Sunday, March 31, 2013

Getting to the pH of the problem

When we fertilize, our goal is to make nutrients available in the soil.  Plants may then absorb and use these nutrients in biological processes.  By taking soil samples we may determine what nutrients may be lacking, or what nutrients may be in such abundance they are creating an antagonistic environment for the plant.  Most soil samples will also return pH.

Soil pH is a measure of the acidity or basicity in soils. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions (H+ or, more precisely, H3O+ aq) in a solution. It ranges from 0 to 14, with 7 being neutral (I copied all of that from Wikipedia).

Soil pH is important. Nutrients can be made unavailable in soil at different pH ranges. Classic examples are iron (Fe) will get tied up in soils at higher pH, while phosphorous (P) will get tied up at lower pH. The pH scale is logarithmic, so there can be a big difference in availability between 5.5 & 6.

Chart of soil pH and Nutrient Availability


When managing trees in a man-made landscape, pH ranges can vary within a few feet of each other. Removal of soil layers, exposure of soil layers, and type of building material can all effect a 'micro pH environment.' Most trees enjoy a pH of around 6 (give or take). Again, depending upon multiple factors, adjusting pH to an ideal range for trees may be difficult. In addition, depending on which way you are trying to tip the scale, and which product you are using, results may take months and may be fleeting.

Take soil samples and pay attention to pH.  All of the nutrients required for growth may already be in the soil and just tied up.  Knowing this information will help arborists choose the correct soil amendments and reduce the chances of creating an antagonistic environment from over applying fertilizers.