The effect of soil pH is great on the solubility of minerals or nutrients. Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it 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. Phosphorus is never readily soluble in the soil but is most available in soil with a pH range centered around 6.5. Extremely and 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 some plants. A pH range of approximately 6 to 7 promotes the most ready availability of plant nutrients.
There are of course exceptions, such as azaleas, rhododendrons, blueberries, white potatoes and conifer trees, which tolerate strong acid soils and grow well. Also, some plants do well only in slightly acid to moderately alkaline soils. However, a slightly alkaline (pH 7.4 - 7.8) or higher pH soil can cause a problem with the availability of iron to pin oak and a few other trees causing chlorosis of the leaves which will put the tree under stress leading to tree decline and eventual mortality. This is also a problem on beans and strawberries.
The soil pH can also influence plant growth by its effect on activity of beneficial microorganisms. Bacteria that decompose soil organic matter are hindered in strong acid soils. This prevents organic matter from breaking down, resulting in an accumulation of organic matter and the tie up of nutrients, particularly nitrogen, that are held in the organic matter.
The bar graph gives the pH ranges at which plant nutrients will be most available. The wider the bar, the more available the nutrient. Calcium, magnesium and potassium--the exchangeable bases--are most available at high pH and unavailable at low pH. Nitrogen and sulfur have similar available pH ranges. Iron, manganese, zinc, and copper are less available at high pH values. Phosphorus and boron are unavailable at both low pH and high pH.
Soybeans and other bean crops are susceptible to micronutrient deficiencies. Here, soybeans are showing iron deficiencies called chlorosis on a calcareous soil. See Soybean Chlorosis Management for further information.
It is impractical to leach the free carbonates out of the soil, so the solution to this deficiency problem is not soil alteration.
There may be considerable variation in the soil pH from one spot in a field
or lawn to another. To determine the average soil pH of a field or lawn it is
necessary to collect soil from several locations and combine into one sample.
In some cases, where changing the pH would be unfeasible, it is better to
find plants adapted to that pH. Your text gives a chart showing the pH
tolerance range of a wide variety of plants. Broad leaf evergreens are noted
for liking acid conditions. On the other end of the scale, alfalfa is one plant
that likes a higher pH range.
Chapter 6 Changing soil pH
Soil pH Chapters
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