Phosphorus (P), potassium (k) and calcium and magnesium are most available (wider width of the bar) between pH 6 and 7.

This has to do with the chemistry of the nutrients and with the fact that a balance of microbial activity occurs in this range, which supports the release of available forms of these nutrients.

When soil pH decreases much below 5.5, fungal growth is favored over bacterial growth and fewer nutrients are released for plant growth.

There are exceptions. Some of the micronutrients, such as iron, manganese, copper and zinc, are more available at lower pH values.

You may see deficiencies of these nutrients, especially iron, when pH is near or above 7.

Plants such as blueberries, rhododendrons, and azaleas require more of these micronutrients for good growth and that is why they do well at lower pHs.

There are a number of reasons why your garden soil may have a pH that is less than optimal.

If you have added compost or organic matter to your soil over a number of years it will cause acidification of the soil.

As organic matter decays to humus, acids (humic and fulvic) are produced that naturally lower the pH. Nitrogen fertilization, whether organic or in the form of ammonium salts, also generates acidity In the soil.

The reason is that when organic matter releases nitrogen it does so as ammonium. This reaction explains why agricultural soils require liming after years of fertilizer application.

Irrigation water can also influence soil acidity. Used over a long time, water can either increase or decrease soil pH depending on whether the water is acid basic or neutral.

As a point of reference, I have been measuring the pH of water sampled from Lake Pend Oreille for two years and have found the pH to be in the 7.5 to 8.0 range. Eventually this water would raise the pH of your soil. Well water in our area could have a very different pH than the lake.

If your soil pH is 6 or below and you want to raise it the following table is a guide to the amount of limestone or dolomite to add. It is dependent ion texture as heavier (higher clay content) soils require more limestone because they have a higher cation exchange capacity (CEC).

The same is true for soils that are high in organic matter content.

The best time to lime a soil is in the fall but it can also be done successfully in the spring.

However, the earlier you apply lime, the better, so that your soil has time to react and equilibrate.

How much agricultural limestone to apply varies by soil texture. For a general guideline, see below:

To raise pH to 6.5 from (in pounds/1,000 square feet)

pH Sand Loam Clay

• 5.0 40 100 150

• 5.5 30 80 100

• 6.0 15 40 60

If you should need to lower your soil pH you can do so by adding aluminum sulfate or ammonium sulfate or elemental sulfur. Again, the greater the CEC the more needed to lower the pH. Information on the type of soil you have can be found at: http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm

Gray Henderson is a professor emeritus of the School of Natural Resources, University of Missouri in Columbia, Mo. He received his bachelor of science degree in agronomy from Iowa State University; and his master’s and doctorate degrees in forest soils at Cornell University, Ithaca, N.Y. He has served in many prestigious venues and capacities over the years, and locally is former president of the Bonner County Gardeners Association, and currently a member of the board of directors for Lake Pend Oreille Waterkeepers. He can be contacted through the BCGA website at bcgardeners.org.