Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityOctober, 1995
A 'Soil' Testing Dilemma
Keywords: soil/media testing, alternative substrates, nutrition, container media, nursery
M ost nursery and greenhouse container growing media that are used today are bark or peat-based, and are free of mineral soil. Hence these are called 'soilless' media. Horticultural researchers that work with soils or media are purists when it comes to such terminology. They might react in horror if someone were to call a soilless growing medium 'soil'! Some no longer even like the term 'media' and now prefer 'substrate'. That's going a bit too far for me, even if I am a University type! Growers are understandably less strict when using media or soil terminology. Actually, for good reasons which you will read below, 'greenhouse soil' is a term that should be avoided, since most greenhouse crops are no longer grown in soil. However 'greenhouse soil' might be used in rare cases when a mix contained a significant proportion of mineral soil.
I have noted that this confusion over whether it's 'soil' or 'media' may cause problems when nursery growers send in soilless container media to the OSU Research Extension Analytical Laboratory (REAL for short). Mineral soils and soilless media should be tested by different procedures. The problem usually arises when growers send in soilless media samples using the 'Horticulture Soil Test' form (REAL yellow form). Any mix submitted with this yellow form will be tested as mineral soil. Apparently there used to be an option on this form for soilless mixes, but no longer. One option at the bottom of the yellow form indicates 'greenhouse or Turf,' but this option should be selected only if the mix contains a large fraction of soil, 20% or more. Probably the only situation that still exists where mixes like this are used is greenhouse ground bed production of roses.
The bottom line is, if you are sending in container soilless media samples, select the soilless media test (REAL pink form). The pink forms say "Floral crop growing medium analysis," which probably has misled nursery growers; nonetheless, this is the correct form to use for nearly all of our container growing media in Ohio. I've been told that they are going to change the title of the pink form to 'Soilless Medium Analysis' some day.
Why different tests for different media? Whether you use the REAL lab or not, it is important that you select the appropriate test for your samples. On first glance at the two tests, we see that there are differences in what is reported, as well as differences in the units of measurement. For example, mineral soil tests usually provide pH, cation exchange capacity (CEC), phosphorus, potassium, calcium, and magnesium. Lime recommendations also are provided, and unless you specify your crop, the REAL lab lime recommendation will suggest lime addition to reach a pH of 6.5. The units of measurement provided by the mineral soil test, e.g., pounds per acre, make sense in the field, but not in container production.
The cation exchange capacity (CEC) is a critical parameter of a mineral soil. The CEC is an indication of the ability of a given soil to store and provide positively charged fertilizer ions (positively charged ion = cation, e.g., calcium, magnesium, potassium). Negatively charged ions such as phosphate and nitrate are not held by the CEC. This can create problems with leaching of nitrates, although phosphates are held in mineral soil by other means. The mineral soil test procedure determines the quantity of calcium, magnesium, and potassium ions that are held on the cation exchange sites by displacing them from these exchange sites, then measuring them.
The concept of CEC is less critical in a soilless medium and is not provided by standard soilless media tests. While soilless media do have nutrient exchange sites, on a weight basis, the CEC is much lower than soil. In contrast to a mineral soil, where we are interested the quantity of nutrients that are both in solution and stored on the CEC, in container media we ignore storage capacity and simply want to know the quantity of nutrients in solution, readily available to the plant. The relative lack of nutrient storage capacity in soilless media is the reason that we must continually supply plants with nutrients (usually in the form of slow-release fertilizers), whereas in the field we may apply fertilizer once or twice a year.
Soilless media tests usually provide pH, soluble salts, and the concentration (PPM) of nitrate-nitrogen, phosphorus, potassium, calcium, and magnesium in solution. In contrast to mineral soil tests in which the analyses are expressed on a weight basis, soilless media are not weighed because the great variety of components that are used in container mixes vary in their bulk density. The most common soilless test, and the one used by the REAL lab, is the saturated media extract procedure (SME). In the SME test, the sample is saturated with distilled water to a pasty consistency. Some soil labs use other dilution methods, such as 2:1 or 5:1 dilution. In these methods, one volume of the sample is moistened with either 2 or 5 volumes of distilled water. Regardless of the dilution method, after the soilless cocktail sits for a while the pH is tested, then the solution is extracted and tested for soluble salts and nutrient concentrations.
Currently the SME is favored over the 2:1 and 5:1 dilution methods. The assumption is that the SME reveals something close to the nutrient environment that roots in the growing medium 'experience' in a fully moist medium. However there is nothing wrong with using the other dilutions; furthermore these are easier for growers to do themselves since solution may be extracted easier from the 2:1 or 5:1 methods. The important thing for a grower to know is that extraction procedures may vary from lab to lab, and a different set of guidelines must be used to interpret test results if this is the case.
The take-home message from this article is to select the appropriate testing procedure for your growing medium or soil. Secondly, be aware that different labs may use different procedures or report results in different units of measurement. Thus it may be difficult to compare results from lab to lab. When sending in samples, include as much information about your crop, stage of growth, fertilization program, etc., because this will assist the specialist in making recommendations. Finally, be aware that all soilless media procedures require a much larger sample volume than mineral soil tests. The REAL lab suggests for soil tests about one cupful (half-pint) of soil. For soilless medium, provide at least three times as much volume of sample. Further, whenever additional tests are requested that are not included as part of the standard package, additional sample may be required. The best thing to do in this case is to call the lab and find out how much sample is needed.
Interpretation of test results from mineral soil tests and soilless media tests will be the topic of future articles. Stay tuned!
