What Is The Impact Of Manure On Soil Organic Matter?
Regular manure applications combined with forage-based rotations are the envy of crop producers when soil organic matter (SOM) levels and soil resilience are considered. What is the actual organic matter benefit from manure to the soil, and how does that vary with different types of manure and organic amendments?
Livestock manure is an excellent source of nutrients and organic matter for the soil. However manure composition, soil characteristics and application management will impact the nutrient and organic matter value.
Factors Impacting Nutrient & Organic Matter Benefits
Manure composition varies with animal type, age, feed ration and the environment.
Manure will add organic matter but also adds nutrients. Over-application of nutrients could lead to crop damage or nutrient losses into the environment. In addition, any benefits from soil organic matter are easily negated if soil compaction results from application on unfit soils.
Building Soil Organic Matter With Manure
Applying manure to the soil will provide other benefits, such as a greater diversity and activity of organisms and better soil structure. Table 1, Effects of 11 Years of Manure Additions on Organic Matter Levels, shows the increase in soil organic matter (SOM) over time. This suggests that at an application rate of 20 tons / acre / year, the SOM level was maintained, while at lower rates and without other additions such as residue or cover crops, the organic matter level gradually decreased.
Table 1. Effects of 11 Years of Manure Additions on Organic Matter Levels
Source: Building Soils for Better Crops, (Magdoff) SARE Outreach, www.sare.org
Note: The original organic matter level was 5.2%. The study was conducted on continuous corn silage on a clay soil adding dairy manure. The manure application also improved soil aggregation and the amount of pore space.
* P ppm (Bray) was converted to % change
Maintaining Soil Organic Matter Example
The question often comes up, "How much organic matter am I adding to the soil?" The answer is complicated, depending on soil texture, existing SOM, cropping practices such as rotation and use of cover crops, tillage, residue management, etc. Figure 1 illustrates how to estimate application rates required to maintain current soil organic matter levels. Assumptions include:
The chart shows the required application rate to match the annual soil decomposition for 2.7% organic matter using typical livestock analysis data. The application rate required to match annual SOM decomposition should consider nutrients added. Solid and liquid cattle manure applied at SOM decomposition rates align nicely with nutrient additions, while liquid hog and broiler manure to meet SOM decomposition rates would apply up to 6 times the nutrients removed from a crop of grain corn.
Figure 1 - Example of Application rates required to maintain SOM levels
Maintaining Soil Organic Matter - Example
Soil Organic Matter (2.7%)
2,000,000 lbs in surface 6 inches X 0.027 = 54,000 lbs organic/acre X 0.03 (3% SOM decomposes each year)
Lost by decomposition = 1,620 lbs OM
Manure Application to Maintain Soil Organic Matter - Example
Gain from manure = 104 lbs
Gain from manure = 90 lbs organic carbon
1,620 ÷ 104 = 15.5 tons solid dairy manure (provides 81 lbs available N, 114 lbs P205; 204 lbs K20)
Adapted from: Magdoff - Building Better Soils for Better Crops
Approximate Application Rates of Various Organic Amendments Required to Maintain Soil Organic Matter
Note: 1 gallon of liquid manure weighs approximately 10 lbs. Assuption: 1000 gallons is equivalent to 5 tons.
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