Humic substances native to agricultural soils typically maintain at constant levels (humus decomposition and new formation are balanced); however, environmental factors can directly effect the amount of decomposition – leading to an overall decrease in soil organic matter. The main factors effecting decomposition of soil humic matter include the effects of vegetation, topography, parent material (minerals), and the effects of climate. (Troeh 2005)
Vegetation: changes in vegetation can modify how organic matter accumulates within the soil (historically forested areas vs. grasslands)
Topography: slopped hills can lead to increased erosion and low of organic matter.
Parent Material: effects nearly all facets of soil characteristics: thickness, texture, and mineral content. Water content of the soil (partially dictated by soil drainage and aeration) is a critical factor in humus decomposition. Better aerated soils encourages microbial action on humic substantiates.
Effects of Climate: several weather conditions effects soil organic matter: Dry climates generally have less organic content in soil due to the lack of plant growth; Soil temperatures also effect organic content levels by effecting microbial activity- warmer climates accelerate decomposition, while areas with cool winters preserve organic content as microbes are inhibited for long period.
Figure 6-8: Typical organic matter accumulations in the A horizon of grasslands soils as influenced by average annual precipitation and temperature (Troeh 2005)
From the information listed above, it is clear that the soils with the least amount of humic material are hot, dry, and well aerated (i.e. sandy). Deserts are not known for their rich, dark soils; however, a large number of farms in the southwest operate in desert like conditions through intensive irrigation practices.