Issue: April 3, 2004

Soil test samples

I would like to test the soil in my garden. How many samples should I take and to whom should I submit them for results? By the way, I garden organically. Will results provide organic suggestions?

Dr. Robert Flynn, NMSU Extension Agronomy Specialist, says that the sample should be "representative"; that is, it should properly represent the soil situation in your garden. If plants in one area of the garden do not perform as well as plants in the rest of the garden, that area should be submitted as a separate sample. Collect samples from a tilled garden to a depth of 12 inches.

He also points out that each sample that is submitted should be a "composite sample" made by collecting several sub-samples from different parts of the sampling area. These sub-samples should be combined and thoroughly mixed before submitting the sample for analysis. He advises that 6 to 12 sub-samples should be sufficient for typical home gardens. Larger gardens will require that you collect more sub-samples to create the composite sample.

The final sample should contain at least two cups of soil. There should be no rocks, mulch, chunks of compost or manure, plant debris, or other contaminants. Of course, manure and compost well mixed into the soil last year is okay.

Dr. Flynn suggests air-drying the soil sample in a shady location before sending. That will reduce the weight and mailing cost and also assure that the package isn't weakened by moisture from the soil. He stated very firmly that the soil sample should not be allowed to sit on the dashboard of the car or any other hot, sunny location.

Once the samples are collected, mixed and dried, they may be placed into plastic or paper bags and mailed to the soil testing laboratory of your choice. NMSU provides soil testing service at the Soil and Water Testing Laboratory. You can contact your local county Extension Service agent for this address or names of other laboratories.

When you submit your sample to the soil testing laboratory, state that your preference is for recommendations for an organic garden. Your county agent can advise you if a specific lab does not give organic recommendations, but most labs will provide that information.

By the way, congratulations for making a wise decision to have your garden soil tested. Without testing, you are only guessing when you apply any nutrient source to your garden. In the challenging gardening environment of New Mexico, it is wise to begin with as much knowledge as possible. Happy gardening!

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 and the NMSU Horticulture Publications page.

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

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