May 7, 2016

1 - Container gardening provides some benefits for tomato growing compared gardening in New Mexico garden soil, but many of the same principles apply.

Yard and Garden May 7, 2016


What are your views on soil and fertilizer for growing our tomatoes in soil and in 15 gallon pots?

- Jim C.



I grow tomatoes (and other vegetables) under both open garden and in large containers conditions. Your question is interesting because the soil situation can differ between container gardens and open garden soil situations.

When gardening in large containers you have a lot of flexibility regarding location and time of planting (if you are able to move the plants into protected locations when our unpredictable springs bring late frosts to threaten our plants). Mobility of container grown tomatoes is a benefit, but a really great benefit is the ability to "engineer" the soil for the tomatoes, or any other plant begin grown. Tomato plants prefer an acid soil (pH 5.5 to 6.0) which is usually difficult to provide in the open New Mexico garden. In containers, the soil can be built from some native mineral soil to which you have added organic matter and sulfur to create the acidity that tomatoes prefer. Test strips for soil pH determination may be purchased in scientific supply stores (such as those where students buy supplies for their science fair projects). The use of native soil provides minerals that the tomatoes will need, especially calcium necessary to avoid blossom end rot. Once the soil is developed, a soil test would be a good idea to determine if other nutrients are needed and what levels of nutrients exist in the soil you have developed. Nitrogen, phosphorus, and potassium as well as micronutrients are needed in the proper quantities for optimum plant growth and crop yield. The soil test will identify any imbalance of these nutrients.

Over time, with the addition of water, minerals dissolved in the water will accumulate in the container soil, but during the course of a growing season this should not be a major consideration, especially if you have started with a proper balance of nutrients. The soil may be re-engineered each year or every few years. It would be a good idea to test the soil annually and to change the crop grown in a container to avoid the build-up of disease organisms in the soil. By rotating to crops that are not susceptible to the same diseases, you can avoid the disease accumulation problem, just as you can do with crop rotation in the garden.

If you have a large percentage of organic matter (compost, or purchased "garden soil") in the container, be careful which fertilizers you use. Some sources of phosphate (super phosphate and triple super phosphate) may advise you not to use them in container gardens with high levels of organic soil.

Many of the same considerations are relevant in the open, native, garden soil. Soil testing and addition of compost to the garden soil will increase your chances of a bountiful crop. You will not as easily alter the pH and you cannot maintain a modified pH as easily in the garden. Water will provide dissolved minerals to create a more alkaline soil in the garden, but minerals in the surrounding soil will also migrate into your modified soil. This will make pH management in the garden somewhat more challenging, but not impossible. The change of soil pH and migration of minerals from surrounding soil may more quickly alter the solubility of plant nutrient minerals, so you may find that your yield may not be as bountiful or you may find blossom end rot and other physiological disorders are more likely in the garden.

Plants in the garden cannot be moved, so planting dates must be later and chosen with knowledge of the likelihood of late frosts. You may need to employ crop covers and other frost protection methods. Crop rotation is still a valuable practice in the garden just as it is in the container garden.

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:, office: 505-865-7340, ext. 113.


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