The acidity or basicity of a soil is referred to as pH and is based on the concentration of the hydrogen ions in the soil solution. As the hydrogen ion concentration (H⁺) in the soil increases, the pH drops increasing the acidity of the soil. If the hydrogen ion concentration drops in relationship to the hydroxyl ion (OH^-) concentration, the pH increases with the soil becoming more basic (or alkaline). Since pH is based on a logarithmic scale of 10, each unit change in pH (i.e. 7 to 6) is a 10 fold change in hydrogen ion concentration.

Soils in the valleys of Western Colorado typically have a pH of 7.5 to 8.5 creating a basic/alkaline condition. Gardeners hoping to grow blueberries (pH preference of 4.5) would need to change the pH accordingly. When changing a soil from a pH of 7.5 to a pH of 4.5, a 1000 fold change in the hydrogen ion concentration would need to be made.

Soil pH is usually reduced by the use of acidifying agents such as elemental sulfur or sulfuric acid. When sulfur is added to soil it combines with water and oxygen through bacterial activity and changes to sulfuric acid. This reaction takes place slowly over 6 to 8 weeks based on soil moisture and temperature conditions. It should be noted that since this process is accomplished through bacterial activity, when soil is pasteurized to eliminate pathogens as is often done with houseplant potting soil, this acidifying process is unlikely to occur as the necessary bacteria are killed. Sulfuric acid is much quicker acting but should be used with caution as previously mentioned.

The soils of Western Colorado are highly buffered with free lime (calcium carbonate - CaCO₃). Any sulfuric acid coming into contact with calcium carbonate will react producing water (H₂O), carbon dioxide (CO₂) and calcium sulfate (CaSO₄). No increase in hydrogen ion concentration occurs in this reaction, consequently no change in soil pH can occur. Instead, an increase in the salt level takes place due to the resulting increase in concentration of the salt gypsum (calcium sulfate - CaSO₄).

If the free lime in Western Colorado soils could be neutralized, any additional applications of sulfur or sulfuric acid would bring about a pH change. Six tons of sulfur (or 20 tons of sulfuric acid) is required per acre to neutralize each percentage of calcium carbonate. Many western Colorado soils contain 5 percent or more of calcium carbonate, thus 30 tons of sulfur or 100 tons of sulfuric acid would need to be applied for each acre. With up to four thousand feet of calcium-based Mancos shale under these valley soils and with the irrigation water containing calcium carbonate, the possibility of reducing pH in gardens and lawns is highly unlikely. Gardeners wishing for an instant remedy for this problem can rest assured that no such cure currently exists.

Greenhouse operators and homeowners working with potted plants can effectively adjust pH as they are working with a finite amount of soil. In those instances where a peatmoss based potting soil is used, it may be necessary to raise the pH due to the acidifying affect of the peat moss. Calcium carbonate (CaCO₃) is typically used to accomplish this task. When the pH of a potting soil needs to be lowered as when growing citrus, azaleas or Easter lillies, the use of sulfuric acid or sulfur may be needed.

Sulfur and gypsum (calicum sulfate) continue to be sold in Western Colorado as remedies to correct the soil problems in gardens and lawns due to a lack of understanding of the soil chemistry in the area. Farmers and gardeners alike should avoid the use of gypsum unless a soil test indicates a high sodium concentration and a low lime content. Such a soil is known as a sodic or alkali soil. Western Colorado soils are alkaline (pH above 7) and the two terms (alkali and alkaline) are sometimes confused. Western Colorado has very few true sodic soils and the use of gypsum should be avoided unless indicated by a soil test.

The addition of sulfur to lawns, however, may have a short term effect of reducing the pH in the upper layers of turf thatch and may affect disease-causing fungi due to sulfur's fungicidal activity. Any reduction in pH due to the addition of sulfur, however, is quickly nullified by the calcium carbonate in irrigation water.

The use of ammonium sulfate as a lawn fertilizer will, however, reduce the pH of the soil immediately around the root (the rhizosphere) due to the release of H⁺ from the root system as ammonium (NH⁺₄ form of nitrogen is taken in by the roots. The use of ammonium forms for fertilizers is a common way of controlling some turf diseases.

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