Salts are defined as dry materials formed when two compounds come together because of charge attractions, in the absence of water. So, a compound with a positive charge is attracted to a negative charged compound, as the “water” is removed from the system.
Water has its own set of weak charges (both positive and negative) (H+: -OH) as it resides in its rare pure form. It’s easy-po-peesy for plants to take up pure water, because they are essentially no chemical forces interfering with the low level charges that pure water resides at.
Now, let’s add salt. Remember that salts are a combination of a positively charged compound and a negatively charged compound (no charge in the dry “salt” form). In the presence of water, the salts now break apart (dissolve). The positive (+) component of the salt groups up and shakes hands with the negative part of the water molecule (salt + -OH), while the negative component of the salt shakes hands with the positive hydrogen + portion of the water. (H+ - salt).
Long story short, our original (H+ -OH) water, is no longer pure water. As the water is increased in dissolved salts, the “less pure” water there is to be taken up by the plant. In their own way, salts limit the uptake of water and cause a special kind of drought stress inside the plant. Because salt stress mimics drought stress, there are a couple of things to look for when investigating a potential salt-stress condition in turf.
For example, a well-watered turf with high soil moisture content is easily penetrated with a Phillips-head screwdriver to a 6-inch depth. As the soil dries out, the penetration is harder to achieve and is shallow.
With salts present, the soil can appear moist with the screw driver, but the grass often wilts when soil moisture is still high – as estimated by the screw driver test. That’s because the water is still in the soil, but the grass can’t take it up because of the soil moisture is now high in salts. The result is a salt-induced drought stress.
Another indicator is before the turf wilts, the grass blades often are a blue-green color. Turf growing in soils with high salt contents are almost always darker in color than otherwise, just like at the beginning of drought stress.
Also, growth is slow, with fewer clippings and slower fill in from damage. This is because salts have their own plant growth regulator effects, as well as lowering the water content of the leaves.
How do turfgrass plants deal with salt? Again, it’s Mother Nature who has come to the rescue, starting hundreds of thousands of years ago. Grasses that originate from areas on the planet which have lots of rainfall, generally have poor salt tolerances – because they don’t need it. Grass plants adapted to areas which get little rainfall, or live on coastal shore lines or by inland lakes that don’t drain – they are usually more salt tolerant. These plants either don’t allow salts to enter the plant at some point, store salts inside special compartments in the plant cells themselves, or dump them out of the leaves (salt glands).
How do you know if I have a salt condition? The screw driver test can lead you to the supposition that you may have a salt condition, but you need at least two items which show you how much salt is present.
A lab test report for water used for irrigation will have information relative to the salinity load of the water itself. It’s generally expressed in two forms.
Electro-conductivity (EC). This is the ability of water to carry an electric charge through it. Believe it or not, pure water is only a moderate conductor of electrical current. As the salt content of water increases, so does its capacity to conduct electrical charges. The higher the salt content in water, the higher the EC reading in the lab. Electrical conductivity is (unfortunately) expressed in different units of charge over a given length or distance. The most common forms of expression are mmhos/cm (millimhos per centimeter), which is the exact same expression as dS/m (decisiemens per meter).
TDS. This is an estimate of total dissolved solids (TDS), or – in this case, the “solids” are the “salts” which are now “dissolved” in the water. Units are always in ppm (parts per million), which is the same thing as mg/liter (milligrams per 1000 cc of water). If we dissolved 0.220 grams of potassium nitrate in 1000 cc (1 liter) of pure water, the water would have a TDS of 220 ppm, or 220 mg/liter, of these dissolved salt components in the water.
The relationship between EC and TDS is fairly well accepted. There are about:
- 640 ppm TDS in 1 mmhos/cm
- 640 ppm mg/l in 1 mmhos/cm
- 640 ppm TDS in 1 dS/m
- 640 ppm mg/L in 1 ds/n
Since we are not growing plants in a tub of water, we need to consider what’s going on in the soil. The “soil” holds more salts than the water does, because there are more “available charge sites” in a given volume of soil than an identical volume of water. As soil particles become smaller and smaller, there is a greater capacity to hold more charges. A lot of these “extra” charges that the soil can hold often come from salty water; Plus, once the soil particle is filled with charges, there are also “free swimming salts” that reside in the water films that surround the “in between” surface areas of soil particles. Yikes! Small soil particles can cling to and hold on to mineral salts, and also be surrounded by a lot water – salty or not.
So, the amount of salts in the soil solution is the important number to assess salt stress. That’s because, the rubber meets the road where the roots interact with the amount of salts in the soil solution – the water around the roots and soil together.
Soils labs have different methods to test for salt load in soils. The best technique is called the saturated paste extract. In this case, the soil sample is carefully wetted until the soil glistens, and then the salt content of the soil water is taken. This is a “click in time” of what the grass roots actually face at the critical point of water uptake – soil/root interface.
Some labs instead will mix a specific volume of soil and water, stir it up, and measure the salt content of the “potion.” From that value, the lab will then estimate how much salt would be in the saturated past extract. When you get a soils test, make sure you ask which method is used in the lab, saturated paste extract, or a soil:water dilution, and what is the dilution factor (1:1, 2:1, for example).
Turfgrass tolerance to salt levels are determined by the grass plants response to the soil salt tolerance, and generally not to salt levels in the irrigation water alone.
The combination of the salt level in a golf facility’s irrigation water (water quality test report) and the turfgrass salt tolerance (based on soil-solution salt levels) determines how to manage salinity in course’s turfgrass system. This is used to calculate how much water must pass into the root-zone. This is called a leaching fraction or leaching requirement (LR).
How it’s calculated and what it means to irrigation delivery amounts at your facility will be discussed in the third and final issue of this series of sodium and salts.