Yard and Garden: Soil pH and Testing

AMES, Iowa -- Early spring is a great time for amending the garden soil, and getting it prepared for the upcoming growing season. Horticulturists with Iowa State University Extension and Outreach talk about changes to soil pH when adding ashes and lime to gardens, soil core aerification and soil testing. To have additional plant and garden questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

Can I dispose of my wood ashes in the garden? 

Wood ashes contain small amounts of several plant nutrients. The nutrient content of wood ashes depends on the type of wood burned, the thoroughness of its burning and other factors. Generally, wood ashes contain 5 to 7 percent potash, 1 percent phosphate and small amounts of other elements. However, the largest component of wood ashes is calcium carbonate. Calcium carbonate is a liming material. Liming materials raise the soil pH.  

The soil pH is important because it affects the availability of essential nutrients. The pH scale runs from 0 to 14. Any pH below 7.0 is acidic and any pH above 7.0 is alkaline. A pH of 7.0 is neutral. Most vegetables, fruits, annuals and perennials grow best in slightly acidic soils with a pH between 6.0 and 7.0. Plants may not perform as well in soils with a pH of 7.5 or above because of the reduced availability of some essential nutrients.  

Avoid applying wood ashes to garden areas with a pH above 7.0.  Applying wood ashes to alkaline soils may raise the soil pH to excessively high levels and reduce the availability of some plant nutrients. If the soil pH in your garden is unknown, conduct a soil test to determine the pH of your soil. 

Should I apply lime to my garden?  

In Iowa, gardeners should apply lime to gardens and lawns only when recommended by a soil test. A soil test will indicate the current soil pH and, if necessary, the amount of lime to apply to the area.  Liming materials include ground limestone which is mainly calcium carbonate (CaCO3) and dolomitic limestone which contains calcium carbonate and magnesium carbonate (MgCO3).  

Lime is applied to acidic soils to raise the soil pH. The soil pH is important because it affects the availability of essential nutrients. The pH scale runs from 0 to 14. Any pH below 7.0 is acidic and any pH above 7.0 is alkaline. A pH of 7.0 indicates a neutral soil. The optimum pH range for most flowers, vegetables and other horticultural crops is between 6.0 and 7.0. Lime is applied to acidic soils with a pH below 6.0 to raise the pH into the optimum range. However, an application of lime to an alkaline soil can raise the soil pH to excessively high levels, reducing the availability of plant nutrients and leading to poor plant growth. 

Will an application of gypsum improve a clay soil? 

Advertisements for gypsum sometimes claim that gypsum will help loosen heavy, clay soils and improve soil drainage. However, the addition of gypsum to Iowa soils is of little benefit. Gypsum is chiefly used to amend sodic soils. Sodic soils are found mainly in arid regions of the western United States.  

Core aerification is the best way to improve growing conditions for lawns established on clay soils. The core aerator should remove soil cores that are approximately three-fourths of an inch in diameter and 3 inches long. There should be 20 to 40 holes per square foot. April and September are the best times to core aerate lawns in Iowa.  

Vegetable and flower gardens can be improved by applying and incorporating organic matter, such as compost, well-rotted manure or peat, into the soil. Work the organic matter into the top 8 to 12 inches of soil. 

How do I have my garden soil tested? 

Soil testing is done by private and state laboratories. Instructions for submitting soil samples to the Iowa State University Soil and Plant Analysis Laboratory can be found at http://www.agron.iastate.edu/soiltesting/


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