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Section 9: Substrate and Tissue Tests for Managing Mineral Nutrition

Substrate testing and tissue testing offer greenhouse managers a way to monitor the mineral nutrient status of the substrate and the crop. Many greenhouse managers maintain pH and E.C. meters so that they can frequently monitor these two variables on-site. Often, especially for longer-term crops or to diagnose a problem, a complete (i.e. pH, E.C. and all mineral nutrients) substrate test needs to be conducted. For a complete test, greenhouse managers typically send substrate samples to a commercial laboratory although sometimes the greenhouse manager may extract liquid solution from the substrate and send the solution to a lab for testing.

When conducting an on-site pH and E.C. test, the greenhouse manager must first extract the liquid phase of the substrate. It is actually the liquid in the substrate that is generally tested (not the solid particles). There are several ways to extract and collect the liquid for testing. These methods include the saturated media extract (SME), the 1:2, the PourThru, and the Press Extraction Method.

Saturated Medium Extract (SME)
The Saturated Medium Extract (SME) technique is based on the procedure typically used to analyze field soils. The SME uses ½ to one cup of substrate that is saturated with distilled water to the point that the surface glistens (making a paste out of the substrate). Using distilled or deionized water is important in avoiding incidental contamination with minerals in the water.  This results in a solution that is about 1 part substrate to 1 part water.  After equilibrating (allowing sample to sit and incubate at room temperature) for 30-60 minutes, the SME can be measured for pH and EC.  Commercial laboratories generally filter the slurry, or extract the liquid from the substrate using a vacuum, in some way prior to testing to protect the equipment from debris in the slurry. One of the negative aspects of using the SME method is that the term “glistening” is subjective and therefore the amount of water added may vary among people conducting the test as well as for a given individual across times.

PourThru
The PourThru method (also known as the Virginia Tech Extraction Method) is a non-destructive method that uses distilled water poured over the surface of the substrate to push (displace) the substrate solution out of the production container.  It is often collected in trays placed under an individual container.  The extract is then immediately ready to be analyzed for pH and EC or to be sent to a commercial laboratory for a complete analysis.

Considerations for the PourThru method include ensuring that there is sufficient substrate solution to be extracted.  To do so, the PourThru is conducted not more than 1 hour after irrigation. Pouring excessive distilled or deionized water over the potting mix will blend with and dilute the potting substrate solution While the volume needed varies depending on substrate components and several other factors, the table  below provides some guidelines for appropriate water volumes for a given container size.

Because the PourThru extract is undiluted, E.C. and specific nutrient values are typically about 1.5 times higher than values obtained with the SME. The table below provides conversion factors for E.C. and values from three extraction methods. 

1:2 Dilutions
The 1:2 dilution is similar to the SME with the exception that rather than adding distilled water to the point of glistening, 2 parts of distilled or deionized water is added to 1 part substrate. This results in a more diluted solution versus the SME and PourThru. The highly diluted 1:2 sample can make micronutrient analysis difficult for commercial labs (micronutrients are often below detection levels). However  the 1:2 method  is a good tool for greenhouse managers to us in-house to monitor pH and E.C.

After the solution is extracted, it may be tested on site for pH and E.C. or sent to a commercial lab. However, usually if a complete test is being conducted, the greenhouse manager sends the actual substrate sample to the lab and the lab technicians use one of the above methods (SME is most common) to collect an extract.

It is important to understand how to use information from substrate tests to adjust the fertilization program. Acceptable ranges for substrate tests using a saturated media extract (SME) are listed in the table below. Properly monitoring the fertility status of a crop can allow for corrections of potential problems before there is significant damage or crop loss and when corrections are more easily made.

Press Extraction Method
The Press Extraction (PE) method is used for plugs or other small substrate volume containers.  Other substrate extraction methods are not well suited for plug and seedling trays because it is hard to collect sufficient leachate or a lot of samples must be taken. Similar to PourThru recommendations, about one hour after a plug tray is fertigated, while the substrate is fairly wet, “press” several individual cells for a single sample.  Collect the pressed solution for measurement.  Results correlate well to SME standards, but pH readings from PE are typically 0.1 to 0.4 units lower and E.C. readings from PE are 0.1 to 0.6 mS/cm higher than for an SME test.

Sometimes a plant tissue test needs to be conducted. Tissues tests are most commonly conducted to diagnose problems that do not readily appear in the substrate test or to confirm results from substrate tests. This type of test is conducted on dried and ashed plant samples (typically leaves) and the actual mineral nutrient levels in the plant tissue are determined. Recommended tissue levels of mineral nutrients for some common greenhouse crops are listed below. These are levels for dried and ashed recently expanded leaves.

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