The soil pH is very important because it exerts strong influences on root development, activity of soil bacteria, fungi, symbiotic nitrogen fixation by legumes and the availability of nutrients.

The pH value reflects the relative number of hydrogen ions (H+) in the soil solution. The more hydrogen ions present, compared to the hydroxyl ions (OH-), the more acidic the solution will be and the lower the pH value. A decrease in hydrogen ions and increase in hydroxyl ions will result in more alkaline or basic conditions.

The ratio between hydrogen ions and hydroxyl ions changes tenfold for each unit change in pH. Therefore, a soil with a pH of 5.0 is 10 times as acidic as a soil with a pH of 6.0.

Soils are becoming more acid as a result of the removal of the cations calcium, magnesium, potassium and sodium through leaching, by growing crops or the use of common nitrogen fertilizers. As the cations are removed from the soil colloids, they are replaced with acid-forming hydrogen and aluminum.

The most common nitrogen fertilizers also contribute to soil acidity, since their reactions increase the concentration of hydrogen ions in the soil solution.

Many agricultural soils are in the pH range 5.5 - 8.0. The growth of crops on these soils is influenced by the favorable effects of near-neutral reaction on nitrification, symbiotic nitrogen fixation and the availability of plant nutrients. The optimum pH range for most crops is 6.0 - 7.5 and for leguminous and other alkaline preferring crops, 6.5 - 8.0. A desirable range for organic soils is 5.0 - 5.5.

Hydrogen ions in the soil solution are increased when the salts increase. This results in a more acid condition or pH. The salts may be a result of fertilizer residues, irrigation water, natural conditions, or microbial decomposition of organic matter. Infertile, sandy, high leached soils usually contain very little soluble salts.

When the soil pH is under 5.5, liming is generally used to raise the pH. Bacteria is low and does not freely fix nitrogen when the pH is under 5.3.

When the soil pH is over 7.5, sulfur is generally used to lower the pH. Bacteria flourishes in a high pH. This can upset the C:N ratio. (i.e. Bacteria will use up organic matter at a very high rate and can deplete the balance needed to grow crops and plants.

(Reference: A&L Agricultural Laboratories Agronomy Handbook - pp 53-57)

[ Home ] [ Guide ] [ What is It? ] [ How It Works ] [ How To Use ] [ Test Results ] [ FAQ's ] [ Contact Us ] [ Distributors ] [ Español ]