Ideal gardening soil pH is between 6.0 and 7.0, which is slightly acidic. Soil acidity is naturally increased through the addition of decaying organic matter. Soil acidity is artificially increased through the use of nitrogen-based chemical fertilizers. Acidification of agricultural soils worldwide is a subject of concern for environmentalists because excess nitrogen distorts the biological processes of soil, plants and people.
Soil acidification increased with the development of nitrate fertilizer in World War I. "Soil acidification is a natural process, recently aggravated by human activities through the emission of sulphur and nitrogen compounds from the combustion of fossil fuels and from industrial processes," according to a report on soil by the Regional Environmental Center for Central & Eastern Europe. Soil becomes overly acidified from the use of nitrogen-based fertilizer and from the airborne nitrates of industrial pollution.
Rivers in the northeastern United States receive twenty times the natural amounts of nitrogen, which distorts wildlife ecosystems. "Nitrogen over-load" is the term used by the World Resources Institute to describe the life-threatening problem of soil acidification. Nitrogen-soaked soil leads to the loss of other essential nutrients such as calcium, magnesium and potassium. Nutrient loss eventually causes soil infertility.
Natural Soil Chemistry
Nitrogen, phosphorus and potassium are nutrients needed in the largest amounts by plants, according to Utah State University horticulturist Jerry Goodspeed. Nitrogen must be replenished yearly. It stimulates green growth. The natural source of nitrogen is the microorganism population in the soil food web. When soil organisms are nurtured with nonchemical care, adequate nitrogen is available to plant root systems in the correct amounts.
Chinese agriculture has contributed to global soil acidification problems through the increased use of nitrogen-based chemical fertilizer. A study in the February 2010 issue of Science magazine reported that "43 percent of the nation's extensive water pollution problem could be linked directly to agriculture." The average soil pH level has decreased 0.30 in Chinese commercial agricultural crops. Each number point on the pH scale indicates a tenfold change in acidity or alkalinity.
Home gardeners prevent acidification of garden soil by using compost to maintain neutral pH, using vermicompost to increase nitrogen, and by nurturing the soil food web. Microorganisms in the soil food web should include beneficial bacteria, many species of beneficial fungi, protozoa, small arthropods and earthworms. Earthworms increase the nitrogen level in soil through their castings.