Many people ask if manure can be applied to economically build soil organic matter. The following article was taken from the Penn State Field Crop News, Vol 01:11, 9/28/2001 and does a good job of explaining the amount and type of manure that can be used to help increase organic matter in the soil.
Organic matter is the bright star in the sky of soil quality. Soil organic matter improves soil tilth, infiltration rate and soil water holding capacity, contributes nutrients to the crop, and increases the CEC of the soil. Manure is an important source of raw or partly decomposed organic matter.
The question is if organic matter levels can be maintained with manure application only. In an 11-year corn silage experiment in Vermont, 20 T/A (wet weight, including bedding) of dairy manure were needed to maintain soil organic matter levels. Applying less than 20 T/A led to a decrease of soil organic matter contents. Twenty tons of dairy manure contains approximately 5200 lbs of dry matter, 200 lbs of N, 80 lbs of P2O5, and 160 lbs of K2O. Assuming a typical yield of 21 T/A, corn silage removes approximately 105 lbs of P2O5, and 230 lbs of K2O, so a 20 T/A manure application is not excessive. The answer in this case is yes, organic matter levels can be maintained with bedded dairy manure in a continuous corn silage system. You would have to apply more than 20 T/A to increase soil organic matter content.
Most dairy farmers use liquid manure which does not contain any bedding material. Liquid dairy manure contains less than 5% dry matter compared to 13% dry matter in solid manure. To apply the same amount of phosphorus as in the previous example, 6200 gallons/A would need to be applied. This amount of manure weighs 25.9 T (assuming 8.35 lbs/gal), contains 2600 lbs of dry matter (high estimate, based on 5% dry matter content), 174 lbs of N, 81 lbs of P2O5, and 155 lbs of K2O. With the same amount of nutrients, one applies approximately half the amount of dry matter in liquid manure as in solid manure. The conclusion therefore is that with liquid manure only, it will not be possible to maintain soil organic matter levels in a corn silage system without over-applying phosphorus.
These results correspond well with the experience of the long-term rotation experiment at the Penn State Research Farm at Rock Springs in Centre County. In this experiment, 15 years of liquid manure application (based on P-needs of the crop) in a continuous corn grain system resulted in an organic matter content of 2.8%, compared to 2.2% in the same system that received fertilizer but no manure. The increase of soil organic matter content was very small in this experiment.
These data show that liquid manure applications without the return of adequate amounts of crop or cover crop residue to the soil will result in a decline of soil organic matter contents.