One important item of the greenhouse production schedule is regular soil testing for both bench crops and potted plants. The grower should not wait until plants display symptoms of difficulty before soil testing. Tests should be run monthly on benches with crops. The frequency of testing needed for potted plants will depend on how long the particular crop takes to reach harvest size. For example, potted azaleas require more production time than most herbaceous plants. They also require an acidic pH. Several soil tests are needed during their production to ensure optimal growth.
Growers need soil tests for three main reasons:
1. To check the pH.
2. To check for nutrient deficiencies.
3. To measure the soluble salt content.
Soil acidity is important in the production of certain crops, such as the azaleas noted above, and needs to be measured whenever field soil is used in the growing medium or in regions where irrigation water may alter the pH. Testing for the level of nitrates, phosphates, and potash in the growing media is usually sufficient unless another deficiency or
excess is suspected. Any of several commercially available soil test kits can provide the grower with same-day results. The do-it-yourself kits are affordable, easy to use, and generally accurate enough to be of use to the grower. More accurate tests can be performed by the state’s college of agriculture or commercial labs, but there is a time lag of several days or longer.
Soil samples need to be selected from several parts of the bench or from random pots of a crop, then mixed, dried, and tested (Figure 20-2). The most reliable results will be obtained if
1. the same person does the testing each time.
2. all glassware is kept clean and rinsed each time with distilled water.
3. the reagents are fresh and kept in a cool, dark storage space when not in use. Regularly scheduled reordering of the reagents, rather than waiting until the old ones are gone, will ensure their freshness.
Knowledge of the soil’s soluble salt levels is considered by many growers and educators to be even more important than pH information for good greenhouse production. The soluble salts of a soil can be increased by steam pasteurization, excessive fertilization, and failure of the irrigation water to drain sufficiently to leach the fertilizers from the soil. Soluble salt levels can be measured easily using a conductivity/TDS meter (Figure 20-3).
figure 20-3. A conductivity/TDS meter is used to test the total soluble salt levels of solutions and media. (Delmar/Cengage Learning. Photo by Jack Ingels.)
figure 20-2. Testing the soil of a greenhouse crop (Delmar/Cengage Learning. Photo by Jack Ingels.)
Most college courses in soil science or greenhouse production include training in soil testing. All the techniques required for greenhouse production are easily learned. Obtaining a truly representative soil sample and maintaining consistency in the testing are the most difficult aspects.