- Gary Shafer, ISP Research
In grower meetings during the past couple of months, an often-asked question concerns the terms “salt”; ”sodium”; and “soluble salts”. It appears there is quite a lot of misunderstanding surrounding these terms. The most common salt of which most of us are familiar is sodium chloride, or common table salt. But this is only one of many potential salts, and many of these other “salts” are necessary in agricultural or horticultural crop production.
Chemically speaking a “salt” is “an ionic compound that is formed by the neutralization reaction of an acid and a base”, or in other words an anionic element (negatively charged) and a cationic element (positively charged). Common salt-forming ions are carbonate, chloride, nitrate, nitrite, oxide, phosphate and sulfate. Common salt-forming cations are ammonium, calcium, iron, magnesium, potassium, sodium and copper. In other words, magnesium sulfate (Epson Salt), potassium nitrate, ammonium sulfate and ammonium phosphate are all “salts”, although none contain sodium unless the grade of fertilizer material is extremely poor. ISP plant foods are completely sodium free.
You’ve mentioned that sodium is increasing on many of your soil analysis reports. This can be even more noticeable if there is hydrogen present on the soil colloids as hydrogen is easily replaced, thus allowing sodium to attach to that site. When looking at your soil analysis reports, many commonly look at sodium as a percent of base saturation and seeing a high sodium percentage (above 1% – 2%), you might think that your soil has a high “salt” content. This may or may not be accurate, although high sodium content in a soil can certainly lead to problems such as crusting, negative soil structure, and a reduction of soil pore space.
A better indication concerning potential issues with your crop production is “soluble salts”. Soluble salts is a measurement of the total ion content (or dissolved minerals) in the soil water. So long as “soluble salts” is below 1.2 to 1.5 there is very little chance of crop damage. If “soluble salts” is above 1.5, certainly greater than 2.0, you might want to address taking actions to reduce overall “salt” content in your soil. This is especially true in greenhouse or high tunnel production, as the plastic or other covering is often not removed until it needs to be replaced. As such, this does not allow for normal leaching activity that is present in outside production. Even so, the question arises, “if sodium is increasing on my soil analysis, where is it coming from?”
There are several possible sources. Sodium is the sixth most abundant element in the earth’s crust, and as a result, most soils will have a certain percentage of sodium. In areas of higher rainfall, sodium will usually be lower than soils found in low rainfall areas, again due to the natural leaching that takes place. Sodium can also be a component of irrigation water, especially if the water source is from city water treatment (although some municipalities are now eliminating sodium chloride as means to “soften” water.) Wells, ponds, rivers and other surface waters can also contain relatively high levels of sodium, and across a period of several years can result in a sodic soil. Finally, sodium may be a component of some manures or manure-based composts. A number of livestock feed additives contain salt, and to a large degree can easily be present in the resulting manures. One thing we can trust … it is coming from one of those three possibilities.
So how do you control or reduce sodium levels in your soil. Again, the most natural method is leaching off, or the introduction of relatively large amounts of sodium-free water to the soil and allowing this water to soak downward. But if the source of sodium build-up is your water, then this of course is not going to be a viable solution. If you can catch a significant amount of rainwater, this would be a wonderful source of water to “flush” your soil.
A second potential issue with flushing is whether or not your soil is compacted, and if so, at what level of depth. It is fruitless to try and flush salts from the growing environment if you have a serious compaction issue at a depth of 8 inches. This can easily result in an extremely high “salt” content at that depth, and of course plant roots will easily grow down into that zone. If using flushing as a method to reduce sodium, or other salts, and you have a serious compaction layer, then it will be necessary to break up this compacted layer prior to flushing.
There is also a large amount of research data indicating that humic acids are effective in reducing the crop damage resulting from sodic soil conditions. These humic substances have the ability to take excessive sodium and form relatively insoluble compounds, thus removing them from the active soil profile. This is one reason that humic materials were much more commonly used in arid croppings areas as compared to those with higher rainfall. Growing in a greenhouse or tunnel, in many respects, is a duplication of growing in an area of no rainfall. Of course, ISP offers a variety of premium humic based products.