June 7, 2014

1 - Organic matter such as compost is the best material to add to clay soils in gardens.

Yard and Garden June7, 2014


I live on a ranch about 15 miles from Fence Lake, atop a sandstone mesa and surrounded by heavy clay soils. My question has to do with modifying the clay soil to make it suitable for vegetable growing.

I am constructing a raised bed 16' x 4' and intend to backfill it with a layer of clean straw, topped with a mixture of purchased potting soil and some native soils as well. As I was looking for possible sites on the ranch that might have sand, I wondered if the material excavated from the large anthills might be useful material.

Do you know what this material is and whether it has horticultural value? Or is it simply more clay?


The key to modifying clay soil to make it a better garden soil is organic matter. Organic matter causes the very small clay particles to bind together into grains or clumps that create channels for air and water. The clay still retains its characteristic to hold nutrients and water. The organic matter also helps hold nutrients to release to plants. The improved structure of the soil caused by the organic matter causing the formation of the grains makes it easier to dig and till the soil. Clay soil modified with organic matter performs more like loam soil, the preferred garden soil.

True loam is a proper mixture of clay, silt, and sand. Sand is the coarsest mineral component and gives the grainy feel to soil. Silt is finer and feels like talcum powder or flour, and clay is composed of even smaller particles. The clay is small, flat plates that, when wet, slide over each other and give clay the slippery characteristics. When dry the clay particles bind together strongly. The small gaps between the clay particles give clay its excellent water and nutrient holding characteristics. Organic matter is not a mineral component of soil, but its effect on clay makes the clay behave like loam. Organic matter can also cause sandy soil to behave like a loam soil, holding more water and nutrients. However, organic matter continues to oxidize over time, turning into carbon dioxide and water. It must be replaced periodically.

As many New Mexico gardeners have experienced the clay soils when dry are more like concrete than soil. Digging and rototilling dry soils can be a major challenge. Even our sandy soils may have enough clay to bind the sand grains together and make the soil difficult to work. Adobe is formed by mixing clay and sand.

In spring and summer the best source of organic matter is compost (potting soil will work for this). In the autumn manure may be used to supply organic matter. The problem with adding manure to the soil during the growing season is high level of salt in manure that can burn plants. Well composted manure from which salts have leached or manure applied in the fall and exposed to winter precipitation will add less salt to the root zone of garden plants.

The material collected from ant hills may be useful or harmful. If the material excavated by the ants is very coarse, like large sand, it can help add porosity to the clay soil. If it is very find sand, it can turn the clay into adobe and may do more harm than good in your garden. You said you live on a sandstone mesa, so the important consideration is the size of the material excavated by the ants. Organic matter, compost, is the best material to add to improve your garden soil. However, I do collect coarse, granular material from harvester ant hills to use in potting soil for plants like cacti that require excellent drainage and good aeration around their roots.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

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