Chemical and Colloidal Properties

Soil pH is one of the most indicative measurements of the chemical properties of a soil. All (bio)chemical reactions in soils are influenced by proton (H+) activity, which is measured by the soil pH. The pH values of most natural soils vary between <3.00 (extremely acid) and 8.00 (weakly alkaline). The solubilities of various compounds (e.g., heavy metals) in soils are influenced by soil pH, as well as by microbial activity and the microbial degradation of pollutants. Optimum pH values for pollutant-degrading microorganisms range from 6.5 to 7.5. Soil pH is influenced by various factors: the nature and type of the inorganic and organic constituents that contribute to the soil's acidity, the soil/solution ratio, the salt or electrolyte content, and the CO2 partial pressure.

The term "heavy metal" refers to any metallic chemical element that has a relatively high density and is toxic or poisonous. Heavy metals are at least five times as dense as water, and light metals have densities that are lower than this. Examples of light metals include sodium, magnesium, and potassium. Examples of heavy metals include mercury, cadmium, thallium, lead, copper, aluminum, arsenic, chromium, and mercury. Fertilizers contain lead and arsenic. Pesticides contain lead, arsenic and mercury. Sewage sludge contains cadmium, arsenic and lead. Irrigation water can transport dissolved metals to agricultural fields, where metals such as cadmium can be incorporated into plants. Copper occurs naturally in soil and plants. It occurs in rocks, soil, water, sediments, and air. Its average concentration in the soil is about 50 parts copper per million parts (ppm) soil. Lead is by far the most the most common contaminant of soils. Lead is virtually a permanent resident in soil. Organic matter, can bind to metals very effectively; for example, the number one source of lead contamination is lead-based paint, which chipped or scraped are off building exteriors over periods of decades or centuries. Other sources are gasoline, exhaust, motor oil, automobiles, tires, industrial activities, coal combustion, and pesticides. Mercury occurs in two forms: organic and inorganic. Inorganic forms most often occur when mercury is combined with chlorine, sulfur or oxygen. Organic forms occur when mercury combines with carbon. Metallic forms of mercury are not absorbed by plants, but are converted by microorganisms to organic forms such as methyl mercury that are taken up by plants. Aluminum toxicity is one of the most common factors that limit plant growth and development in many acid soils. Aluminum is found in clay soils, in aluminum silicates and aluminum oxides, and plays a role in soil acidity.

Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment