All about soils: An area gardener's guide

Recently, I asked Gray Henderson, an expert on soils and their importance in gardening, to write a column on the subject for readers who might be interested. He agreed, and I submitted his article for print. Unfortunately, a series of problems – partly due to the use of a flash-drive with different programming than mine – brought about a confusing mélange of information that was sadly garbled.

Here, with apologies, is the properly scanned information on this timely subject, as written by Mr. Henderson.

For the most part the soils used for gardening in Bonner County are “young” by geologic standards. They are influenced by glacial activity that occurred less than 30,000 years ago, inputs of volcanic ash from the eruption of Mount Mazama (Crater Lake) about 6500 years ago and to a lesser extent from the recent eruption of Mount St. Helens. And, they have also received inputs of wind-blown silt (loess) similar to that found in the Palouse of Eastern Washington and parts of Central Idaho.

The two soil properties most important to gardeners are soil texture and soil pH (acidity or alkalinity). Soil texture is important because it influences how sell soils drain and therefore how early soils warm up in the spring. Silty soils and those with a lot of clay hold more water than sandy soils and therefore are more difficult to work in the spring. When you see water standing in fields around the area you can be sure there is a clay or silt layer in that soil that is restricting drainage and making it difficult to cultivate. Sandy soils drain well; however, they have the disadvantage of being “droughty” – that is, they need more frequent irrigation during the summer because they store less water available to plants.

Short of importing soil or using raised beds to garden, soil texture is hard to change. The only practical way to alleviate poor soil texture is to add organic matter to the soil. The mantra is the more organic matter in the soil the better. It helps loosen heavy silt and clay soils and allows sandy soils to hold more water.

The other important soil property is its acidity, measured by a value called soil pH. The generally accepted notion is that our soils are acid because most of them formed under forest vegetation. But I have not found this to be the case. A soil with a pH of 7 is considered neutral, neither acidic nor basic. I have found the pH of the soils in our gardens to range from 4/5 (very acid) to 7.5 (somewhat alkaline). Most vegetables, flowers and shrubs grow best at a pH between 6 and 7. Exceptions are potatoes, rhododendrons and azaleas which do better at slightly lower pHs and blueberries which require acid soil with pHs between 4.5 and 5.

The reason pH is important us that it controls the availability of nutrients. As shown in the illustration, the major nutrients nitrogen (N), 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 (NH4). 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 on 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.

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

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

The map of soil types are based on the ‘Soil Survey of the Bonner County Area, Idaho’ (USDA Soil Conservation Service) done in the 1980s but is still valid today.

Gray Henderson is a Professor Emeritus of the School of Natural Resources, U, of Missouri, Columbia, MO. He received his BS in Agronomy from Iowa State University; MS and PhD in Forest Soils, 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 Lake Pend Oreille Waterkeepers. He can be contacted through the BCGA website at: bcgardeners.org.

Valle Novak writes the Country Chef and Weekend Gardener columns from the Daily Bee. She can be reached at bcdailybee@bonnercountydailybee.com or by phone at 208-265-4688.