Special test is needed to measure greenhouse soil health.
Traditional soil testing relies on the theoretical basis that natural soil systems typically provide plant available nutrients via the soil solution. These nutrients are then replenished via the reserve nutrient pool of exchangeable cations held on the surfaces of the clay mineralogy and organic matter. Therefore, all traditional field tests of agricultural soils are based on that soil’s ability (via Cation Exchange Capacity) to hold onto and supply nutrients over one or more seasons. This is known as the nutrient quantity or capacity factor.
Nutrient Quantity | Capacity Factor
To determine these factors, the soil sample is typically dried and then mixed with an acid solution to extract the reserve nutrients for analysis and quantification. These acid solutions are often region, soil type, or pH based with specific names, such as Morgan, Mehlich, or Bray. While these tests have been calibrated to work well for long term nutrient management and yield expectations, these field soil tests do not give a measure of the short/medium term nutrient availability. Nor are they well calibrated to work with specialty or soil-less growing media typically encountered in bench top greenhouse production systems or compost based production.
Saturated Media Extract
The Saturated Media Extract (SME) test (also known as Greenhouse Media or Saturated Paste) was developed at Michigan State University to analyze moist samples of greenhouse media (potting soils) or compost in order to determine the nutrient “intensity” factor of that media. The SME test determines what nutrients are available to the plant from that media in the short to medium term. This is especially important in containerized production where the plant has access to a limited amount of media but are given nutrients via fertigation or slow release fertilizers. It may also be a better test for production systems primarily growing in compost or in long term covered protected culture systems.
Rather than using an acid extraction process, the SME extraction uses distilled or deionized water to extract nutrients from the media being tested. This approximates what the plant roots have access to in the short to medium term, rather than the full complement of nutrients that might be released from the sample over a longer period via the acid extraction. Because the extraction methods vary significantly, there is a separate set of recommendations for nutrient application based on SME test results.