How to conduct onsite soil testing for pH, EC

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Terms including EC, PourThru and SME have been tossed around a lot lately, so here’s the first in a 3-part series on the basics of soil testing.

Growing media pH and electrical conductivity (EC) are the most important soil measurements that you can test for in your nursery. If you can maintain media pH and media EC within acceptable limits and select an appropriate growing medium and fertilizer, you will avoid most nutritional problems.

Growing media pH

Media pH affects how soluble nutrients supplied by a fertilizer will be in growing media, especially micronutrients such as iron. Soluble nutrients are the only ones available for uptake by roots. As media pH increases, solubility of most of the micronutrients decreases. Therefore, micronutrient toxicity symptoms are common in plants grown at excessively low pH, whereas micronutrient deficiency symptoms can occur at high pH.

Growing media electrical conductivity

Media EC is a generic measurement that tells you how much salt is contained in growing media. In most cases, the salt is from fertilizers applied to the crop, but it can be made up of non-fertilizer ions such as sodium or chloride (table salt). A low media EC indicates lack of available nutrients (leading to nutrient deficiency), whereas a high media EC indicates excess nutrients (leading to nutrient toxicities, or damage to sensitive roots).

Why consider in-house testing

You can send samples into a commercial laboratory for pH and EC testing, but unless you operate a very small nursery, it would be better (and cheaper) to purchase a pH and EC meter for in-house testing and use the lab for more detailed analysis of specific nutrients in the media or for water-quality testing or tissue testing.

The advantages of in-house testing of media pH and EC are the low cost and the ability to quickly handle many samples on a regular basis. The goal is to keep the pH and nutritional levels within an acceptable range and to spot problems early on. This is a far better strategy than having to take dramatic steps to rescue stressed crops.

You can use media pH and EC results to make fertilizer decisions in a simple, systematic way. For example, if the pH is too low (acidic) then you may want to change fertilizers to one that contains a higher level of nitrate nitrogen. If that does not work, then a more aggressive strategy of drenching with liquid lime or potassium bicarbonate may be needed.

If the pH is too high (basic), then you may want to change fertilizers to one that contains a higher level of ammoniacal nitrogen. If that does resolve the situation, then using a fertilizer with very high ammoniacal nitrogen level (21-7-7) along with supplemental iron drenches may be necessary.

If the media EC is too low (deficient in nutrients) then the applied fertilizer concentration can be increased. If the media EC is too high, then you may need to use lower fertilizer concentrations, alternate the fertilizer with some clear water applications, or remove the salts from the pot by increasing the leaching rate.

Choosing a soil testing method

In general, there is no one best method for measuring media pH or EC. Consider how much experience that you have with a particular method as well as how much help and advice you can receive from other people who are located nearby such as other growers, extension agents, university personnel or soil testing laboratories.

Here are the soil testing methods commonly used to measure media pH and EC in nurseries.

PourThru. The growing medium is irrigated thoroughly and allowed to drain for one hour. At testing, a small volume of distilled water is poured on the medium at the top of the pot. A small sample of the resulting leachate is collected at the base for testing.

This method is good for rapid, nondestructive measurement in a nursery. It is especially good for large or specimen pots or pots that contain slow-release fertilizer prills that when using more destructive soil testing methods can break open and give false readings.

1 part soil:2 parts water dilution (1:2). Growing medium is removed from the lower two-thirds of the pot. At testing, one volume of soil is mixed with two volumes of water, and this slurry is used for testing.

This method is good for running many samples and guidelines are well established. Although the dilution method is common in other countries, the ratio of soil:water used is often different and can range from 1:1 to as much as 1:10.

Saturated medium extract (SME). Growing medium is removed from the lower two-thirds of the pot. At testing, just enough water is added to a soil sample to saturate air spaces.

This is generally the most consistent testing method but more training is required than with other testing methods. It also takes slightly more time to prepare the samples than with other methods. This is the testing method used by most commercial laboratories for soilless media.

Plug squeeze method. The growing medium is irrigated thoroughly and allowed to drain for 30 minutes to an hour. At testing, the soil solution is squeezed out of the plug and collected. This method is used for plugs where samples are difficult to handle with the above methods. EC results tend to be more variable than pH results using this method.

Direct measurement. The growing medium is irrigated thoroughly and allowed to drain for 30 minutes to an hour. At testing, hardened sensors with fine tips are placed directly into the medium. This method is used mainly with plugs and in hydroponic slab culture. Direct measurement is faster but more variable (especially for pH) than other methods.

Consistency is the key

To make nutrient-related decisions, you need good data. This means purchasing a good quality pH and EC meter, and calibrating the meters before use. Consistency also starts with having a single, trained person conducting the tests. Finally, measuring nutrients with two or more methods often leads to confusion. Choose one method, and stick with it.

After you measure the soil sample, use the correct standards to check whether pH and EC are in the acceptable range. Each soil testing method can provide different results, especially for EC, because methods vary in how much the soil solution is diluted. For example, diluting a sample by half with distilled water will result in half the EC level.

Media-pH is less sensitive to the testing method, but can still vary by ±0.5 pH units between testing methods. This means that you need to use the correct tables to interpret the results and be consistent.

The optimum media-pH varies depending on the tendency of a plant species to have problems at low pH or at high pH. The optimum pH is designed to make nutrients soluble enough for healthy uptake without the risk of excess and toxicity.

Optimum media EC

Media EC is a measure of the total salt concentration in the extracted solution. The EC measurement does not indicate the concentration of any individual plant nutrient. The only way to determine exactly what ions make up the EC is to use a more detailed commercial laboratory analysis.

Caution is needed in interpreting an acceptable EC when deciding whether plants have received sufficient or excess fertilizer. A low media EC does not necessarily indicate a problem. This is because vigorously-growing plants can rapidly (within hours) take up fertilizer nutrients into the tissue, especially in small containers such as plugs or liners. Observing the overall appearance of the plant, checking the fertilizer concentration being applied, and using your grower experience are equally important. Also check for trends from one week to the next.

If EC is tending to increase over time this indicates excess fertilizer application. If EC is stable, plant uptake and applied fertilizer are probably in balance, and if EC is decreasing then the plant is taking up more nutrients than the fertilizer level currently being applied.

Interpretation of media pH levels for container-grown crops


Acceptable pH range

Examples

Iron-inefficient group

5.4 to 6.2

Plants are prone to micronutrient deficiency (particularly iron) when grown at high media pH.

General group

5.8 to 6.4

Plants are not generally affected by either micronutrient deficiencies or toxicities.

Iron-efficient group

6 to 6.6

Plants are prone to micronutrient toxicity (particularly iron and manganese) when grown at low media pH.

Values are the same for all testing methods, although subtle differences in soil testing methods can vary results (by up to 0.5 pH units).

Adapted from Understanding pH management of container grown crops. W. Argo and P. Fisher, 2002. Meister Media, Willoughby, Ohio.

Interpretation of media electrical conductivity (EC) or soluble salt levels


1:2

Method

Saturated

media extract

method

PourThru

method

Plug

squeeze

method

Low fertility

0 to 0.5

0 to 1.5

0 to 2.4

0 to 2.4

Acceptable range

0.6 to 1.50

1.6 to 3.0

2.5 to 5.0

2.5 to 4.0

High fertility

>1.50

>3.0

>5.0

>4.0

Values are reported in milliSiemens per centimeter (mS/cm). The units of measure for EC can be mMho/cm, dS/m, mS/cm,

µS/cm or mMho x 10-5/cm. The relationship is 1 mMho/cm=1 dS/m=1 mS/cm=1000 µS/cm=100 mMho x 10-5/cm.

- Paul R. Fisher, Amy C. Douglas and William R. Argo

Paul Fisher is associate professor in the Environmental Horticulture Department at the University of Florida; pfisher@ufl.edu. Amy Douglas is an assistant breeder at NovaFlora Inc., adouglas@novaflora.com. William R. Argo is technical manager, Blackmore Co., (800) 874-8660; bargo@blackmoreco.com.

The authors thank the University of New Hampshire Agricultural Experiment Station, and our Young Plant Research Center partners Blackmore Co., Center Greenhouses, Costa Nursery, D.S. Cole Growers, Ellegaard, Fafard, Four Star Greenhouses, Glass Corner Greenhouses, Greencare Fertilizers, Knox Nursery, Kube-Pak Corp., Lucas Greenhouses, Pindstrup, Pleasant View Gardens, Premier Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and Welby Gardens for supporting this research.

The authors and their organizations do not assume any liability for use of this information. Use of trade names does not imply endorsement, preference or recommendation for any products listed or omitted.