Manure has been a source of nutrients in grain crop production for millennia. Archaeologists have found evidence that European farmers have used animal manure to fertilize cereals and pulse crops for at least 7,900 years. Manure inputs remain an integral component of many agricultural systems worldwide. But what do we know about benefits to crop yields, soil properties, and environmental impacts when combining the use of commercial fertilizers and manures? Why might growers want to manage their soil fertility with both fertilizer and manure sources?
The longest-running agricultural experiment in the world indicates that the combined use of nitrogen (N) fertilizers and manure support high crop yields. The long-term experiments in Rothamsted, England, have shown that, for almost 50 years, N fertilizers supplied with either manure or mineral phosphorus (P) and potassium (K) fertilizers have sustained greater winter wheat yields in a diversified crop rotation than continuous winter wheat receiving livestock manure or NPK fertilizers alone. The research at Rothamsted highlights not just the importance of crop rotation but also demonstrates that balanced plant nutrition is critical to optimizing crop yield, soil health, and environmental performance.
Manure sources can complement NPK fertilizers by also providing other nutrients, such as calcium, magnesium, and sulfur, and micronutrients. While a lot of previous research indicates that balanced NPK fertilization improves crop production, the addition of manure with these fertilizers can sometimes but not always further improve yields. Some studies suggest that supplying NPK fertilizers is more critical to increasing yields regardless of manure applications. However, other studies have found that crops receiving NPK fertilizers along with manure consistently out-yielded crops receiving NPK fertilizers alone.
But if crop yields were to increase by combining fertilizer and manure sources, does this mean that the N requirement also increases? In other words, do higher-yielding crops need more N from these sources? The likelihood that the crop’s N requirement will increase in an integrated management system largely depends on the N dynamics in the system. Specifically, changes in N requirements depend on the magnitude of yield increase over time, the accuracy of the recommended N rate, the immobilization potential of the manure, the release of manure N in subsequent years, the degree to which N use efficiency is improved by including manure, and inherent soil properties.
If the N requirement is not expected to increase, farmers can substitute N from manure for a portion of their fertilizer N inputs. In North America there is a notable gap in the scientific literature on the optimal ratio of fertilizer and manure sources. In other parts of the world, research demonstrates that substituting a portion of fertilizer N with fresh or composted manure can enhance grain crop yields, crop N uptake, and N use efficiency. However, the yield response declines as the substitution proportion increases, and a high substitution ratio (>75%) tends to decrease crop yields and entails a number of risks. First, the availability of the N in manure can be difficult to predict. The ammonium in manure is immediately available, but the release of the organic N depends on mineralization. The timing of the manure application may not be as optimal for synchronizing with crop uptake as can be achieved with commercial fertilizers. Second, if manure is used to supply most of the crop’s N demand, excess P levels often accumulate in soils and may result in higher losses of P in drainage water, a risk of harm to the environment.
Integrating NPK fertilizers and manure also has clear benefits on various chemical, physical, and biological properties in the soil. Combining fertilizers and manure increases the carbon content in the soil, including various labile carbon and recalcitrant fractions. The additions of manure with NPK fertilizers increase the soil’s total N content, as well as N, P, and/or K availability for crop uptake. Biological benefits include enhancing the microbial biomass carbon, microbial activity, or the activity of specific enzymes. Physical benefits include a reduction in bulk density, improvements in water stable aggregates, moisture retention, and water infiltration. Integrated management can also improve environmental outcomes, such as reducing N leaching, runoff, and ammonia emissions, as well as increasing the rate of carbon sequestration.
Integrating nutrient sources, however, is not without its challenges. The nutrient availability of manure varies widely, which is a major challenge for nutrient management. As a result, the fertilizer equivalents of the manure’s nutrients can range from zero to almost 100% in the year of application. Solid manure typically has a lower mineral fertilizer equivalent than slurry, although both sources continue to provide nutrients in subsequent years. As a result, the nutrient availability of manures must be analyzed before their application to utilize the sources appropriately. At Rothamsted a high rate of manure applied long-term tripled soil organic carbon but also resulted in the highest rates of nitrate loss through tile drains as compared to N fertilizers applied without manure. Fertilizers resulted in modest increases in soil carbon but did not increase nitrate losses nearly as much and attained the same level of productivity.
Another challenge is that livestock and cropping systems are largely decoupled in many parts of the world, such as the U.S. After World War II, the agricultural landscape changed dramatically in the U.S. A growing population required greater food production, so farm size increased and energy use intensified. As a result, crop and livestock operations separated as cropping systems became increasingly specialized. These changes resulted in a high reliance on fertilizer inputs due to large nutrient imbalances in these specialized cropping systems. Today cropping systems require high amounts of nutrient inputs, while feeding operations accumulate such nutrients that were traditionally recycled in cropping systems. However, spatial separation presents a major logistical challenge to reintegration of these systems. Technologies to make the return of manure nutrients to the land more efficient and effective are needed.
Many studies have been published in scientific journals demonstrating the agronomic and environmental benefits of integrating fertilizer and manure sources. The research conducted at Rothamsted is available online in the research center’s Electronic Archive for data of the Broadbalk long-term winter wheat grain yields. A large volume of research has shown that balanced NPK nutrition is critical to optimizing crop yields, and that manure sources can complement NPK fertilizers by also providing secondary and micronutrients while building soil carbon. The positive agronomic effects reported here are based on 20 peer-reviewed studies, and the reported impacts of integrated management on biological, chemical, and physical soil properties are based on 18 published research findings. The recommendation of partially substituting fertilizer N (50% or less) with fresh or composted manure is based on a meta-analysis that looked at 141 peer-reviewed studies selected from around the world. However, the optimal combination of NPK fertilizers and manure may depend on many factors, such as the type of manure, the crop yield response, and the cropping system (e.g., soil type, crop rotation, etc.). Challenges associated with manure nutrient availability and decoupling of animal and crop systems have also been reviewed extensively in the literature.