Chloride in Soil
Chloride: An Essential Nutrient
Chloride is rarely thought of as an essential nutrient. But it is!
Chloride’s function in the plant is primarily for osmoregulation, (the maintenance of constant osmotic pressure in the fluids of an organism by the control of water and salt concentrations.) It is often concentrated in the vacuole of cells and it is moved across membranes to help draw water into a cell or into a cell’s organelle. It's important in turgor pressure, or the rigidity of cells. It also helps with the opening and closing of stomata.
It is an anion (negatively charged ion) so it is very responsible for charge balance across membranes. It's often a counter anion. There are a lot of positively charged cations (calcium, magnesium, potassium) that most people are more familiar with. But there are also anions that help maintain charge balance, which is the primary role of chloride. The plant has a certain intelligence in chloride uptake from the soil. The chloride goes through the symplast and the tonoplast in the plant and is able to regulate how much it takes up.
Chloride on Soil Tests
Chloride will show up on Logan Labs soil paste tests but not on standard soil tests (order tests here). You'll also see it on irrigation suitability analyses.
Chloride Toxicity
I tend to see high levels of Chloride on laboratory plant sap tests. Labs will often send you down a rabbit hole trying to reduce chlorides in your plant sap. I am not sure how this happens, but I continue to believe it’s simply acting as a counter ion to balance the uptake of cations.
(Note: if you want to start sap testing or you want to go from 0 to 60 in your understanding of sap testing, check out my “Guide to Plant Sap Analysis”).
It is typically in the highest concentration of all the anions on a laboratory sap analysis. Chloride, sulfate, nitrate, and phosphate as your primary anions. Chloride is always going to be in highest concentration followed by sulfate. Theoretically, because it's an anion it could start blocking phosphate in the plant. But I have yet to see in organic production a chloride toxicity or a chloride deficiency. It could still be a hidden hunger, but I haven't seen physical symptoms of either of those things.
In conventional agronomy, chlorides are more common in fertilizer. Similar to sodium, chloride based fertilizers can definitely burn plants as they contribute significant osmotic stress compared to other ions. Again, it can theoretically block phosphate and it’s possible to see excess chloride on lab tests accumulate in the bottom leaves. I see levels over 5,000 ppm in plant sap quite frequently. Levels above 5,000ppm are definitely high in my opinion.
On the soil test I've seen it as low as 7ppm and as high as 300ppm. I feel that 300ppm is too high. However, the crop at 300ppm was healthy!
Note of Humility: I'm still confused about chloride because I've heard from other consultants that when they've lowered the chloride level in their water, they see a drop in the plant sap and they've seen beneficial effects.
My understanding physiologically is that chloride is actually very beneficial in the plant. The plant selectively uptakes it in the quantities that it needs.
How Chloride Operates
In soil, most importantly, chloride is a bit of a “free agent”. It doesn't precipitate out very easily. It doesn't absorb to other soil particles, organic matter, or the clay colloid. It just kind of floats in the soil solution. Because of that, it actually leaches very easily with a lot of precipitation. I can look at a soil test and predict whether it's from an arid or wet climate based on how much chloride is on the test. If there's really low chloride on the paste test, usually that's indicative that there's been a lot of water running through that soil profile and leaching it out. It's fairly easy to down-regulate in the soil through leaching.
Sources of Chlorides
Usually the sources of chloride are:
irrigation water
fertilizer
dust
sea spray aerosols
In organic production, unless chlorides are really high in your water, it doesn't make sense to me that there would be excess chloride. There are products that address this, such as carbon-based products that claims to tie up excess chlorides. If possible, the better solution is to leach your soil.
Irrigation water is likely the primary source of chlorides. When it comes to municipal water, chlorine is often used as a disinfectant and antibiotic. I have not seen any evidence or any studies that show that using municipal water decreases biological activity in the soil.
The concentrations that are used are fairly low and I have not seen accumulation in the soil. But if that's something of concern and you're seeing high chlorides on your paste test then giving it a flush with clean water would definitely help reduce Cl levels.
Reverse Osmosis will take chlorides out of the water, although I'm not a huge fan of RO water as it produces a huge amount of waste water, usually 3:1. And often times you're removing beneficial nutrients from the water as well.
If you want help with chloride, get in touch. The first step is almost always a Soil Test and an Irrigation Water Test.