Current Methods of Greenhouse Media Testing and How They Differ
Many growers are experimenting with various ways of performing their own soil tests for pH and soluble salts on-site. Others are sending samples to different labs in search of timely and inexpensive media testing. Often growers end up with a confusing collection of tests done different ways with widely different "numbers". This article is meant to review current ways of testing media on-site or at a commercial lab and how they are differ.
The major difference between the current methods of greenhouse media testing is the way plant-available nutrients and soluble salts "extracted" from the media samples for analysis. Differences between the actual methods of pH, soluble salts, and nutrient analysis and the laboratory equipment used to do the analyses are not as important as how the plant-available nutrients are extracted from the sample.
When most people think of "soil testing", they might imagine some procedure which analyses the solids in a mix for pH, nutrients, and soluble salts. However soil samples themselves are not actually analyzed during a soil test, but rather plant-available nutrients are pulled out or "extracted" from the sample using an "extracting solution".
A massive amount of research has been done over the years to determine what the best chemical extracting solutions are for outdoor field soils used in farming. At this time the situation is much simpler for soilless greenhouse media: plain water has proven to be a very effective extracting solution to determine fertility status. However, one of the main difference between testing methods is how and in what volume the water is used to produce an extract to test for pH, soluble salts, and nutrients.
Current testing methods
There are three commonly used methods of testing soilless media based on the use of water as an extracting solution, they are: saturated media extract, 1:2 dilution method, and leachate pourthru.
Saturated media extract (SME). SME is currently "the" method of testing soilless greenhouse media and it is almost universally done by commercial and university labs, including the UMass Soil and Plant Tissue Testing Lab. In this test a paste is made using soil and water and then the liquid portion (the extract) is separated from the solid portion for pH, soluble salt, and nutrient analysis. Special skills and laboratory equipment are required to perform this test. SME is probably not suitable for a grower to use unless the greenhouse operation is large enough to support a lab, a technically trained person is hired to carry out the tests, and there is a committment to frequent testing and tracking of the results.
1:2 dilution method. This method has been used for many years and has good interpretative data to back it up. In this test an air-dried sample of soil and water are mixed together in the volume ratio of 1 part soil to 2 parts water (e.g., using a measuring cup, 1 fl. oz. of soil + 2 fl. oz. of water). The liquid extract is then separated from the solids using laboratory grade filter paper or a common coffee filter. The extract is then ready for analysis. This is a very easy test to master and quite suitable for on-site greenhouse testing of pH and soluble salt using the so-called pH and EC "pens" available from greenhouse suppliers. In my opinion the 1:2 method is a very good choice for occasional pH and soluble salts testing by growers on-site. In the reference section you can find a web address of a UMass Fact Sheet describing the use of pH and EC pens and the 1:2 dilution method of analysis.
Leachate PourThru. This is the newest way of testing soilless media and its apparent simplicity has caught the interest of many growers. One of the major advantages to leachate pourthru is that there is no media sampling or preparation. Unlike SME and 1:2 methods, plants do not have to be sacrificed or disturbed for testing because the extract is the leachate collected from the container during routine irrigation. The leachate can be analyzed on-site using the pH and EC pens or it can be sent to a commercial laboratory for a complete nutrient analysis. In the reference section there is a Fact Sheet from North Carolina State which gives detailed information on the leachate pourthru method.
Leachate pourthru is best used for continuous monitoring and graphical tracking of pH and soluble salts. To make this method work best an irrigation and leachate protocol must be established and carefully followed when sampling takes place. Based on my observations, leachate pourthru is not a good choice for casual checks (use 1:2 method for this). Some growers like to "whip out" the old pH or EC pen now and then and check the drainage from some pots. Unfortunately, with casual use like this, the "numbers" are often quite variable, inconclusive, and probably unreliable. The NC State Fact Sheet tells exactly how to correctly use the leachate pourthru method.
Comparing the numbers
The amount of water used to extract plant-available nutrients and other details of the tests outlined here can give large differences in the results of the tests. This is illustrated by a comparison of soluble salts values made by North Carolina State researchers (Table 1). Note that the 1:2 method results in the lowest levels. Results of SME and leachate pourthru are closer, but some differences exist. Always use the interpretative data that match the test you made otherwise you could make an incorrect interpretation of the results. For example, a result of 2.6 would be "extreme" (too high) for the 1:2 method, "normal" for SME, and "low" for leachate pourthru. What a difference in how you would think about your results! Interestingly the differences in pH results between the tests is much smaller.
Optimizing Soil Test Results
If used correctly, the three methods of soil testing outlined here give valuable and useful results for greenhouse crops. To optimize the value of soil tests, care in taking and describing the samples is very important.
Sampling. The goal of sampling for a soil test is to efficiently collect samples which best represent the nutrient status of the crop or the problem to be diagnosed. The first step is to identify the crop unit(s) to be sampled - bench, greenhouse, etc. In a mixed greenhouse, crops of different species must be sampled separately for the tests to have any value. If a problem is being diagnosed, it is best to have a sample from both normal and abnormal plants for comparison.
After selecting and recording the crop unit, 10 pots or flats should be chosen at random from each unit to sample. The "subsamples" from the 10 are then mixed into one sample analysis. Sampling in this fashion is important because a sample from one pot or flat could be an anomaly (values too high or too low) which does not represent the crop as a whole. Sampling and analyzing soil separately from 10 different pots would be the best way but also the most expensive way!
For the 1:2 and SME tests the actual soil sample is taken by either a core or composite sample from all depths in the pot or from the root zone only (i.e., portion where roots are most active). Never sample from just the surface 1-2" of the pot - nutrient and soluble salts levels will be always be much higher here than in the root zone and composite samples and, as a result, will overestimate fertility.
Sample about 2 hours after fertilizing or at least on the same day. If slow-release fertilizer pellets are present try to pick them out. If the pellets are left in and they break during testing this may result in an overestimation of fertility.
Finally, be consistent in all sampling procedures each time you sample. A lot of variability can be introduced to tests due to inconsistent sampling and this diminishes the value of testing especially if you are trying to track fertility.
Sample description. Be sure to collect and record a practical amount of information describing the sample. This should be sent along with the sample to the lab (keep a copy for yourself!) and is particularly important if you expect useful, specific recommendations from someone associated with the lab who has not seen your plants or may not have talked to you.
You should answer the following questions:
- What is the crop?
- What is the age or stage of development?
- What is the growth medium (soil or soilless, commercial brand)?
- What is the fertilizer program (i.e., specific fertilizer, rate [ppm], frequency of application)?
- Is there slow-release fertilizer mixed in?
- Is there a plant problem and if so what is it?
- Having answers to these questions provides you with a framework for interpretation of the test results later on regardless of how the soil sample was analyzed.
- Cox, D.A. How to use pH and EC pens to monitor greenhouse crop nutrition. UMass Floriculture Fact Sheet.
- North Carolina State fact sheets on the leachate pourthru method.
- Greenhouse Soil Testing and University of Massachusetts Soil Test Laboratory
Plant and Soil Sciences
University of Massachusetts
Amherst, MA 01003