Tuesday 23 February 2010
THE EFFECTS OF SOIL pH ON NUTRIENT AVAILABILITY
Few things in gardening are so essential and so misunderstood as soil pH. Simply put pH is a measure of the acidity in soil. A basic soil test will give you all the information you need to make informed decisions on plant selection based on pH. The balance point for pH where a soil is said to be neither acidic or basic if a pH reading of 7. The acidity increases as the pH number decreases for example, a pH of 5 is more acidic than a pH of 6. Because the scale is logarithmic, rather than arithmetic, an increase of one point increases the acidity of a soil by 10 times, 2 points on the scale would increase acidity by a factor of 10 x10.
In the opposite direction on the pH scale, a soil with a pH of 8 is ten times more basic or alkaline than a soil with a pH of 7. The effects of soil Ph are far from simple, the acidity and alkalinity of soils directly effects the way most nutrients are taken up by trees and shrubs. It also effects the essential soil organisms that grow, thrive and interact with each other in soils. Fungi, bacteria, and other microorganisms depend on certain pH levels to function within soils.
Generally moist soils with lots of decomposing plant material, like the soils on forest floors, will be acidic. In contrast dry soils, from areas where grasses are the predominant vegetation will be less acidic and more alkaline. Parent material, the underlying decomposing rock that composes the gritty portion of the soil, also has an effect on the soils inherent pH. Soils that originate from volcanic rock sources like granite or basalt, tend to be acidic. Soils that originate from sedimentary rocks like limestone tend to be more alkaline.
With the basic background in place we can talk about the effects of soil acidity on plant growth.
Acidity in soils can be pictured as a key as one increases or decreases pH, certain soil chemicals will become unlocked and available, even to the point of toxicity. Similarly some soil chemicals will become locked or bound at certain pH levels and will be deficient and unavailable to trees and shrubs. All plants have ideal ranges of pH where they and their supporting microorganisms
can grow and thrive.
In your garden certain trees that prefer acidic soils, in the 4.5 to 5.0 range include blue berry, azaleas, hydrangeas, sweet gum and pin oak. As the soils become less acidic in the 5.0 to 5.5 range, most conifers including Scots pine, irises, ferns and similar plants will do well.
As we further decrease the acidity to the 5.5 to 6.0 range trees like white oak, bass wood, sugar maple and shrubs like boxwood or raspberry will do better. As you approach neutral to basic or alkaline pH's dry land species like ash, box elder, lilac as well as dogwoods and crab apples are in their ideal pH range. It is much better to pick a tree or shrub based on its pH requirements than to try to adjust the pH of the soil on your site. It is difficult but not impossible to raise or lower your soil pH. These processes take time and skill and will have to be monitored and adjusted on an ongoing basis as each site has its inherent pH based on parent material and existing vegetation.
Certified Arborists are knowledgeable in issues related to soil pH and are available to help with plant selection and diagnosing nutrient deficiency based on soil pH