Yard and Garden: Preparing Garden Soil for Spring Planting

AMES, Iowa – Although snow is on the ground in much of Iowa, spring is upon us. It’s time to think about planting gardens, but before that happens, proper care must be taken to ensure the soil is ready for growth. That means fertilizing soil, testing it and, perhaps, applying materials like lime.

Iowa State University Extension and Outreach horticulturists can help answer questions about proper spring garden preparations. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

What type of fertilizer should I apply to my garden?

To determine the garden’s fertility needs, submit a soil sample to a soil testing laboratory in fall or early spring. Apply and incorporate the recommended type and amount of fertilizer into the top 6 to 8 inches of soil prior to planting. If the fertility level of the soil is unknown, apply and incorporate one pound of 10-10-10 or a similar analysis fertilizer per 100 square feet. 

How can I have my garden soil tested?

Soil testing is done by private and state laboratories. In Iowa, gardeners can submit soil samples to the ISU Soil and Plant Analysis Laboratory. A soil sample information sheet and instructions for taking soil samples can be found at http://soiltesting.agron.iastate.edu/Forms.html.

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 plant 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 lawns and gardens in Iowa 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 aeration 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 aerate lawns in Iowa.

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