A County-Level Assessment of Manure Nutrient Availability Relative to Crop Nutrient Capacity in Iowa: Analysis of Spatial and Temporal Trends

During the twentieth century, agricultural production strived to achieve increased food production in order to satisfy both local and export demands. In many cases, this led to increased farm sizes and an operational separation of crop and livestock production. Society fears that the trend of increasing centralization and industrialization of agriculture, specifically animal agriculture, has resulted in concentration of waste products associated with their production (manures, wash-down water, process waters, etc.) over relatively small geographic regions that are spatially segregated from crop production areas. Since the distance that manure can be economically hauled for land application has practical limits, this could lead to over-application, of manures near animal feeding facilities, potentially increasing nutrient losses to ground and surface waters. A statewide analysis of crop and animal production in Iowa suggests that about 25% of current nitrogen and phosphorus requirements for crop production could be supplied from manures and litters, while around 40% of the required potassium could be provided. However, neither livestock nor crop production is uniformly distributed across all counties. This unequal distribution suggests that a more disaggregated analysis of crop nutrient requirements and manure nutrient supply is necessary to estimate the risks of excess nutrient loss to the environment. Results indicated that in general all counties had sufficient nutrient utilization capacities to value manure as a resource; however, counties in Northwest Iowa are becoming progressively more manure rich, while counties in Southwestern and Central Iowa are becoming progressively more manure poor. This separation of crop and livestock production is becoming more pronounced, indicating that solids separation and nutrient (especially phosphorus) recovery systems that can concentrate manure nutrients for transport could become more important to help counties maintain nutrient balance and to return manure nutrients to the soil if these trends persist.