pH and Soluble Salts Monitoring of Greenhouse Substrates



Dr. J. Raymond Kessler, Jr.

Auburn University



What if your car had no gas indicator gauge? How could you depart on an extended trip and know you had enough gasoline? What if your bank never mailed you an account statement? How would you know you had enough money to pay the bills? Certainly in each case, life would be a greater challenge and involve more risk. However, many growers fertilize greenhouse crops without knowing the fertility status of the growing substrate. This also involves risk because a failed crop not only means loss of investment and potential profit but threatens customer relationships. Fortunately, recommended fertilization regimes from academic and industry sources work well in many circumstances if followed carefully. However, plants constrained to a limited soil volume in a container are subject to a variety of cultural and environmental variations that can lead to fertility problems even when a recommended regime is followed. Changes in greenhouse facilities, substrate components, and grower style can make "the way we've always done it" less effective than before. Like the gasoline gauge in a car, what is needed is a means of monitoring substrate fertility so the grower can know if "the tank is full or empty".

Periodically collecting substrate samples from a greenhouse crop and sending them to a laboratory for analysis has been recommended for years. This method provides the highest degree of reliability and the greatest amount of detail on the nutritional status of the crop, especially when combined and interpreted with tissue analysis. However, growers often argue that this method requires too much time (labor), costs too much, or is too slow for short-term crops. Too often, soil testing is used as a last resort when nutritional problems become visible and crop quality has already decreased rather than as a tool to detect and correct problems before quality suffers. While these arguments may have some validity under certain circumstances, the grower must weigh the expenditures involved against the potential for crop loss.

One viable supplement to laboratory soil testing that can be more expedient is "in-house" testing of substrate pH and soluble salts. This has the following advantages:

1) Management has control over when and how the tests are performed.

2) Results can be available in less than an hour rather than days.

3) Investment in equipment and costs per sample are relatively small.

4) Tests can be performed frequently so corrective action can be taken before crop quality suffers.

5) Results can be graphed over time so that trends become evident.

Keep in mind that testing soluble salts and pH does not provide information on the status of individual nutrients and should be combined with at least monthly laboratory testing.

Why pH

One major property of substrates that can have a major impact on the root's ability to acquire and utilize nutrients is the pH. Substrate pH is a measure of the hydrogen ion concentration of the soil solution, and is measured on a logarithmic scale ranging from 0 to 14 with 7 being neutral. The importance of soil pH to roots is its effect on nutrient uptake and the availability of nutrients in the soil solution. Most greenhouse crops grow within a pH range of 5.4-6.8. However, plants vary in their pH optimum for growth and each species operates well within a specific pH range, e.g. pansy, pH 5.4-5.8. Too low a pH results in increased micronutrient availability that can lead to phytotoxic responses and decreased availability of calcium and magnesium. Conversely, high pH can lead to micronutrient deficiencies. Rapid changes in pH can affect both the availability and solubility of individual nutrients in the soil solution. Recommendations are available providing optimum pH ranges for most major greenhouse crops.

Why Soluble Salts

Greenhouse fertilizers are composed of a combination of fertilizer salts. Substrate soluble salts consist of all the organic and inorganic components in the soil solution that will conduct electricity. These include the salts applied during fertilization, slow-release fertilizer, salts from poor quality water, residual fertilizer, dissolved salts from the media components, and compounds resulting from microbial decomposition of organic matter. Some fungicidal drenches also contribute soluble salts. Soluble salts is a fairly reliable measure of the nutritional status of a crops as long as the major source of salts is from the fertility program. Soluble salts is determined by measuring the ability of a sample solution from the substrate to conduct electricity. Crops vary in their tolerance to soluble salts. Seedling and rooted cutting are the most sensitive. Some crops such as African Violets and Azalea are particularly sensitive and should not be grown at high soluble salts levels. Conversely, poinsettia and chrysanthemum require high fertility and can tolerate high soluble salts.. Recommendations are available providing optimum soluble salts ranges for most major greenhouse crops.

Monitoring

Every greenhouse should purchase pH and soluble salts testing equipment. However, many growers only check soluble salts and pH when a problem is perceived. Scheduled sampling and measuring allows the grower to track trends in these factors and make corrections before problems arise. Ten plants from a given crop can be sampled weekly and the results for soluble salts and pH averaged. These results are marked on a chart over time that indicates the optimum and acceptable ranges for these two parameters. Values outside the optimum range should be a concern, values outside the acceptable range should require the grower to take corrective action.

Tracking soluble salts and pH is probably the simplest and least expensive way to monitor crop fertility. However, generating this iinformation requires a commitment by management to purchase the equipment and allocate the labor to get the task accomplished each week. This resource allocation should by considered against the consequences of crop damage or loss.