Media, Tissue and Water Testing for Good Growing

November 1, 2003 2:38 pm

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When do most growers test media, tissue or water in the greenhouse? If your answer is “only when there is a problem”, you are probably right. However, that is not the only time that growers should be testing. Growers should be testing at least for media pH and EC on a weekly or bi-monthly basis (on-site testing) and should also be conducting complete tests 2 or more times per crop for major crops (complete lab analysis).

Tests should not be limited to media pH and EC, but also to water alkalinity, EC and pH. This is especially important if growers are having problems maintaining pH in their mixes. Knowing the alkalinity of the water and fertilizing according to the alkalinity goes a long way in helping assure a successful, high quality crop. This is a separate topic in and of itself.

Growers should be testing fresh, unused mix as well as mix in production. Knowing the pH and EC of the mix prior to use in production can help with decisions on fertilization and watering practices that may be needed. At least 80% of nutritional problems that are encountered by growers can be traced back to improper media pH.

The biggest investment for continual on-site testing is not the equipment. Good quality pH and EC meters can be purchased for a few hundred dollars. Alkalinity test kits cost about $35. Containers for sampling and sample preparation can be purchased at a relatively low cost. Plastic cups can be used for sampling and larger cups or bowls can be used for sample preparation. Physical properties can be tested using pots and a quality scale. The biggest investment for continual on-site testing is going to be time. Taking the proper samples and testing them takes a lot of time. Evaluating the results and making recommendations on changes based on these results will also take time. A moderate to large operation can easily need to have one person dedicated to doing only this. There are many growers who have more than one person dedicated to this.

Procedure for Water and Fertilizer Solutions

Water for testing should be collected at the point of use. In other words, collect water from the end of the hose. It should be allowed to run for a few minutes before collection. The same is true for fertilizer solutions. Once the sample has been collected, alkalinity, pH and EC of the water can be tested using alkalinity test kits and pH and EC meters.

The most important information from the water test will be the alkalinity value and the EC. If this is the first time the water is being tested, a sample should also be sent to a commercial or state lab in order to obtain a complete analysis. Water test results are then used to determine the proper fertilizer to use and also whether or not acidification is necessary. The crop being grown must also be taken into consideration when determining fertilizer use and acidification. This will be discussed in future articles.

The EC of a fertilizer solution can be used to determine if the proper fertilizer concentration is being applied to the crop. It also can help determine if the injector is working properly. The EC of the clear water must be subtracted from that of the fertilizer solution to determine the EC that is due to the fertilizer. This can then be compared to a table associated with the specific fertilizer used in order to determine the actual fertilizer concentration that is being applied.

Sampling Procedure for Media

What is the best way to conduct in-house tests? The two most common media testing methods are the Saturated Media Extract (SME), which is the common one used in commercial labs, and a 2:1 extract (2 parts distilled water to 1 part growing media). In general, the 2:1 is a little easier to teach someone to conduct and is a little faster to do. The SME is the one of choice for those growers who want to have a more direct comparison to tests conducted at commercial labs. The SME may be a little more affected by operator error due to interpretation of when the media

is completed saturated. Media pH will not differ much between SME and 2:1 extracts. However, EC of the mix tested using SME will generally be about 2 times that of media tested using the 2:1 extract.

Samples should be taken from a number of pots and combined. Media should not be taken from the top of the pots, but rather the middle area. Media from several pots should be combined to make one sample. The combined samples should all be the same media and plant variety and should come from the same general location in the greenhouse.

Combine media and distilled water, stir and let stand for at least 20 minutes. It is important that all samples stand for the same amount of time. This time interval should be used any time samples are tested. After the allotted time, minutes, samples can be filtered (especially if SME) or pH and EC probes can be directly inserted into the slurry for testing. Media pH and EC should be recorded and tracked over time.

These results need to be correlated to plant growth and quality. It is a good idea to plot these values over time on a graph with desired highs and lows marked on the graph. This can be a good tool in determining if changes in fertility regimes (or other corrective actions) are needed. Sampling of plug media can be difficult. However, a suggested method for plugs is to drench a flat with distilled water to the point where leaching is just beginning (a few drops at most coming out of plug cells). Let the tray stand for 20-30 minutes, then squeeze the solution out of the plugs and test for pH and EC. Results from this type of testing would correlate closer to SME than to the 2:1 method. These results should also be tracked.

Regardless of the method used for media testing, consistency is key. Sampling should be done at about the same time in relation to watering and slurries should be allowed to stand for the same amount of time at each test. Media should be sampled from the same varieties and pot sizes over time. The results will likely not exactly match those of a commercial lab (especially if 2:1 extract is used), but can be compared if taken from the same samples, in order to get a better idea of what is going on in the mix. It is also important to make sure that all testing equipment is working properly. Equipment should be calibrated at least at the beginning of each day of use. Calibration solutions should be fresh. When in doubt, send a comparative sample to a commercial lab for testing.

Closing Points

The use of on-site test results for media and solution are another tool for growers to use for successful crop production. They are not meant to provide full information when a problem occurs. Rather, they are meant to be used to help keep media pH and EC in a desired range. This will help to avoid nutritional problems. It should be remembered that most nutritional problems can usually be traced back to media pH problems. Therefore, if adequate information is available regarding the trend of media pH and EC, then corrective measures can be taken before the crop is affected.

Even with all this investment in time and equipment, on-site testing is not meant to replace outside (commercial lab) testing. It is meant to complement it. In-house testing results may not exactly match those from a commercial lab, but with adequate equipment and consistency in testing, it will provide a valuable picture of the nutritional needs of the crop.

~ Dan Jacques


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