University of Maryland Extension

How to interpret salinity test results

Author: 
Neith Little and Andrew Ristvey

Salinity matters for high tunnels and growth media

Neith Little, Urban Agriculture Extension Agent
Dr. Andrew Ristvey, Extension Specialist for Commercial Horticulture

Salinity is an important consideration for management of healthy soil and growth media, particularly in high tunnels or hoop houses. Salinity measures the total amount of soluble salts (minerals) in the soil or growth media.

One of the benefits of growing in a high tunnel is that it protects your crops from excessive rain and keeps their leaves dry, which can reduce the spread of disease. However, soaking rains serve the beneficial purpose of leaching salt accumulated from fertilizers, compost and minerals in the irrigation water, down below the root zone. So over time a lack of soaking rains can result in a build-up of minerals in high tunnel soil, increasing the soil’s salinity. You can sometimes visually see a build-up of these minerals as a white crust on the surface of your high tunnel soil.

Salinity is also an important measure of the quality of growth media (mixes of substrates such as peat, coir, compost, sand, vermiculate, etc.). Some composts can have a high salt content, so if you are mixing your own growth medium, or purchasing one, it’s a good idea to make sure that the salinity of your mix is not too high.

What happens if the salinity is too high?
Basically, too much salt makes it harder for your crops to pull water and nutrients into their roots, and at very high levels it can stunt root growth. This is the same thing people talk about when they say a high-nitrogen fertilizer can “burn” seedlings’ roots.

How do you measure salinity?
You may be able to visually see very high salinity as white crusting on soil, but how do you spot the problem before it gets that bad?

You can send a sample off to the lab, or use an Electrical Conductivity (EC) probe to measure salinity in soil and growth media. A simple probe costs about $150, and measures both EC and pH.

Electrical Conductivity is a measure of how easy or difficult it is for electricity to pass or conduct through the material. In soil or growth media, it is a measurement of salts or minerals dissolved in solution (the soil water). The more salt in the soil, the higher the conductivity. The procedure to measure EC in your soil or growth media is very simple. Mix equal parts by weight of soil and distilled water, let it sit for at least 30 minutes, stir again and insert the probe.

Soil mixed with water for EC measurement

 

A note about units:
Electrical conductivity is reported in a variety of units, but presently the Siemens is the derived metric unit of electrical conductivity.

Electrical conductivity probes typically measure in mS/cm (milliSiemens per centimeter). You may also see salinity reported as µS/cm (microSiemens per centimeter) or dS/m (desiSiemens per meter). Converting between units of Siemens is relatively simple, because while the units are different, the scale is the same. For instance,
1 mS/cm = 1 dS/m
1mS/cm = 1000 µS/cm

Occasionally old reports use the unit of mho for electrical conductivity. A mho is the reciprocal of ohm, the unit of resistance (and you thought chemists didn’t have a sense of humor).
1 mho/m = 1 mmho/cm = 1 mS/cm = 1 dS/cm = 1000 µS/cm

In aquaponics and hydroponics, people sometimes use measure Total Dissolved Solids as an estimate of salinity, instead of electrical conductivity (EC). However, TDS measures all dissolved solids in water, not just salt, so electrical conductivity is a more accurate measure of salinity specifically.

Occasionally EC is used as an estimate of nitrogen because available nitrogen is usually in a salt form. However, EC measures all salts—nitrogen, sodium, potassium, potassium, magnesium, calcium, and more—so a high EC reading may be due to high nitrogen, or it may be due to high sodium, and you will not be able to tell the difference. For this reason, we do not recommend using EC to estimate nitrogen availability on urban farms.

How do you interpret the results?
At salinity above 2 mS/cm seedling germination starts to be inhibited and roots of larger plants may have difficulty accessing nutrients and water. Above 4 mS/cm roots are physically damaged and may die. This factsheet from Utah Extension explains it very well.

Irrigation water and high-tunnel soils
A farmer using high tunnels should also get a water quality test. Irrigation water can carry dissolved minerals which will accumulate in time, potentially increasing the salinity and pH of the soil in time. If salinity is a problem, the high-tunnel soil should be exposed to rainfall if possible, which can leach the minerals out of the root zone.

Conclusion:
If you are growing in a high tunnel, or mixing your own growth media, measuring the salinity of your soil or substrate is a good idea. In a high tunnel, we recommend measuring salinity annually, so that you can monitor changes over time. If salinity measurements increase over time, or are above 2 mS/cm, it’s time to assess why and what to do. Penn State Extension has a helpful article  on next steps for salinity management in high tunnels.

 

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