Manure Application to Soybeans

During the last decade, the number and size of confined animal feeding operations has continued to increase. In the Midwest, corn is the primary recipient of liquid manure from these facilities. However, while the density of production animals has continued to increase, the corn acreage available for manure application has not. To avoid over-application of manure to corn land, producers are pursuing other crops such as soybeans and alfalfa as alternative crops to receive manure.

Why Consider Applying Manure to Legumes?

The justification often applied for manure use on legumes is their ability to reduce N fixation when a readily available N source such as manure is applied. In addition, crops such as soybeans and alfalfa can utilize the phosphorus and potassium applied with the manure, thus reducing the costs of commercial fertilizer. For example, a 60 bushel/acre soybean crop in Iowa may remove up to 228 lbs of nitrogen, 48 lbs of phosphorus (P2O5), and 90 lbs of potassium (K2O) per acre. More…

While there may be some economic, practical, and environmental reasons to apply manure to both corn and legumes such as soybeans and alfalfa there are also some disadvantages of such practices. Issues related to manure use on alfalfa will be discussed in Manure Application to Alfalfa. Here, we discuss manure use for soybeans.

Effects of Manure Application to Soybeans

One area of concern is related to the environmental consequences of manure application to row crops such as soybeans, and specifically concerns about nitrate losses through subsurface drainage systems. Relative to environmental considerations, it should be noted that application of manure on corn residue prior to soybeans may have some benefit compared to application of the manure prior to corn on soybean residue since sufficient soil residue cover may be maintained with injection into cornstalks. In addition, there are questions on whether there are any negative impacts of manure application on soybean yields.

Subsurface drainage throughout much of the Midwest U.S. accelerates nitrate loss to downstream waterbodies. (Photo courtesy of the USDA-NRCS)

Yield Impacts

Several studies have been performed in the Midwest region of the U.S. resulting in positive yield increases related to liquid swine manure application on soybeans. However, there is no single conclusion as to why an increase in yield occurs. The studies identify yield increases from manure as the potential result of in-field initial nitrate, P, K, or other nutrient deficiencies. So, manure provided the nutrients that were deficient resulting in a yield increase and offsetting costs for purchased fertilizer. However, not in all cases was the yield increase sufficient to overcome application costs.

In addition to potential environmental concerns some studies have noted rare occurrences of reductions in soybean yield when manure is applied prior to soybeans and higher occurrences of common soybean diseases. A Minnesota study recommended that application of manure be avoided on fields with a history of white mold due to potential yield suppression due to manure application. Others have noted that manure application prior to soybeans can increase certain soybean diseases, specifically Pythium and Phytophthora damping off and Phytophthora root rot.

Another precaution that has been raised relative to liquid swine manure application to soybeans is that soybean seed germination and emergence can be sensitive to salts, so that if manure is applied close to planting time, there is a potential for injury especially if the soybean is planted into the manure or very near the manure.

Environmental Impacts

There have been few studies that have documented the environmental impacts of manure application to soybeans. A Minnesota study in the 1990’s evaluated the impact of liquid swine manure application on nodulating and non-nodulating soybeans. They found that applying manure at greater nitrogen rates than needed for maximum soybean yields did not adversely affect soybean yield. However, they found that application of nitrogen from the liquid swine manure increased post harvest soil nitrate levels. They also found greater increases in soil nitrate levels early in the growing season than post harvest.


Manure application rates supplying from 0 to 446 lb N/acre in 89 lb N/acre increments were used in the study. Post-harvest soil nitrate levels were on average 37.7 lb N/acre (0-48 in) when no manure was applied and increased to 39.9, 44.4, 51.0, and 60.0 lb N/acre at applied nitrogen application rates of 50, 100, 150, and 200 lb N/acre, respectively. So, at an applied nitrogen application rate of 100 lb N/acre which might be about one-half of crop removal (soybean) the post-harvest soil nitrate was increased by about 15% compared to when no manure was applied.

Two drainage water quality studies in Iowa have evaluated the impact of liquid swine manure application to both corn and soybeans within a corn-soybean rotation. For a four-year study (2001-2004) at the Gilmore City research site in Pocahontas County, applying liquid swine manure at the rate of 150 lb N/acre (total nitrogen) before both corn and soybeans did not increase either corn or soybean yields compared to a rate of 200 lb N/acre of manure applied every other year before corn. In addition, the total of 300 lbs (two years of 150 lb N/acre) versus the 200 lb N/acre two-year-rate resulted in nitrate-N concentrations in tile drainage increasing on average from 17 to 23 mg/L, a 35% increase that was statistically significant.

For a six-year study (2001-2006) at the ISU Northeast Iowa Research Farm, applying liquid swine manure at the rate of 150 lb N/acre (total nitrogen) before corn and 200 lb N/acre (total nitrogen) before soybeans increased corn and/or soybean yields slightly some years (on average 3 and 2 bu/acre for corn and soybeans, respectively) compared to 150 lb N/ac of manure applied every other year before corn. The total of 350 (one year of 150 lb N/acre and one year of 200 lb N/acre) versus the 150 lb N/acre two-year-rate resulted in nitrate-N concentrations in tile drainage increasing on average from 21 to 38 mg/L, an 81% increase.

Both of these studies applied a relatively high nitrogen rate to the soybeans, but at these rates when liquid swine manure was applied every year in a corn-soybean rotation there was an increase in nitrate-nitrogen concentrations in the subsurface drainage water. However, it is unknown what direct water quality risk there would be with lower application rates specifically at rates ranging from 100-125 lb N/acre to soybeans. While the results discussed above were for studies on tile drained soils it is expected that there would be similar risks on non-tile drained soils relative to nitrate concentrations moving below the crop root zone.


The application of manure to both corn and soybean, as noted above, could increase the risk of nitrate loss. Additionally the annual application of manure could increase the buildup phosphorus which could be of concern mainly from a surface runoff perspective. Considering a 60 bu/acre soybean crop the phosphorus removal (P2O5) might be 48 lb/acre and the potassium removal (K2O) might be 90 lb/acre, and a 200 bu/acre corn crop might remove 75 lb/acre of phosphorus (P2O5) and 60 lb/acre of potassium (K2O). This might result in a two-year removal of 123 lb/acre of phosphorus (P2O5) and 150 lb/acre of potassium (K2O).

Applying liquid swine manure at a nitrogen application rate of 150 lb N/acre to corn and 100 lb N/acre to soybeans (250 lb N/acre in two year rotation) might result in an overall phosphorus application of 172 lb/acre and an overall potassium application of 194 lb/acre (using values for liquid swine manure from a grow-finish operation (wet/dry). More… These application rates could be a long-term concern relative to phosphorus build up since crop removal might be 123 lb/acre for phosphorus with a phosphorus application of 172 lb/acre. A phosphorus buildup could have implications relative to the phosphorus index.

Pros, cons, and recommendations for manure application to soybeans


  • Manure can supply phosphorus (P), potassium (K) and other nutrients;
  • Manure application on cornstalks can provide greater crop residue cover and lower erosion potential when injected or incorporated into cornstalks instead of soybean stubble on erosive land;
  • Manure application to soybeans can provide flexibility in application plans; and
  • Manure application to soybeans may improve soybean yields in some case.


  • Manure application to soybeans has the risk to increase nitrate in the soil profile which may increase the risk of nitrate loss;
  • Manure application to both corn and soybeans at an N rate for both crops could lead to a buildup of phosphorus; and
  • Manure application to soybeans under certain conditions may increase the risk of soybean diseases which could negatively impact yield.


  • Limit manure application to soybeans to a rate that compensates for N that would not be fixed by the soybean – this may be in the range of 100 lb N /acre;
  • Possibly limit manure application to soybeans to rates that fulfill the P and/or K requirements of the soybean crop or two-year corn-soybean rotation; and
  • Avoid manure applications when there is low crop utilization (i.e. fall applications).

Additional Resources

Author: Matt Helmers, Iowa State University Reviewers: Rick Koelsch, University of Nebraska, Quirine Ketterings, Cornell University and John Lory, University of Minnesota

Manure Application to Alfalfa

Reasons to Apply Manure to Alfalfa

  • Alfalfa requires high phosphorus (P) and potassium (K) soil fertility and has a high demand for these nutrients.
  • Alfalfa can benefit from the micronutrients in manure, particularly boron.
  • Alfalfa removes large amounts of nitrogen (N) and K from the field when harvested as hay.
  • Alfalfa has the ability to draw down nitrate levels within its root zone.
  • Forages like alfalfa have low erosion, nutrient runoff and nutrient leaching potential.
  • Alfalfa offers opportunities for manure application throughout the year.

Challenges In Applying Manure to Alfalfa

  • Poorly timed applications can damage the alfalfa stand through physical damage to plants.
  • Manure applications can increase weed competition particularly in mixed grass-alfalfa stands.
  • Alfalfa is a legume so has no requirement for applied N limiting manure nutrient value.
  • Manure can be an unbalanced fertilizer (i.e. does not exactly match crop needs for all nutrients) so applying manure based on N often results in the build up of excess P and K in the soil.
  • Manure applications at the end of stand life can result in soil N supply in excess of the following crop needs potentially increasing N losses to water and the atmosphere.

Alfalfa Corn Rotation

Alfalfa is predominantly grown in alfalfa-corn rotations associated with dairy production. The alfalfa stand can last five years or more and then the field is rotated to corn for two to three years before returning to alfalfa. In these systems the crop rotation has the dual role of utilizing the fertilizer value of manure and providing a place to apply manure to prevent overflow of manure storage facilities.

One challenging characteristic of alfalfa-corn rotations is the relatively low demand for external inputs of N. Alfalfa, as a legume crop, fixes the N it needs and typically requires no N applications to maximize yield. Residual benefits from the N fixed by alfalfa will provide most of the N needed by first-year corn following alfalfa and frequently reduce N needs in the second-year crop following alfalfa. See state-specific recommendations on the N value of an alfalfa stand to the subsequent crop. In most states the only significant fertilizer N need in alfalfa-corn rotations is on second- and third-year corn following alfalfa.

The best opportunity to maximize the value of manure in alfalfa-corn rotations is to apply manure to second- and third-year corn following alfalfa to at rates that meet N needs of corn. In this system all the N, P and K fertilizer value will be realized. Manure eliminates the need to purchase N, P and K fertilizer for corn and any excess P and K reduces fertilizer need of the following alfalfa crop.

Manure on established alfalfa can increase yields. In this study, swine slurry (4.5% dry matter) was surface-applied to alfalfa at four rates (0, 300, 450 and 600 kg/ha/year) in 1994 and 1995. Manure was applied four times per year typically a few days after cutting. Alfalfa yield was increased in the years of application and the two years following application. Reference: Ceotto, E and P. Spallacci. 2006. Pig slurry applications to alfalfa: Productivity, solar radiation utilization, N and P removal. Field Crops Res. 95:135-155.

Farmers benefit from maximizing manure applications to this portion of the rotation but there are limitations. The optimal timing of application is limited to a relatively short period in spring prior to planting and early in crop growth. There also typically is not enough land in second- and third-year corn following alfalfa to utilize all manure produced by the farm. Consequently dairy farmers frequently are looking for opportunities to apply manure on the alfalfa phase of alfalfa-corn rotations. However, this can result in a steady buildup of P and K in the soil.

Repeated applications of manure to meet both the nitrogen need of corn and the nitrogen removal capacity of alfalfa can excessively raise soil test P and K levels. Use nutrient management planning to balance manure application rates with rotation P and K needs. Manure on second- or third-year corn following alfalfa has the added benefit that the crop needs N. Figure from D. Beegle, The Pennsylvania State University.

image:Crop rotation 1.jpg image:Crop rotation 2.jpg

cc2.5 manure nutrient mgmt group

Opportunities to Apply Manure on Alfalfa

A prerequisite to making smart decisions about applying manure on alfalfa requires an understanding of the impact of applied N on alfalfa stands. Alfalfa has the capacity to fix N from the atmosphere to meet its needs. Each ton of harvested alfalfa can contain 50 pounds of N and in low N-supply soils most of this N will be derived from N fixation. Total N fixation can reach hundreds of pounds of N per acre per year. There is an energy benefit to the alfalfa plant to use N from the soil in preference to fixing N from the atmosphere. Alfalfa plants that have access to alternative N supplies will reduce N fixation and preferentially use the alternative N supply. This buffering process means manure applications that meet alfalfa N need will not lead to over-application of N. Such applications are not economically beneficial because there is no yield value to the applied N. But there is no water quality cost from applying this N even though the alfalfa does not need it.

There are three potential opportunities to apply manure on alfalfa:

  1. Preplant applications.
  2. Application to established productive stands.
  3. Post production applications prior to destroying the stand.

The first two strategies can be conditionally recommended as long as farmers are aware of the limitations and challenges of manure applications during these periods of alfalfa production. The final strategy, post production applications, cannot be recommended because manure applied at this time is likely to lead to potential increases in N loss.

Preplant Applications

Preplant manure applications for alfalfa can meet P and K requirements of manure for the first years of production. The application rate is limited by the lower yield and N demand during alfalfa establishment. Care must be taken insure that the seed is not in direct contact with fresh manure through injection or incorporation of manure. The N in the manure can promote weed competition during establishment. It also can promote lodging of oats if oats are used as a nurse crop for establishing alfalfa.

Application to Established Stands

Manure applications to established stands can provide needed P, K and boron. Alfalfa also provides windows of opportunity for manure application through the whole growing season. High yielding alfalfa has a high capacity to buffer high amounts of manure N. The primary concern with manure applications to established alfalfa is damage to the stand from the manure. Alfalfa plants can be damaged by high salt or ammonia concentration in the manure, by physical damage to the crowns by manure application equipment or by water deficits induced by high salt concentrations in the manure. The greatest danger is from slurry or solid manure that is applied with large manure spreaders. Lagoon water from unagitated lagoons typically posses less risk because nutrient and salt concentrations are lower. Another concern is manure may increase competition from grass or weeds if they are present in the stand.


Legumes like alfalfa have the capacity to buffer applications of manure N. When manure is applied alfalfa fixes less nitrogen. Figure used with permission from US Dairy Forage Research Center. cc2.5 John Lory

To minimize potential damage to the stand:

  • Make sure application equipment breaks up large lumps of solid manure and applies manure in a uniform pattern on the field.
  • Limit manure application rates. High rates increase potential for stand damage.
  • Apply manure immediately after cutting alfalfa and before budding on the alfalfa crowns. The alfalfa plant is less vulnerable to salt damage when no green leaves are showing. This is particularly important for surface applications of slurry.
  • When using manure that has a high potential to damage the stand apply to older stands that have a high grass or weed component. Mistakes with manure applications on these stands are less costly.

Applications at or Near the End of Stand Life

Applications at or near the end of stand life cannot be recommended because they lead to applications that exceed N utilization capacity of the rotation. Farmers frequently want to apply manure to alfalfa in the fall just prior to killing the stand. Manure applications at this stage occur after alfalfa N demand. It is really an application that will supply the subsequent crop with N; typically there is limited need for added N in the crop following alfalfa. Under these conditions manure applications will easily lead to excess N in the soil profile. Such conditions promote N losses to water and air resources.

Other resources

There are excellent extension publications further detailing the opportunities and challenges of managing manure for alfalfa. These include:


  • In alfalfa-corn rotations maximize manure value by applying to second- and third-year corn following alfalfa.
  • Applications to established alfalfa stands are conditionally recommended to meet P, K and boron needs. Manure N will have no value but alfalfa can buffer applied N minimizing over application concerns.
  • If manure is applied based on N need of the corn and N uptake by alfalfa, excess P and K buildup can be a problem. Therefore, try to balance P and K over the whole rotation.
  • Steps need to be taken to minimize potential damage to established alfalfa stands particularly when applying slurry or solid manure.

Author: John A. Lory, University of Missouri