Horticulture: June 2005

Market gardeners in the southern Great Plains continue to discover the benefits of growing in hoop houses. Some of these benefits include season extension, storm protection, yield and product quality enhancement and reduction in foliar disease.

Salt injury on a young strawberry plant.

Producing crops in hoop houses is not, however, without problems. At the Noble Foundation, the most persistent problem we have in our hoop houses is the accumulation of salt in the raised beds. Salt's presence is indicated by the white crust that forms on the soil when it is allowed to dry. Salt tends to accumulate with regular application of fertilizer and as a result of mineralization of organic matter. Normally, rainfall would leach excess salt from the beds; however, because we do not remove the poly-film from our permanent houses, the soil becomes saltier over time.

Excess salts in the root zone hinder plants from withdrawing water from the soil. This lowers the amount of water available to plants, regardless of the amount of water in the root zone. Although the water is not held tighter to the soil, the presence of salt in the soil solution causes plants to exert more energy extracting water from the soil. More energy spent extracting less water causes stress, resulting in reduced growth and yield.

The conversion process of soil organic matter to plant available nutrients (mineral salts) is referred to as mineralization. Given adequate soil oxygen and moisture, the rate of mineralization is increased as soil temperature increases. While mineralization is normally beneficial, salts can quickly accumulate to harmful levels during solarization (the practice of using solar energy to pasteurize the soil to control soil-borne disease organisms).

To test for soluble salts, take a composite sample of several cores, 6 to 8 inches deep. Submit a minimum of one pint of the mixture to a soil testing lab. Soluble salts are measured by taking a small amount of the sample, adding enough pure water to the sample to completely saturate it and extracting water from the saturated soil. The amount of soluble salts present is measured by determining the electrical conductivity of the extract. The soluble salt content is proportional to the electrical conductivity of the solution. See Table1 for interpreting test results and tolerance ratings for selected crops.

To manage soil salinity in the hoop house, consider the following:

1. Don't over-fertilize. Follow crop-specific fertilizer schedules to ensure crops receive the needed amount of fertilizer and no more.

2. Use only good-quality irrigation water low in salts. Most soil testing labs perform irrigation water quality analyses.

3. Maintain adequate soil moisture content. As water is lost from the soil by evapotransporation, soil water salinity increases because salts become more concentrated in the remaining water. Use a moisture meter such as an irrometer to monitor soil moisture and schedule irrigations as needed.

4. Use compost derived from plant materials instead of animal manures. Plant-based composts generally contain fewer salts compared to manure-based composts. Don't over apply.

5. Flush salts from bed using sprinkler irrigation. While it is possible to leach using drip irrigation, it is not as uniform as sprinkler irrigation. Sprinklers apply water to the entire surface of the bed, enabling uniform leaching. Drip irrigation tends to concentrate salts along the edges of the beds. The amount of water to apply will depend on the initial salt level, desired salt level and the desired depth to leach. Eight to 10 inches of water may be required to remove 70 percent of the total salts for each 12 inches of soil to be leached. The only sure way of determining if leaching has been successful is to test.

6. Remove the poly film covering from the house. This should be your last option as the house is out of commission for as long as it takes precipitation to adequately leach the beds. 7. Soil test for salts on a regular basis. If leaching is required, be sure and test again following leaching. Also remember to test soil following solarization.