A Microbial activity

Soil-agricultural waste material microbial composition and microbial activity greatly influence the rate of organic waste mineralization. Soil moisture, temperature, and aeration regulate soil microbial activity and thus are factors that influence the rate of waste mineralization.

Table 5-1 Common exchangeable soil cations and anions

Elements

Cations

Anions

Aluminum

Al+3

Boron

BO3-3

Calcium

Ca+2

Carbon

CO3-2, HCO3-

Chlorine

Cl- 3 3

Copper

Cu+, Cu+2

Hydrogen

H+

OH-

Iron

Fe+2, Fe+3

Magnesium

Mg+2

Manganese

Mn+2, Mn+3

Molybdenum

MoO4-2

Nitrogen

nh4+

no2-, no3-

Phosphorus

hpo4-2, h2po4-

Potassium

K+

Sulfur

SO3-2, SO4-2

Zinc

Zn+2

Soils that are warm, moist, and well aerated have the highest potential microbial activity and the highest potential rate of organic waste mineralization. Lower potential rates should be expected when soils are dry, cold, or saturated with water. (See figs. 5-1 & 5-2.)

Average annual soil surface temperature and seasonal temperature variations have a significant impact on the duration and rate of soil microbial activity. Average annual soil temperatures in the conterminous United States range from less than 32 °F (0 °C) to more than 72 °F (22 °C). Microbial activity is highest in soils that have high average annual soil temperature and lowest in soils that have low temperature.

In many areas, the mean winter soil temperature is 9 °F (5 °C) or more below the mean summer soil temperature. Microbial activity and organic waste mineralization in the soils in these areas are greatest during the summer months and least during the winter months. Thus, microbial activity decreases or increases as mean monthly soil temperature changes throughout the year.

Agricultural wastes applied to cold or frozen soils mineralize very slowly, are difficult or impossible to incorporate, and are vulnerable to surface runoff and erosion. Potential agricultural waste contamination of surface water is highest when agricultural wastes are applied under these conditions.

Microbial activity is also highly dependent on the soil moisture content. Soils that are dry throughout most of the growing season have a low organic matter mineralization rate. Microbial activity in these soils is greatest immediately after rainfall or irrigation events and decreases as soil moisture decreases. Conversely, soils that are moist throughout most of the growing season have higher microbial activity and more capacity to mineralize organic waste. Wet soils or soils that are saturated with water during the growing season have potentially lower microbial activity than moist soils. This is not caused by a lack of soil moisture, but is the result of low soil aeration when the soils are saturated.

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