Building Soil Organic Matter With Manure

There has been a lot of talk about nutrient management and the development of Nutrient Management Plans. There have been comments that some growers may avoid using manure to avoid doing Nutrient Management Plans. Manure has significant fertilizer value but it can also supply vital organic matter to the soil. Manure is one of the few ways to bring in an "outside source" of organic matter. We are also hearing more about greenhouse gases these days. Increasing soil organic matter levels is a way of sequestering (storing carbon) in the soil. Carbon in the soil can help make up for greenhouse gas emissions.

Soil Organic Matter

Soil organic matter exists in three pools in the soil. These pools are often described as the living, the dead and the very dead. Organic matter cycles between these pools. Crop management practices have the greatest influence on the living and the dead organic matter. As part of the cycle, organic matter is gained and lost. Additions to the soil are generally in the form of plant residues, manures and other organic materials. Organic matter is lost through soil erosion and burned off or oxidized through tillage.

Bringing In Manure

If you plan to use manure to build soil organic matter it is important to know the characteristics of the manure. There are several types of manure and many different feeding and bedding systems that impact its composition. Table 1 illustrates some of the differences in dry matter content.

What does it take to maintain or increase soil organic matter levels with manure? A long-term study was setup in Vermont to look at the impact of manure on soil properties. The research was conducted on a clay soil in a continuous corn silage rotation. This lack of rotation and the crop returns very little crop residue to the soil. Solid dairy manure was added at 3 different rates and conventional tillage was used. Table 2 shows the results.

Magdoff and Amadon, 1980

The 20 tons/acre/year rate was sufficient to counter the impact of continuous cropping a crop with few residues returned. Organic matter levels were maintained and the pH was kept close to the original level. Dairy and poultry manures have liming effects. This rate of manure supplied more nutrients than the crop removed. The organic matter additions improved soil aggregation, thereby increasing pore space.

How much organic material was being added at the 20 ton/acre rate? The dairy manure used had 13% dry matter, so 5,200 lbs. of solids were added. After decomposition about 25% of this would be added to the organic matter pool. In a 6 inch soil sample depth this would add about 0.065% organic matter per year.

The management of the field in this study is close to a worst case scenario. Cropping systems that return residues to the soil, protect the soil from erosion, and the use of no-till or reduced till would not require as large an organic matter addition to maintain or increase soil organic matter levels.

There are other organic materials that can be added to the soil and influence different management systems. This will be discussed in future articles this winter.

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