Compost and humus comprise the majority of brown earth soil. The terms humus and compost are frequently used interchangeably. Compost originates from decayed organic matter. Microbial activity degrades compost to a stabilised form of soil, called humus. Varying particle sizes in humus result from the decomposition of the soil. These particles aggregate and provide pathways for oxygen and water to flow and for dissolution of nutrients. Humus absorbs and holds three times more positively charged particles, water and carbon compounds than other soils and provides these properties in forms plants can absorb.
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Dark soils predominately contain carbon. Plants create carbon from photosynthesis. Decomposing plants and animal manures transfer carbon to soil. Carbon may remain in soil for up to 1,000 years.
Combined with other organic substances, carbon forms starches and sugars, creating energy for plants.
Carbon sources for compost include sawdust, newspaper, dried leaves and pine needles.
Plants take up nitrogen when dark brown soils have a carbon to nitrogen ratio of less than 25:1. Plants use nitrogen for leaf and stalk growth. Nitrogen enters the soil from crop residues, animal manures and precipitation. Nitrogen-fixation is the process of nitrogen conversion from nitrogen gas into a form plants can access, such as urea. Legume crops fix nitrogen in garden soil. Dark soils release nitrogen slowly at a rate plants absorb efficiently. This slow release prevents water pollution associated with other, faster-release, nitrogen fertilisers.
Dark soil contains phosphorous in small quantities in the form of phosphate. Phosphorous plays a role in photosynthesis, plant maturation and stress resistance. Phosphorous aides in bloom and root growth processes and effects plant growth rates. Phosphorous leaches easily from many soil types and is often associated with water pollution. Humus provides an environmentally-friendly, slow release form of phosphorous at a rate plants can absorb. If a soil test indicates a deficiency of phosphorous in your garden, amend with compost or bone meal.
Humus contains sulphur in small quantities. It can enter the soil from precipitation and plant and animal residues. Sulphur regulates photosynthesis and nitrogen fixation. Sulphur affects plant root growth, growth rate, seed production and cold resistance. Soils with a pH range of six to seven generally have adequate amounts of sulphur present in the soil. Adjust pH if the soil is deficient in sulphur.
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- USDA Natural Resource Conservation Service: Basic Biological Factors of Soil Carbon and Nitrogen
- Oregon State University Extension: Composting
- University of Missouri Extension: Compost Analysis
- Cornell University Cooperative Extension: Sulfur for Field Crops
- College of the Virgin Islands Extension: Soil Fertility