P fertilization rates. Extra N and potassium fertilizer was applied to the entire area of the small plots to mask any effect of these nutrients in the manure. The manure application rates were planned to apply approximately one-half and the full amount of estimated N needs of corn (based on analyses of total N in the manure). Details about manure nutrient concentrations and corn response to manure were shown in the Spring 2003 newsletter article.
Table 1 shows corn response to four supplemental P fertilizer rates after applying manure. Because the actual P amount varied across sites and treatments, the results across locations are summarized for several ranges of N-based manure rates. The lower manure-N application range (70–100 lb N/acre) applied on average an amount of P equivalent to the P removed by a corn yield of about 150 bu/acre. The higher manure rates applied amounts of P that were up to 4 times the P usually removed by an average corn crop. The yield data showed no significant yield response to supplemental P fertilization, although there was a small responsive trend for the lower manure application range. The initial soil-test P values were highly variable within a site, but at most sites the average initial soil-test P before applying manure tested in the optimum (16–20 ppm, Bray-1 test) or higher interpretation classes for corn. These results demonstrate that manure application based on N needs of corn (usually 100–150 lb N/acre) supply excessive P for corn and sometimes enough P for two crops.
In these fields, manure and fertilizer P often increase early-season corn growth and plant P uptake (not shown), but these responses did not translate into higher grain yield. The P uptake response was mainly due to increased early growth compared with P tissue concentration. Previous research based on P fertilization also showed early growth responses at soil-test P levels higher than levels needed to maximize grain yield; however, factors other than P from the manure could explain early growth responses seen at some field sites.
Effects of manure applications on soybean yield were tested at eight locations in 2000–2002 (Table 2). Because most fields tested optimum or higher in soil-test P, a lack of soybean yield response at most fields is reasonable. There was a statistically significant response to manure application in one low-testing field (Clay County, in 2001). However, there was also a significant yield response in one high-testing field (Washington County, in 2002) and small responsive trends in other fields testing optimum or higher. These results coincide with results from other studies showing small soybean yield increases from manure application when soil-test P is high. Soybean yield response in high-testing soils is not observed when fertilizer P is applied. The response to manure is most likely due to complex, poorly understood nutritional and physical factors influenced by manure application (not the manure P itself).
Table 3 shows the soybean response to four supplemental P fertilizer rates after manure application. Because the actual manure P applied varied across sites and treatments, the results across locations are summarized for several ranges of manure P application rates. The lower manure application range (40–60 lb P2O5/acre) applied an amount of P equivalent to the P removed by a soybean yield of about 60 bu/acre. The higher manure rates applied as much P as 3.5 times the P usually removed by an average soybean crop. The yield data showed no significant yield response to supplemental P fertilization. These results also demonstrated that manure application ahead of soybean can be used to supply the needs of this crop and to build up P if needed, but also will apply unneeded high N rates. Evaluation of the effects of manure application at rates greater than P removal in grain of one crop on the yield of second-year crops (not shown) indicates that the manure-P is available in the second year and that producers should account for it when planning for the next crop.
Effect of manure applications on soil-test P. An additional component of the demonstration is to evaluate manure effects on soil P measured by commonly used agronomic soil tests and environmental P tests. Although P losses from fields are not being measured in this project, there is the need to assess the impact of manure application on soil P because of possible impacts on P loss from fields. Environmental tests are not designed to assess plant-available P, and relationships between these tests and P loss from fields are being assessed in other projects conducted by Dr. Mallarino and his graduate students. For example, a test based on P extracted by shaking soil with water could provide better estimates of amounts of dissolved P lost from manured fields with surface runoff or tile drainage than the agronomic tests. A test based on P extracted by iron-oxide–impregnated paper gives a different estimate of bioavailable P than routine agronomic tests. Preliminary results of this project summarized in Figure 1 (averages across all sites) show that all tests detected little change in postharvest soil-test P levels after low manure application rates. These rates were planned to maintain soil-test P levels based on expected P removed in grain harvested from one crop. However, manure application rates that supplied more P than one crop year of the rotation increased postharvest soil-test P levels measured by all tests. Increases in soil-test P provide an indication of the high crop availability of P in liquid swine manure. The results demonstrate that excess manure P applied for one crop increased available P for the second crop of the rotation.
The three agronomic soil P tests used in Iowa (Bray-1, Olsen, and Mehlich-3)
and two environmental P tests provided similar estimates of the relative
effect of manure application on soil P, even though the tests extracted
widely different amounts of P. Correlations among all agronomic and environmental
P tests were high (Figure 2). The trend lines also reveal no difference
in soil test performance for non-manured and manured soils other than
the soil P level. Agronomic and environmental tests seemed similar in
estimating P availability in fertilized or manured soils. However, the
water environmental P test was less sensitive to changes in soil P caused
by manure P application compared with the other tests.
These preliminary results suggest that all soil P tests will adequately evaluate the impact of swine manure on soil P (once amounts of P extracted are considered through appropriate field calibrations). Previous research showed that the agronomic soil P tests are better correlated to yield response from soil nutrient additions. Producers are advised to use the currently recommended routine soil tests (Bray-1, Olsen, and Mehlich-3) for both agronomic and environmental assessments of the impact of manure on soil P.
Summary. This project is documenting the importance and value of liquid swine manure as a nutrient source for crop production in Iowa. Following a comprehensive approach of preapplication manure sampling and laboratory analyses, manure sampling during application, and calibrated rate applications, it is feasible to agronomically provide crop N and P nutrient needs of crops from swine manure. Soil testing to determine crop-available P and to provide information for environmental P management can be accomplished with routine agronomic soil P tests on soils receiving swine manure. Results from these 3 years also confirm that best management of liquid swine manure should consider practices that enhance achieving desired manure rates for providing N or P, minimize potential for loss, and closely estimate rates of N or P needed for crop production.
The ISU Swine Manure Nutrient Utilization Project, part of the Integrated Farm/Livestock Management (IFLM) Demonstration Program, receives funding from the Iowa Department of Agriculture and Land Stewardship, Division of Soil Conservation, USDA Natural Resources Conservation Service, and the Leopold Center for Sustainable Agriculture.
This is the fourth and final article in this series. The first three articles highlighting efforts of this project can be accessed in the Fall 2002, Winter 2002 and Spring 2003 Odor and Nutrient Management Newsletters available online at http://www.extension.iastate.edu/Pages/communications/EPC/
