Closing the Loop: How Well Could Fertilizer Derived from Dairy Manure Meet Crop Nitrogen Demand in Whatcom County, Washington
Posted by Karen Hills | January 6, 2021
Manure can play a valuable role in crop production because of its ability to build soil fertility and soil health. Ironically, manure can pose a waste disposal issue for livestock producers. Why does this situation exist even in places where dairies are surrounded by ample cropland? It arises from the disconnect between crop and livestock agriculture that afflicts modern specialized farming systems. Two years ago, I jumped at the chance to get involved in a project called “Dairies to Berries” that aimed to make inroads into this agricultural conundrum through recovering nutrients from dairy manure for use in cropping systems.
In previous posts, we explored two different aspects of this project: food safety in application to raspberries, and interest from crop producers in using manure-derived fertilizer. Here we delve into a more theoretical realm, discussing the potential impact that a nutrient removal system (dissolved air flotation, or DAF) could have on the transportation costs and the balance of nitrogen (N) in a crop and dairy heavy area, like Whatcom County, (e.g., 96 dairies and 35,810 hectares [88,488 acres] of cropland, according to 2018 data). We focused on N, specifically, because it is the limiting nutrient in many cropping systems, is the nutrient of greatest concentration in DAF solids, and is important for the adoption of DAF solids as an alternative to synthetic fertilizer.
I was involved with a project, led by Dr. Nathan Stacey, Postdoctoral Research Associate at the Center for Sustaining Agriculture and Natural Resources, to answer these questions using publicly available data on cropland and dairies from the Washington State Department of Agriculture (WSDA). Nitrogen is a fickle nutrient and manure nitrogen is prone to transformation and loss via processes that occur during storage and application. However, using textbook values, we estimated that raw dairy manure can provide between 3,810 and 6,708 Mg N annually in Whatcom County (Figure 1) The wide estimate is due to manure storage, where N loss can range anywhere between 20 and 50 % of total N and is directly related to the type of facility utilized for manure storage.
Calculation of crop nutrient needs was somewhat more straightforward, given the data available from WSDA and the availability of extension guidance on nitrogen needs for most crops (though in this hypothetical exercise, we did not account for soil conditions of any specific site). We estimated that annual crop nitrogen needs in Whatcom County are about 4,639 Mg N, but this estimate is valid only if producers follow recommended fertility guidelines, it does not account for the actual nutrient removed by a crop during a growing season. If DAF nutrient recovery were practiced at all dairies, the resulting DAF solids could theoretically provide about 2,100 Mg N, or roughly two thirds of the nitrogen needs for Whatcom County.
The barriers to using manure to fertilize cropland are not simple to overcome. They include food safety concerns, the fact that manure nutrients may not be easy to incorporate in all cropping systems (for example, in perennial berries or fruit systems the soil is not often turned) and the significant cost of transporting raw manure, which consists mostly of water. This is where the DAF process could provide a significant advantage by concentrating nutrients and lowering water content. In this study, we compared the costs to transport nitrogen contained in DAF solids and raw manure.
Notably, transportation costs for N incorporated into DAF solids were only 13% of transportation costs of N in untreated manure. This difference of over seven-fold could be significant in improving the ability to distribute N from dairy manure to cropland throughout Whatcom County, which has a fairly even distribution of dairies and croplands, or possibly even further if market demand existed. While N was the nutrient examined here, use of DAF solids would also have helpful implications for distribution of phosphorus, which tends to accumulate in soils receiving manure and is the nutrient of primary concern in freshwater systems.
For more information on the larger “Dairies to Berries” project, funded through a Conservation Innovation Grant from the Natural Resources Conservation Service, see the project website.
Stacey N, Hills K, Yorgey G (2020). Estimating and comparing cropland nitrogen need with dairy farm nutrient recovery: a case study in Whatcom County, WA. Renewable Agriculture and Food Systems 1–8. https://doi.org/10.1017/S1742170520000198