Building Soil Organic Matter

SOM is in a constant state of flux – additions and losses are simultaneously occurring. To maintain SOM levels, one must reduce the rate of SOM loss while increasing the rate of SOM additions.

SOM can be lost from soil by both wind and water erosion. However, the primary loss mechanism for SOM is microbial decomposition. Soil microbes use SOM as a source of energy and nutrition, converting SOM into carbon dioxide and mineral elements. The rate of microbial decomposition of SOM is controlled by a number of factors including: soil temperature, moisture, aeration (oxygen), and the quality or characteristics of the SOM. The rate of decomposition can be greatly influenced by management. For example, aggressive tillage and cultivation increases aeration and destroys the soil aggregates that protect SOM from microbial decomposition. Reducing tillage and cultivation is an effective management strategy to maintain, or even increase, SOM content (see Reduced Tillage).

There are a number of ways to increase or maintain SOM. Increasing the quantity of plant residues returned to the soil is one of the most sustainable methods of maintaining or increasing SOM additions. Most vegetables leave little residue in the field and SOM will usually decrease if only vegetable residues remain in fields.  However, significant amounts of biomass can be added to the soil by including cover crops in the rotation. Although cover crops can add enough biomass to maintain SOM, it is difficult to increase SOM with most cover crops. Sod forming crops included in the rotation, however, can increase SOM. Another more rapid and direct method of increasing SOM is the application of organic amendments such as organic mulch and compost. While the application of organic amendments is among the most effective methods of rapidly increasing SOM, extractable soil phosphorous (P) concentrations must be monitored to avoid excessive applications. 

The amount of organic amendments that can be added without building up excessive phosphorus depends primarily on: 1) the existing soil test P level of the field; and 2) the P2O5 content of the amendment.  Table 7 shows the effect of both soil test P categories and the P2O5 concentration of an organic amendment on the suggested maximum amount of material to apply. If these rates of amendments are applied every year, it is recommended to analyze the soil for extractable P annually to ensure that soil test P has not risen to excessive levels. Phosphorus should not be accumulating in the soil if the soil test P level is Very Low or Low, but rapid accumulation when the soil test P level is Optimum could increase the soil test P to Above Optimum levels, which would preclude the application of more compost until the soil test P level fell back into the Optimum range.

Table 7. Maximum Compost or Organic Amendment Application and total P2O5 per Soil Test Category and  P2O5 Concentration1

 

Soil Test Phosphorus Category

Compost/organic amendment P2O5 content

Very Low/Low Optimum

Optimum Above Optimum

% P2O5 (dry wt.)

P2O5 (lbs/acre)

Compost (tons/acre)

P2O5 (lbs/acre)

Compost (tons/acre)

 

Low (0.1 to 0.5%) 0.25%2

330

120

82

30

No application

Medium (0.5 to 1.5%) 1%

330

30

55

5

No application

High (1.5% to 3.0%) 2%

330

15

No application

No application

1 Assumes moisture content of the compost or organic amendment of 45%.

2 Percentage used to calculate amounts of P2O5 applied for various rates of compost applications.