Out
On A Limb / August 2007
Tipping the Scale
Soil pH refers to the acidity or alkalinity of soil. The pH of soil or, more precisely the pH of the soil solution, is important because soil solution carries nutrients like Nitrogen, Potassium and Phosphorus that plants need in specific amounts to grow, thrive, and fight off disease. Fourteen of the seventeen essential plant nutrients come from soil. Soil pH affects the solubility of minerals or nutrients and knowing the pH provides useful clues about soil properties.
The pH scale is actually a representation of relative hydrogen proton concentration. On the pH scale, 7 is neutral, 0 is the most acidic and 14 is most alkaline. But, unlike a linear scale that progresses in a smooth, incremental manner, a value shift on the pH scale represents a tenfold difference in the concentration. A change in pH from 2 to 3 represents a 10-fold decrease in concentration. A shift from 2 to 4 represents a one hundred (10 x 10)-fold decrease in concentration and from 2 to 5 represents a thousand (10 x 10 x 10)-fold decrease.
Before nutrients can be used by plants they must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils. If the pH of the soil solution is increased above 5.5, Nitrogen is made available to plants. Phosphorus, never readily soluble in the soil, is most available in soil with a pH range around 6.5.
The majority of food crops prefer a neutral or slightly acidic soil but blueberries, white potatoes, strawberries do well in strong acid soils. Some plants, azaleas, rhododendrons, foxgloves, birches and conifer trees, also grow well in strong acid soils. But, strongly acid soils, pH 4.0-5.0, can have high concentrations of soluble aluminum, iron and manganese which may be toxic to the growth of certain plants. In acidic soils, plants are more likely to take up toxic metals and some plants eventually die of toxicity.
A pH range of approximately 6 to 7 promotes the most ready availability of plant nutrients. Some crops, like cabbage, cauliflower and mustards, do well in moderately alkaline soils. Dogwoods, clematis, lilac and honeysuckle also enjoy a little alkalinity, but a slightly alkaline soils, pH 7.4-7.8 or higher, may have an inadequate availability of iron, manganese, copper, zinc, phosphorus and boron.
The soil pH also influences plant growth by affecting activity of beneficial microorganisms. Strong acid soils inhibit bacteria that decompose organic matter, preventing organic matter from breaking down. The result is an accumulation of organic matter with important nutrients, particularly nitrogen, locked up.
Most pesticides are labeled for specific soil conditions. If soils have a pH outside the allowed range, the pesticides may be ineffective or change to an undesirable form.
The pH value of a soil is influenced by the kinds of parent materials from which the soil was formed. Soils developed from basic rocks generally have higher pH values than those formed from acid rocks.
Soils can become acidic as a result of rainwater leaching away basic ions like calcium, magnesium, potassium and sodium. They are replaced by acidic elements such as aluminum and iron. Soils formed under high rainfall conditions are more acidic than those formed under dry conditions. Sometimes, carbon dioxide from decomposing organic matter and root respiration can dissolve in soil water to form a weak organic acid. And, sometimes, either decaying organic matter or the oxidation of ammonium and sulfur fertilizers can be responsible for formation of strong organic and inorganic acids.
The addition of lime replaces hydrogen ions and raises soil pH. Lime also makes phosphorus added to the soil more available for plant growth and increases the availability of nitrogen by encouraging decomposition of organic matter.
The Danish chemist that invented the pH scale was interested in how enzymes in the yeast used to make beer required a certain acidity to function. In his experiments, the chemist discovered most beers are in the 4.5 range and that acids and alkalis affect plant color.
There may be considerable variation in soil pH from one spot in a lawn to another. And, pH isn’t constant in soil or water, but varies due to rainfall, evaporation, biological growth within the soil and temperature changes.
Knowing the pH value of soil before planting is important because it has a direct influence on the health of the plant. Every plant has a recommended soil value range since each plant has different nutrient needs and the soil pH affects availability of the nutrients.
Gardens graced with Hydrangea macrophylla allow a rough reckoning of pH. A soil pH of 6 or below produces blue blooms, while a pH of 6.8 or higher produces pink flowers. PL
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