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                                                SOIL pH
                                                 Edited by Len Phillips

Soil pH is a measure of acidity or alkalinity in soil, represented by a number on a scale in which 1 is very acidic, 7 is neutral, and 14 is extremely alkaline. For optimum plant growth, efforts should focus on maintaining a nearly neutral soil pH. “pH” means the power of hydrogen and is a measurement of hydrogen atoms in the soil. Acidic soil contains many H⁺ ions and alkaline soil contains many OH^– ions.

The most accurate method of determining soil pH is by a pH meter. A second method, which is simple and easy but less accurate than using a pH meter, consists of using certain indicators or dyes.

Why pH is Important

Soil pH is important because it influences plant growth, beneficial bacteria and nutrient availability, toxic elements, and soil structure:

• A pH determination (soil test) will tell whether the soil will produce good plant growth or whether it will need to be treated to adjust the pH level. For most plants, the optimum pH range is from 5.5 to 7.0, but certain trees prefer a more acidic soil and others may require a more alkaline level.

• Bacterial activity that releases nitrogen from organic matter and certain fertilizers is particularly affected by soil pH, because bacteria function best in the pH range of 5.5 to 7.0.

• Plant nutrients leach out of soils with a pH below 5.0 much more rapidly than from soils with values between 5.0 and 7.5 and are generally most available to plants in the range of 5.5 to 6.5.

• Aluminum, iron, and manganese may become toxic to plant growth in certain soils with a pH below 5.0.

• The structure of the soil is affected by pH. Clay soils for example, are granular and are easily worked at the optimum pH range (5.5 to 7.0), but if the soil pH is either extremely acid or extremely alkaline, clays tend to become sticky and hard to cultivate.

• Raising the pH will add calcium and reduce the effects of calcium leaching.

• Raising the pH will also raise phosphorus, molybdenum, and magnesium levels.

The pH is not an indication of fertility, but it affects the solubility and availability of nutrients to be taken up by plant roots. A soil may contain adequate nutrients, yet growth may be limited by a very unfavorable pH. Likewise, builder's sand, which is virtually devoid of nutrients, may have an optimum pH for certain plant growth.

Descriptive terms commonly associated with certain ranges in soil pH are:

• Extremely acid: < than 4.5; lemon=2.5; vinegar=3.0; stomach acid=2.0; soda=2.0–4.0

• Very strongly acid: 4.5–5.0; beer=4.5–5.0; tomatoes=4.5

• Strongly acid: 5.1–5.5; carrots=5.0; asparagus=5.5; boric acid=5.2; cabbage=5.3

• Moderately acid: 5.6–6.0; potatoes=5.6

• Slightly acid: 6.1–6.5; salmon=6.2; cow's milk=6.5

• Neutral: 6.6–7.3; saliva=6.6–7.3; blood=7.3; shrimp=7.0

• Slightly alkaline: 7.4–7.8; eggs=7.6–7.8

• Moderately alkaline: 7.9–8.4; sea water=8.2; sodium bicarbonate=8.4

• Strongly alkaline: 8.5–9.0; borax=9.0

• Very strongly alkaline > than 9.1; milk of magnesia=10.5, ammonia=11.1; limestone=12

Soils tend to become acidic as a result of:

• rainwater leaching away basic ions (calcium, magnesium, potassium and sodium);

• carbon dioxide from decomposing organic matter and root respiration dissolving in soil water to form a weak organic acid;

• the formation of strong organic and inorganic acids, such as nitric and sulfuric acid, from decaying organic matter and oxidation of ammonium and sulfur fertilizers. Strongly acid soils are usually the result of the action of these strong organic and inorganic acids.

How to correct pH

Normally, limestone (calcium carbonate), dolomitic limestone (calcium carbonate and magnesium carbonate), burnt lime (calcium oxide), or slaked lime (calcium hydroxide) are used to increase the pH, or "sweeten" the soil. Limestone and dolomitic limestone are less likely to "burn" plant roots, while burnt and slaked lime are not recommended around plants. The amount of these materials necessary to change the pH will depend on the soil type. Table 1 shows the amounts of limestone needed to raise the pH. The greater the amount of organic matter or clay in a soil, the more limestone or dolomitic limestone required to raise the pH.

Table 1. Pounds of limestone per 100 sq. ft. needed to raise the pH to 6.5
	Sandy loam	Loam	Clay
From pH 4.5	12.6	25.3	34.8
From pH 5.0	10.6	21.2	29.0
From pH 5.5	4.2	8.4	11.6
From pH 6.0	1.7	3.3	4.5

If a soil is tested as too alkaline, determine if this is due to recent application of limestone or whether it is due to an inherent characteristic of the soil. It is quite difficult, if not impossible, to change appreciably the pH of naturally alkaline soil by use of acid-forming materials. If a high pH is due to applied limestone or other alkaline additives, ammonium sulfate, sulfur, or similar acid-forming materials can be applied. Table 2 shows the amounts of sulfur needed to lower the pH.

Table 2. Pounds of sulfur per 100 sq. ft. (9 sq. m.) needed to lower the pH
	To pH 6.5	To pH 6.0	To pH 5.5	To pH 5.0
From pH 8.0	3.0	4.0	5.5	7.0
From pH 7.5	2.0	3.5	5.0	6.5
From pH 7.0	1.0	2.0	3.5	5.0
From pH 6.5	None	1.0	2.5	4.0

Not more than 1 pound (0.46 kg) of sulfur per 100 sq. ft. (9 sq. m.) should be applied in one treatment. If the soil is clay loam, heavier applications of sulfur will be necessary. Repeat applications of sulfur should not be made more often than once every 8 weeks. Sulfur oxidizes in the soil and mixes with water to form a strong acid that can burn the roots of plants and should be used with caution. It is easier to raise the pH of soil than it is to lower it.

Organic Matter Effect

As organic matter decomposes, minerals are slowly converted to salts that dissolve in water and become available for plant roots to absorb. Using overly acidic compost won't usually do any long-term damage to the soil, but using one that's too alkaline might. Regular applications of good-quality compost help maintain neutral soil pH. High-pH composts often contain carbonates, usually in the form of limestone (calcium carbonate). In naturally alkaline soil (most common in drier regions), avoid using high-pH compost because other nutrients, such as phosphorus and zinc, will become unavailable.

Tree Growth

Trees grow within a limited range of pH values. It is difficult to change the pH in a landscape situation as compared to an agricultural situation where the soil is turned frequently and soil amendments are easily added. In the landscape situation, limestone or sulfur can be conveniently added only during the planting process. Therefore, it is better to plant trees that tolerate the existing pH rather than trying to change it after planting, unless the entire root zone is replaced with non-alkaline soil. To do otherwise may result in nutrient deficiencies that would affect plant growth and survival.

If trees are planted in alkaline soils, trees that are tolerant of alkalinity can get the needed nutrients through a process that acidifies the soil around their roots. When planting "moderate water-use" plants, try to take steps to counter the alkalinity and promote better plant health. At planting time include well-decomposed organic matter and soil sulfur in the planting mix of an over-sized planting hole. The organic matter naturally tends to moderate the alkaline conditions, especially over time.

Sulfur is only available in pellet form. When intact, the pellet has a limited effect and is overly concentrated in that spot. To counter this condition, add the sulfur to the backfill piles and turn the pile once. The pile must be at least somewhat moist. Allow the pile to sit for a few hours so the soil moistens the sulfur pellets and the sulfur becomes softer and prone to disintegration. When the soil is turned a second time and again in the planting hole, the pellets break apart and disintegrate. This process of allowing the sulfur to soften greatly enhances its effectiveness.

Sources:

• Bickelhaupt, Donald, “Soil pH: What it Means”, State University of New York College of Environmental Science and Forestry, 2008.

• Mathers, Dr. Hannah, "Field Planting of Caliper Trees", Nursery News, Vol. 20 No. 7, July 2005.

• Nesmith, J. and E.W. McElwee, Circular 352, Florida Cooperative Extension Service.

• Organic Gardening staff, “Testing for 5 Key Compost Characteristics”, Organic Gardening Magazine.

• Perry, Leonard, “pH for the Garden”, University of Vermont Extension, 2003.

• Schilling, Norman, President, Schilling Horticulture Group, personal communication, 2007.

• Trowbridge, Peter J. and Nina L. Bassuk, “Trees in the Urban Landscape”, John Wiley & Sons, Inc. 2004.

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