Building Soils for Better Crops, Third Edition

Effects of Manuring on Soils


Effects on Organic Matter

When considering the influence of any residue or organic material on soil organic matter, the key question is how much solids are returned to the soil. Equal amounts of different types of manures will have different effects on soil organic matter levels. Dairy and beef manures contain undigested parts of forages and may have significant quantities of bedding. They therefore have a high amount of complex substances, such as lignin, that do not decompose readily in soils. Using this type of manure results in a much greater long-term influence on soil organic matter than does a poultry or swine manure without bedding. More solids are commonly applied to soil with solid-manure-handling systems than with liquid systems, because greater amounts of bedding are usually included. A number of trends in dairy farming mean that manures may have less organic material than in the past. One is the use of sand as bedding material in free-stall barns, much of which is recovered and reused. The other is the separation of solids and liquids with the sale of solids or the use of digested solids as bedding. Under both situations much less organic solids are returned to fields. On the other hand, the bedded pack (or compost barn) does produce a manure that is high in organic solid content.

The Influence of Manure on Many Soil Properties

The application of manures causes many soil changes—biological, chemical, and physical. A few of these types of changes are indicated in table 12.2, which contains the results of a long-term experiment in Vermont with continuous corn silage on a clay soil. Manure counteracted many of the negative effects of a monoculture cropping system in which few residues are returned to the soil. Soil receiving 20 tons of dairy manure annually (wet weight, including bedding—equivalent to approximately 8,000 pounds of solids) maintained organic matter and CEC levels and close to the original pH (although acid-forming nitrogen fertilizers also were used). Manures, such as from dairy and poultry, have liming effects and actually counteract acidification. (Note: If instead of the solid manure, liquid had been used to supply N and other nutrients for the crop, there would not have been anywhere near as large a beneficial effect on soil organic matter, CEC, and pore space.)

High rates of manure addition caused a buildup of both phosphorus and potassium to high levels. Soil in plots receiving manures were better aggregated and less dense and, therefore, had greater amounts of pore space than fields receiving no manure.

Table 12.2: Effects of 11 Years of Manure Additions on Soil Properties
Application Rate (tons/acre/year)
Organic Levelnone10 tons20 tons30 tons
Organic Matter5.
CEC (mg/100g)19.815.817.017.818.9
P (ppm)*
K (ppm)*129.0121.0159.0191.0232.0
Total pores Space (%)ND44.
* P and K levels with 20 and 30 tons of manure applied annually are much higher than crop needs (see table 21.3A).
Note: ND = not determined.
Sources: Magdoff and Amadon (1980); Magdoff and Villamil (1977).

When conventional tillage is used to grow a crop such as corn silage, whose entire aboveground portion is harvested, research indicates that an annual application of 20 to 30 tons of the solid type of dairy manure per acre is needed to maintain soil organic matter (table 12.2). As discussed above, a nitrogen-demanding crop, such as corn, may be able to use all of the nitrogen in 20 to 30 tons of manure. If more residues are returned to the soil by just harvesting grain, lower rates of manure application will be sufficient to maintain or build up soil organic matter.

An example of how a manure addition might balance annual loss is given in figure 12.1. One Holstein “cow year” worth of manure is about 20 tons. Although 20 tons of anything is a lot, when considering dairy manure, it translates into a much smaller amount of solids. If the approximately 5,200 pounds of solid material in the 20 tons is applied over the surface of one acre and mixed with the 2 million pounds of soil present to a 6-inch depth, it would raise the soil organic matter by about 0.3%. However, much of the manure will decompose during the year, so the net effect on soil organic matter will be even less. Let’s assume that 75% of the solid matter decomposes during the first year, and the carbon ends up as atmospheric CO2. At the beginning of the following year, only 25% of the original 5,200 pounds, or 1,300 pounds of organic matter, is added to the soil. The net effect is an increase in soil organic matter of 0.065% (the calculation is [1,300/2,000,000] x 100). Although this does not seem like much added organic matter, if a soil had 2.17% organic matter and 3% of that was decomposed annually during cropping, the loss would be 0.065% per year, and the manure addition would just balance that loss. Manures with lower amounts of bedding, although helping maintain organic matter and adding to the active (“dead”) portion, will not have as great an effect as manures containing a lot of bedding material.

Table of Contents