Living in a region where crops thrive without irrigation is a stroke of luck. However, for most of us, irrigation is indispensable for crop growth. Yet, the water used for irrigation often comes with its own set of problems.

One of the most common issues with irrigation water is its high bicarbonate content. Bicarbonates infiltrate water as it passes through calcium carbonate or magnesium carbonate (limestone or dolomite) rock formations. When these rocks dissolve, they release calcium and/or magnesium ions along with bicarbonate ions. While this might seem harmless, bicarbonates elevate the water’s pH level, causing disruption in both soil and plants.

If your water has a high bicarbonate content, you might want to consider putting your soil on a low-carb diet, just like we did!

So, what problems do bicarbonates bring? Let’s count the ways…

– Bicarbonate exists only in dissolved form in water. When water evaporates, the bicarbonates combine with soluble calcium and magnesium in the soil, forming insoluble calcium carbonate (limestone, CaCO3), and magnesium carbonate (found in dolomitic limestone, MgCO3), seen as the white crust around irrigation emitters. This results in less soluble calcium and magnesium being available for plants.

– A considerable amount of bicarbonate can be introduced to the soil through irrigation water. For instance, with a bicarbonate level of 300 ppm (very high), one inch of water contains about 30 kgs/acre of bicarbonate. If the soil is irrigated at a rate of 1 inch per week over a 30-week season, this accumulates to 1000 kgs/acre of bicarbonate. This is almost equivalent to adding nearly a ton per acre of lime, significantly increasing the pH, especially in lighter soils like our loamy sand.

– Bicarbonates raise the soil’s pH level, making many micronutrients such as iron, manganese, and zinc unavailable to plants at higher pH levels. While bicarbonates can enhance phosphorus availability by tying up calcium, it’s crucial to note that phosphorus tends to strongly bind with calcium at pH levels over 7.3, which might not be as desirable as it sounds.

– Bicarbonates, when combined with calcium and magnesium, form lime deposits on leaf and fruit surfaces if overhead sprinklers are used. These unsightly white spots reduce the marketability of the produce.

– Bicarbonates and calcium in the water can combine to form lime deposits, clogging drip irrigation emitters. The risk of clogging is highest when bicarbonate levels exceed 120 ppm and the water pH exceeds 7.5.

– Bicarbonates react more readily with calcium to form insoluble calcium carbonate at higher temperatures. That’s why the hot water faucet develops a white crust more quickly than the cold one.

– Plants can directly absorb bicarbonates. Inside the plant, they can block iron assimilation pathways, leading to iron chlorosis despite iron presence in the plant tissue.

– Bicarbonates affect plant roots, reducing their ability to absorb nutrients. As a result, plants are smaller, chloritic, and less effective in photosynthesis.

Alkalinity Levels:
Bicarbonates are often expressed as alkalinity in a water test report because bicarbonates (and carbonates if the pH is over 8.3) in water have the most significant influence on alkalinity. Alkalinity measures water’s ability to neutralize an acid and is usually expressed in ppm of calcium carbonate (CaCO3) equivalent.

So, what bicarbonate levels in water pose a problem? It depends on how the plants are grown and how much irrigation water they receive. In small pots like those used for seedlings, bicarbonates can be more problematic than in larger pots or in the ground. Therefore, the bicarbonate level of concern is much lower.

However, water can also have too low a level of bicarbonates. Water without any bicarbonates doesn’t resist pH changes much, and pH can fluctuate wildly depending on the fertilizers used.

Alkalinity in the range of 100–150 ppm should be the maximum limit for ground crops.

An irrigation water test will provide a value for the total alkalinity expressed as ppm of calcium carbonate and a value for bicarbonates expressed as ppm bicarbonate, meq/liter. Other labs or tests may only provide alkalinity, usually as ppm of equivalent calcium carbonate. To convert from ppm alkalinity to ppm bicarbonate, multiply by 1.22. To convert from ppm bicarbonate to ppm alkalinity, divide by 1.22. Note that these equations only work for water pH less than 8.3 because above that, carbonate is also present.

Treatment Options:
So, what can be done about it?

Some soils are naturally high in carbonates (such as calcareous soils, which are high in calcium carbonate). If this is the case for your soil, there may be other more pressing issues to deal with than irrigation water. Understanding your soil type is essential before addressing the water. Doing nothing is also an option!

Another option is to add soil amendments to counteract the problems. Gypsum (calcium sulfate) provides soluble calcium to counteract calcium tie-up. However, gypsum alone cannot lower soil pH. Only elemental sulfur can permanently lower soil pH. Microbes in the soil convert elemental sulfur into sulfuric acid, which reacts with carbonates, turning them into carbon dioxide and water. Problem solved, right?

Not quite. The amount of elemental sulfur needed to lower soil pH is considerable. As mentioned earlier, sulfur turns into sulfuric acid in the soil, which in large amounts has a detrimental effect on soil biology. We advocate for practices that promote healthy soil biology, therefore we limit elemental sulfur additions to 50 kgs/acre in most situations, and up to 150 kgs/acre if the pH needs significant adjustment. As shown in the table below, depending on the required pH change and soil type, over 500 kgs/acre may actually be necessary, requiring applications to be spread over several years. For instance, if we’re applying irrigation water with bicarbonates at, say, 200 ppm, for every 12 inches of water we apply, we’re also applying 250 kgs/acre of bicarbonate. It takes one molecule of elemental sulfur to neutralize one molecule of bicarbonate, so that works out to 125 kgs/ac sulfur per 12″ water just to keep up with the bicarbonate, let alone lower the pH. All this is to say that applications of elemental sulfur might not be sufficient to lower pH and reduce bicarbonates, even in the long term.

Adding more iron and other micronutrients to the soil to “fix” chlorosis might seem tempting. However, as soil pH increases, micronutrients become less available. While there may be plenty of micronutrients in the soil, they simply aren’t available to the plant at higher pH levels.

One possibility is to foliar feed micronutrients to offset chlorosis.

Soil organic matter helps buffer soil pH changes, making higher soil organic matter (SOM) soils less prone to pH changes. SOM has many exchange sites for H+ ions, which may release if the pH is high or capture H+ if the pH is low. As organic matter decays, different effects come into play, raising or

lowering the pH. Initial decay increases soil pH as cations are released. Further breakdown of plant material into ammonium temporarily increases the pH, while ammonium conversion to nitrate decreases pH. If the nitrate leaches, a permanent lowering of pH can occur. Soil organic matter also helps chelate and make plant-available the micronutrients iron, zinc, and manganese that are suppressed at higher pH levels. While soil organic matter may not eliminate the effects of high bicarbonates in water, it can help. Unfortunately, some composts, especially those from manure sources, can be high in sodium or salts or have a high pH. Obviously, these are not helpful.

The other option is to treat water to reduce bicarbonates before it is applied to the soil.

Water Treatment:
One way to treat water is to run it through a reverse osmosis filter. Reverse osmosis (RO) removes contaminants, minerals, and bicarbonates from water. However, it has some drawbacks. Systems are expensive and require maintenance. RO removes so many minerals and bicarbonates that some need to be added back in to balance the pH. Additionally, they produce an immense amount of wastewater. Our kitchen RO unit produces four times the amount of wastewater compared to filtered water.

A more common treatment for high alkalinity water is to mix in an acid before it is delivered to the irrigation lines. The acid releases H+ ions, which combine with the bicarbonate, breaking it down into carbon dioxide and water. By adjusting the amount of acid in the mix, one can adjust the amount of bicarbonate in the irrigation water and lower the pH to a desirable range.

Commercial farmers, greenhouse growers, and even golf course operators may use large-scale acid injection devices. These may inject sulfuric acid, nitric acid, phosphoric acid, or “n-pHuric” acid (urea sulfate). All of these are dangerous to handle (some are extremely dangerous), and none of them are certified for organic use.

The least expensive acid certified for organic use is anhydrous and non-synthetic citric acid. Citric acid is a weak acid and is safe to handle with precautions. It is a white crystal and is used as a food additive to provide a lemony flavor. This may be too expensive for use in a large-scale operation, but it works for a large garden. One bag of 25 kgs will last about 3 months in the summer.