By Rebecca Muenich of the Sustainable Phosphorus Alliance
Precision agriculture, the practice of employing monitoring systems, software, and equipment to modify farming inputs in a productive and efficient manner, is applied everywhere in modern agricultural systems, except when it comes to a majority of manure applications. Compared to inorganic fertilizer sources that have clearly documented, consistent compositions, manure and its associated nutrient values are not well appreciated and are even treated as waste by many farms in the US. This is especially true for large farms with a lot of liquid manure, including dairy and swine operations.
Research has shown the potential value of these recycled fertilizer products (Schröder 2004; Choudhary et al. 1996), so it is perhaps surprising that, in many places in the US, manure is treated as waste product and contaminant source rather than a nutritional opportunity. Understanding manure composition is key to addressing this problem. This is because variable composition has been identified as a barrier to farmers’ distribution or sales of their manure to other farms, especially non-livestock farms (Battel and Krueger 2005).
There are many references available to help calculate the nutrient content of manures, including the American Society for Agricultural and Biological Engineer’s “Manure Production and Characteristics” Technical standard ASAE D384.2, generalized table values available in many online extension publications (University of Maryland Extension; Ohio State Extension), and in permit information for regulated facilities. However, the micro- and macronutrient content of recycled fertilizers can vary dramatically by source and even over time. For manure specifically, it is well known that factors such as the feed an animal eats, the age, size, and species of animal, and the method of manure storage can all affect the nutrient content. Composition may also be affected by such practices as the addition of compost or water from other parts of the farm. In addition, the variation in methods for calculating nutrient content alone creates discrepancies.
In my work to understand manure application practices in the Lake Erie basin with colleagues at Ohio State University and the University of Michigan, we compared lab-measured nitrogen and phosphorus contents of manure to estimated “book values” for large, regulated animal operations where data were available. This comparison supported what others have found: that using book values alone may not be enough to understand the nutrient composition of excreted and/or stored manure. This variability can have important implications for farm economics as well as environmental management. Putting on too little or too much nutrients can put a farm at risk. Therefore, lab testing of manure nutrient content should be a preferred source for composition information.
We demonstrated on a small scale with very detailed data that there is a great potential to redistribute (i.e. recycle) manure by applying it to lands that have lower soil nutrient contents. It is often the case that cropland currently receiving manure applications is not in need of more, as a history of consistent manure applications can lead to a buildup of nutrients in the soil (Sharpley et al. 2013). Yet redistributing and applying manure at appropriate rates will require improvements in measuring and conveying the nutrient content of manure products to potential end users.
More frequent, replicated samples and more nuanced reporting are called for. In terms of reporting, manure composition tests may not break down the phosphorus in the sample into organic and inorganic forms, though they may indicate how much is available in the first year. Giving farmers a better idea of the total amount of phosphorus and its speciation (to assess bioavailability) can improve nutrient management. More testing is needed too. While testing manure compositions is required for large, regulated operations on typically an annual basis, it is not a widespread technique used in manure management, at least in the Lake Erie Basin states. On small farms with fewer resources, manure composition testing (~$30/sample) may not seem economically beneficial at first glance. However, if manure nutrients can contribute to crop nutrient needs, overall fertilizer additions may be decreased, saving money while decreasing the risk of nutrient pollution.
Improved and more frequent testing and communication of manure compositions is key in achieving the “Right rate” in 4R Nutrient Stewardship.
[Open source] Battel RD, Krueger DE. 2005. Barriers to change: Farmers’ willingness to adopt sustainable manure management practices. Journal of Extension, 43(4). https://archives.joe.org/joe/2005august/a7.php
Choudhary M, Bailey LD, Grant CA. 1996. Review of the use of swine manure in crop production: Effects on yield and composition and on soil and water quality. Waste Management and Research, 14(6). https://www.sciencedirect.com/science/article/abs/pii/S0734242X96900567
Schröder J. 2004. Revisiting the agronomic benefits of manure: a correct assessment and exploitation of its fertilizer value pares the environment. Bioresource Technology, 96(2): 253-261. https://doi.org/10.1016/j.biortech.2004.05.015
[Open source] Sharpley AS, Jarvie HP, Buda A, May L, Spears B, Kleinman P. 2013. Phosphorus legacy: Overcoming the effects of past management practices to mitigate future water quality impairment. Journal of Environmental Quality, 42(5): 1308-1326. https://acsess.onlinelibrary.wiley.com/doi/full/10.2134/jeq2013.03.0098
University of Maryland Extension. Manure Summary Report. Available at: https://extension.umd.edu/sites/extension.umd.edu/files/2021-05/Manure_Summary_Report_2016%20update_final.pdf
Ohio State Extension. Ohio Livestock Manure Management Guide. Bulletin 604. Available at: https://agcrops.osu.edu/sites/agcrops/files/imce/fertility/bulletin_604.pdf