Misconceptions of Manure Nutrient Availability

• Manure can be an important part of a soil fertility program, especially as fertilizer prices rise.
  
• Manure nutrient availability should be analyzed carefully and planned accordingly with soil sample information.

Manure is a key part of a crop fertility program for many farmers and may become a component of more fertility programs as fertilizer prices rise. If managed correctly, manure can help reduce input costs. However, there are factors of manure nutrient availability that should be understood.

Types of Manure and Their Nutrient Levels

Manure nutrient availability can vary depending on the animal source, how it is stored, water dilution and the bedding and diet of the animal. It is recommended to have the manure source regularly tested by a laboratory (typically offered through university agriculture programs) for more precise measurements of nutrient levels. There are some resources available online to help provide average ranges of manure nutrient percentages, as well as the percent moisture of various manure sources. Knowing the moisture percentage of a manure source is helpful in calculating the quantity of nutrients applied at a given rate because water has a diluting effect on the final nutrient concentration of manure.¹

Manure Nutrient Efficiency

Nutrients from manure can be found in both organic and inorganic forms, so they may not always be as readily available to plants as commercial fertilizers. Some nutrients, such as potassium and certain nitrates, may be obtainable from the soil quite rapidly, but the availability of other nutrients may take much longer. When building a nutrient management plan using manure, 100% of the nutrients should not be considered available in the first year.² It can take up to 4 years for nitrogen, phosphorus and potassium to be fully available for the crop when applied as manure. As a rule of thumb, 80% of potassium and 90% of phosphorus are likely available during the first year.¹

Predicting nitrogen availability can be more complex as it is dependent on both animal species and application method.¹ Nitrogen from raw manure is available as ammonium, which is immediately available to plants and in an organic form, which must be mineralized into ammonium before being useful. Ammonium also has the potential to convert to ammonia and be lost through volatilization. Incorporating manure as quickly as possibly greatly reduces the amount of nitrogen lost with volatilization. Due to delays in mineralization, it should be expected that a portion of plant-available nitrogen from manure will not be available until 1 year after being applied. Table 1 illustrates how second-year nitrogen credits could range from 15-25% of the total nitrogen applied, depending on method of application and animal species.

Management Considerations

Soil Sampling

Soil testing and understanding current soil nutrient levels is important. If a field has high phosphorus and/or potassium, caution should be used in applying excessive amounts of manure. Excess levels of phosphorus and potassium in a manure application can interfere with the uptake of copper and/or zinc, which may lead to deficiencies in the crop, such as decreased moisture uptake (zinc) and decreased stalk strength (copper). Excess potassium may also interfere with the uptake of boron and magnesium. This may also be a cause for concern, as boron is important for cell structure, pollination and grain fill, while magnesium plays a key role in chlorophyll and enzyme production. Thus, a deficiency in these may lead to stunted crop growth and increased drought stress.

Soil sampling is also important the years following the manure application to understand the plant-available nutrients. Different soil types and soil properties can affect the rate of mineralization of the nutrients in the manure.

Manure Sample Testing

Manure sampling is very insightful to understanding the nutrient content to ensure accurate application rates. Nutrient value in manure can vary based on many factors, including storage, so multiple samples throughout the storage system are recommended. Another solution to the variability in manure is to agitate the product in the storage system prior to application for a more even distribution of nutrients.  

A possible concern to keep in mind from manure applications is the potential for a liming effect to occur. This tends to be a more common occurrence in feedlot manures, as they are more likely to contain higher levels of calcium carbonates – a common additive to feedlot cattle diets. To determine the percentage of calcium carbonate equivalent more precisely in manure, it is recommended to request an effective calcium carbonate (ECC) commercial fertilizer test to aid in determining optimal application rates.

Application Timing and Method

The timing of a manure application influences the amount of nitrogen loss in a cropping system. If the application occurs on a warm day or in dry soil conditions, there can be significant nitrogen loss to volatilization which reduces the amount of nitrogen available to the crop. 

How quickly manure is incorporated and the method used can have dramatic impacts on nitrogen loss rates. As an example, plant-available nitrogen in year 1 from swine manure can be as high as 80% when injected with sweeps and as low as 35% if broadcast and incorporated 4 days later. 

Table 1 outlines the variability of nitrogen availability by animal species and application method.

Manure has high nutrient value and soil health benefits, but proper management, such as application rate, timing and placement is important. Consistently collect manure samples to understand the nutrient content and soil samples to know the appropriate application rate.

References:

1 Zhang, H. 2017. Fertilizer nutrient in animal manure. Oklahoma State University Extension. https://extension.okstate.edu/fact-sheets/fertilizer-nutrients-in-animal-manure.html
2 James, R. et al. 2006. Ohio Livestock Manure Management Guide. Ohio State University Extension, Bulletin 604.  https://agcrops.osu.edu/sites/agcrops/files/imce/fertility/bulletin_604.pdf
3 Wilson, M. 2021. Manure characteristics. University of Minnesota Extension. https://extension.umn.edu/manure-management/manure-characteristics#nitrogen-817860