March 6, 2016

1 - Organic matter can make clay garden soils much more pleasant for gardening.

Yard and Garden March 6, 2016

Q.

Any tips for gardening in clay soil?

-Charlotte M.

A.

Understanding soils in New Mexico is a very important factor in gardening successfully here. For gardeners who have never taken a class in soils, here is a brief introduction to soil in general. Soil, not dirt, is what we need to grow our vegetables, fruits, and flowers. Soil is composed of 3 mineral components - sand, silt, and clay. Sand is the coarsest component. It gives a gritty feel to soil. It also allows water and air to enter the soil so that plants roots will have access to them. Silt is composed of smaller particles and when rubbed between your fingers has a feel much talcum powder. It holds water and nutrients better than sand because of its smaller size. Clay particles are the smallest and are flat, plate-like mineral components. Because they are flat plates they give clay its sticky characteristic and also makes clay slippery. Clay holds water and nutrients best of all the mineral components of soil.

Clay soil can make gardening difficult because it is hard to work with and difficult to irrigate properly. However, clay provides advantages over easy to garden sandy soil in that it holds more water (once it is moistened) and holds more nutrients for plants.

The ideal garden soil is called "loam", an appropriate mixture of sand, silt, and clay. Such soils are great for gardening, but not common in New Mexico. The best way to make clay soil more manageable and perform like loam in your garden is to add organic matter to the soil. I prefer to use well-made compost, but in the case of clay soil, if there are small pieces of undecomposed bark or other coarse material in the compost, this will help open the soil and facilitate irrigation. The organic matter may also be manure, but if manure is applied in the spring or fall, there is a chance that it will have excess mineral salts that can cause salt burn in the plants and damage roots of new seedlings. Manure may be more successfully applied in the autumn, especially if the winter provides good precipitation to leach away surplus mineral salts that can cause burning.

The organic matter you add will cause the very small clay particles to clump together in manner that improves the characteristics of the soil. Because clay particles are very small, flat mineral plates that resist the entry of water, both rainwater and irrigation water may run off without soaking in properly. The organic matter changes the "structure" of the soil that allows better water infiltration. The change in structure involves clumping together of the small particles to make "crumb-like" structure that has air spaces between the crumbs. This provides channels for entry of water into the soil. The organic matter is also sponge-like and can help hold water and minerals to keep them available to your garden plants.

If you add sand to "loosen" the clay soil, you can end up making adobe and creating greater problems in your garden. It would take a very large amount of sand to alter the properties of the clay.

In some situations, it may make more sense to build raised garden beds and fill these beds with better soil. This is the case in places with high mineral salt content in the soil and poor drainage to allow leaching of these salts. Such locations are in valley locations to which water drains and brings additional mineral salts to the site. As the water at such a location evaporates, the salts accumulate and create problems. By building raised beds you create a situation where you can better control the characteristics of your garden soil and where you can provide the drainage necessary to allow the proper ratio of water and air that plant roots need. You can adjust these factors to meet the needs of different crops which have different water and aeration needs.

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.

Links:

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.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!