VALLEY — When preparing a garden site, one of the hardest things to manage can be the pH (measure of acidity) of the soil. In the San Luis Valley, the natural soil often has a pH of 7.5 or higher. We will find pH’s in the 8.0 to 8.6 often, and even some above 10.0 pH. 

All plants have a pH range that they will grow best in. Most plants will grow in a soil with a pH of 6.0 to 7.5. When the pH is outside this preferred range the growth or fruit productions can be drastically reduced or stopped completely. A pH of 7.0 is neutral with a pH of 6.8 to 7.2 called “near neutral.” Soils with a pH below 7.0 are called acidic. These high pH soils are called basic or alkaline.

Soils with a pH of 7.5 or above generally have a high calcium carbonate content. The high calcium carbonate is known as free lime. This is important as it is impractical to lower a soils pH if it contains free lime. This free lime buffers the soils against pH changes. If the soil pH gets above 8.3 it is referred to as a sodic soil. It can be very difficult to lower the pH of a sodic soil. Most plants won’t grow in a sodic soil.

The soil pH is a product of the soil that it comes from and the climate. The quality of the irrigation water can also have an effect on the soils pH. Many areas of the world where there is limited or small amounts of rainfall have alkaline soils. 

The soil pH can be adjusted with the right conditions and the right soil amendment.  Changing the soil pH starts with a soil pH test.

To attempt to manage these soils with a pH above 7.5, start by increasing organic matter in the soil. Also, manage the soil moisture by using organic mulches and proper irrigation. Overly wet or dry soils can amplify the problems of working with a high alkaline soil. 

Over time (decades) you may see a slight decrease in soil pH if proper irrigation is used. Many fertilizers can add acid to the soil which may help lower the soil pH over time. 

Lowering the pH of a soil can occur. If there is no free lime (pH below 7.5), applications of elemental sulfur can lower the pH. If there is free lime, lowering the pH in not practical. To test for free lime, crumble up some dry soil, place a heaping tablespoon of the soil in a cup. To the cup add some vinegar. If the soil-vinegar mixture bubbles, the soil has free lime. 

For soils without free lime, you can add elemental sulfur, aluminum sulfate, some ammonium fertilizers or acid sphagnum peat prior to planting. The higher the pH, the more elemental sulfur will need to be added. To change the pH from a 7.5 to a 6.5, add 1.5 pounds of sulfur per 100 square feet. To move from an 8.3 pH to a 6.5 pH, you would need to add 4.0 pounds of sulfur per 100 square feet. This assumes you don’t have free lime in the soil.

Aluminum sulfate can be used but it has the potential to cause aluminum toxicity in plant roots. Fertilizers should not be used at rates higher than what is needed for the plant nitrogen requirements. Acid sphagnum peat should be incorporated before planting. The positive effects of acid sphagnum peat will last several years.  However, over time additional measures must be taken or the soil pH will begin increasing.

Take time to learn about your soil before planting a garden to improve your chances for success.

For more information, you can call the Colorado State University Extension, San Luis Valley Area Office at 719-852-7381. Please feel free to visit our website at: http://sanluisvalley.colostate.edu for information about services provided. Extension programs are available to all without discrimination, Colorado State University Extension, U.S. Department of Agriculture and Colorado counties cooperating.