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Soil tests and pH are important

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By Dennis Morgeson

I have had numerous people ask me what to fertilize with and how much.

I often tell them it depends on the plants they are growing and that I can’t give them an accurate recommendation without a soil test. 

Granted, we don’t test for nitrogen because it is always present and volatile. However timing and amounts to apply are generally constant for particular plants. 

If you want an accurate recommendation, a soil test is your only answer.  Soil tests contain valuable information about what is present in your soil including phosphorus, potassium, calcium and pH levels. 

Most people are aware that the most important nutrients or at least the nutrients needed in the largest quantities for plants are nitrogen, phosphorus, and potassium but what they don’t know is that the pH level plays a large role in availability of nutrients.

pH is a measurement of acid-forming or hydrogen ion activity in the soil.  If you are wondering what that means, it simply means either the soil is acid or alkaline. Older people call this sour or sweet. 

The reason for this is acidic things taste sour like lemon juice or vinegar.  pH is measured on a scale with the lower the number the more acidic and the higher the number the more alkaline. 

The pH scale range is from 1-14 with seven being neutral which is neither acidic nor alkaline. 

The pH scale is logarithmic. 

This means that for each number you move on the scale, the pH increases or decreases by 10 times.  For instance, if your pH goes from seven to eight the pH is now 10 times more alkaline or sweet than before. 

If the pH goes from eight to 10 it is now 100 times more alkaline or sweet.  This is also true for acidity. For instance if the pH goes from six to four it is now 100 times more acidic or sour. 

If it goes from 6 to 9, the soil has went from being acidic or sour to alkaline or sweet and is now actually 1000 times more so.  Basically what all this means is that there is a large difference between each number on the scale.  So why do we care?

Well, the pH of the soil is one of the primary factors impacting plant growth.  When the pH is acidic or alkaline nutrients are sometimes tied up in the soil and the plants can’t use them. 

This means that a plant may show a phosphorus deficiency by looking at it, but the soil may actually have plenty of phosphorus but the plant just can’t get it because of the pH.  If the pH isn’t fixed, the plant actually starves to death, while other plants that prefer that particular pH grows excessively.

Most plants prefer a slightly acidic pH of around 6.8 or so.  This just happens to be the pH that most nutrients are readily available. 

With that said, all plants have an ideal pH range at which they grow best. 

This is most evident in the landscape with acid-loving plants such as azalea, rhododendron, most oaks, holly, magnolia, dogwood and river birch.

When the pH is too high, acid-loving plants will first show signs of iron chlorosis. 

The leaves will be a pale green color with darker green veins. 

Over time, the plant will grow slowly and the leaves will eventually even turn almost white before they brown out and drop.  Now, this doesn’t happen overnight. It usually occurs slowly over several seasons.  These plants are displaying an iron deficiency, so without a soil test, one might just apply iron fertilizer.  Well, in most cases, there is plenty of iron in the soil. It’s just not available because the pH is too alkaline or sweet.
Now you are wondering what to do and how do we know. 

Well, enter the soil test. 

A soil test will tell us the pH.  If the pH isn’t at least 6.5 or lower, we know that there is probably enough iron but the plant can’t get to it. However, we will also know how much iron is actually present in the soil from our test results. 

So the recommendation would be to apply sulfur or aluminum sulfate to lower the pH, the amount will be determined by how much we need to lower the pH.  

One note of caution: don’t apply sulfur or aluminum sulfate to plants just because you know they are acid loving.  The pH can get too acidic even for acid- loving plants.  In other words, get a soil test.  It is recommended that you test your soil every few years.

Every two to three years is sufficient for landscapes, flowerbeds and vegetable gardens. 

Fall is a great time to test as well because over the winter months, your pH can be corrected if needed.

For more information about soil tests or iron chlorosis, call the Washington County Extension Office.