If you live in a place where your crops don’t need irrigation, count yourself lucky. For most of us, irrigation is necessary in order to grow a crop. And for many of us, our irrigation water comes with problems.
One of the most common problems with irrigation water is that it comes with an excess of bicarbonates. Bicarbonates get into water when it passes through a calcium carbonate or magnesium carbonate (limestone or dolomite) rock formation. The stone dissolves into calcium and/or magnesium ions and bicarbonate ions. This sounds innocuous enough, but bicarbonates raise the pH of the water and cause havoc in soil and plants.
If your water is very high in bicarbonates, you may want to do what we did – put your soil on a low carb diet!
Bicarbonate only exists dissolved in water. When water dries out, the bicarbonates in it combine with soluble calcium and magnesium in the soil, locking them up into insoluble calcium carbonate (limestone, CaCO3) and magnesium carbonate (present in dolomitic limestone, MgCO3), the white crust seen around irrigation emitters. This means there will be less soluble calcium and magnesium available for the plants.
An enormous amount of bicarbonate can be delivered to the soil via the irrigation water. With a bicarbonate level of 300 ppm (very high), one inch of water contains about 70 lbs/acre of bicarbonate. If the soil is being irrigated at the rate of 1 inch per week, over the course of a 30 week season that adds up to 2100 lbs/acre of bicarbonate. That is equivalent to adding almost a ton and a half per acre of lime, which can significantly raise the pH, especially on lighter soils. This is what we have seen on our loamy sand.
Bicarbonate raises the pH of the soil and since many of the micro-nutrients such as iron, manganese and zinc become unavailable at higher pH, these elements can become deficient in irrigated plants. Because bicarbonates tie up calcium, they can actually make phosphorus more available, however it is important to recognize that phosphorus tends to strongly tie up with calcium at pH over 7.3, so this effect is perhaps not as desirable as it sounds.
Bicarbonate combines with calcium and magnesium to form lime deposits on leaf and fruit surfaces if overhead sprinkling is used. The white spots are unsightly and reduce the marketability of the product.
Bicarbonate and calcium in the water can combine to form lime deposits and clog drip irrigation emitters. The potential for clogging is highest when bicarbonate exceeds 120 ppm and the water pH exceeds 7.5.
Bicarbonate reacts with calcium to form insoluble calcium carbonate more readily at higher temperatures. That’s why your hot water faucet get a white crust on it more quickly than the cold.
Bicarbonates can be taken up directly by the plant. Inside the plant they can block iron assimilation pathways, causing iron chlorosis despite the presence of iron in the plant tissue.
Bicarbonates have an effect on plant roots that reduces their ability to take up nutrients. Plants are smaller, chloritic and do not photosynthesize as effectively as they could otherwise.
Bicarbonates are commonly expressed as alkalinity on a water test report, as bicarbonates (and carbonates if the pH is over 8.3) in the water have the greatest influence on alkalinity. Alkalinity is the ability of water to neutralize an acid. Units are ppm of calcium carbonate (CaCO3) equivalent.
So what levels of bicarbonates in water are a problem? That depends on how the plants are being grown and how much irrigation water they receive. In small pots such as seedlings are grown in, bicarbonates can be more of a problem than in larger pots or in the ground. Consequently the bicarb level of concern is much lower.
However, water can also have too low a level of bicarbonates as well. Water without any bicarbonates does not have much resistance to changes in pH, which can then swing wildly depending on what other fertilizers are used. U Mass has provided a table with recommended minimum and maximum values for alkalinity.
Container | Minimum Alkalinity (ppm) | Maximum Alkalinity (ppm) |
|---|---|---|
| As CaCO3 equivalent (ppm) | As CaCO3 equivalent (ppm) | |
| Plugs or seedlings | 37.5 | 65 |
| Small pots / shallow flats | 37.5 | 85 |
| 4-5 inch pots / deep flats | 37.5 | 105 |
| 6 inch pots / long term crops | 37.5 | 130 |
From Ref 3, Water Quality for Crop Production
There seems to be consensus that alkalinity in the range of 100 – 150 ppm should be a maximum limit for crops in the ground.
An irrigation water test from Logan Labs 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 and lbs/ac-in. 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.
So, what to do about it?
Some soils are high in carbonates to begin with (such as calcareous soils which are by definition high in calcium carbonate). If this is the case for your soil there may be other more pressing issues to deal with than the irrigation water. It is worthwhile understanding your soil type ( find your soil type here) before tackling the water. One option is to do nothing!
Another option is to add soil amendments to counteract the problems. Gypsum (calcium sulfate) supplies soluble calcium to counteract the effect of calcium tie up. However, gypsum by itself cannot lower the soil pH. Only elemental sulfur can lower soil pH on a permanent basis. Microbes in the soil turn elemental sulfur into sulfuric acid and the acid reacts with the carbonates to turn them into carbon dioxide and water. Problem solved, eh?
Not quite. The amount of elemental sulfur needed to lower the soil pH is a lot. As mentioned above, sulfur turns into sulfuric acid in the soil which in large amounts has a detrimental effect on soil biology. We are promoting practices that promote a healthy soil biology and therefore we limit additions of elemental sulfur to 100 lbs/acre in most situations and up 300 lbs/acre if the pH needs to be lowered. As can be seen in the table below, depending on the amount the pH needs to be changed and the soil type, levels over 1000 lbs/ acre can be actually be needed, and so applications would need to be split over several years. If we are applying irrigation water with bicarbonates at say, 200 ppm, then for every 12 inches of water we apply, we also apply 545 lbs/acre of bicarbonate. It takes one molecule of elemental sulfur to neutralize one molecule of bicarbonate, so that works out to 175 lbs/ac sulfur per 12″ water just to stay even with the bicarbonate, let alone lower the pH. All this is to say that applications of elemental sulfur might not be enough to lower pH and reduce bicarbonates, even over the long term.
(lbs/acre applied S) | (lbs/acre applied S) | (lbs/acre applied S) | |
|---|---|---|---|
From Ref 2, Soil Acidification: How to Lower Soil pH
It may be tempting to add more iron and other micronutrients to the soil to “fix” the chlorosis problem. The trouble is that as the soil pH gets higher, the micronutrients become less available. There may be plenty of micronutrients in the soil, but they just aren’t available to the plant at higher pH.
One possibility is to foliar feed micronutrients to offset chlorosis.
Soil organic matter helps to buffer soil pH change, so higher SOM soils are less prone to pH runaway. SOM has many exchange sites for H+ ions which may release if the pH is high or which may capture H+ if the pH is low. As organic matter decays, different effects come into play which raise or lower the pH. Initial decay increases soil pH as cations are released. Further breakdown of plant material into ammonium temporarily increase the pH while ammonium conversion to nitrate decrease pH. If the nitrate leaches, a permanent lowering of pH can occur. Soil organic matter also helps to chelate and make plant available the micronutrients iron, zinc and manganese that are suppressed at higher pH. Soil organic matter may not eliminate the effects of high bicarbonates in water, but it can help. Unfortunately some composts, especially those from manure sources, can be high in sodium or salts or high in pH. Obviously, these are not helpful.
The other option is the treatment of water to reduce bicarbonates before it is put on the soil.
One way to treat water is to run it through a reverse osmosis filter. Reverse osmosis (RO) will remove contaminants, minerals and the bicarbonates. However it does have some drawbacks. Systems are expensive and need to be maintained. RO removes so many of the minerals and bicarbonates that some need to be put back in, in order to balance the pH. And they produce an immense amount of waste water. Our kitchen RO unit produces 4 times the amount of waste water as 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 will release H+ ions which combine with the bicarbonate and break 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 citric acid, anhydrous and non-synthetic. Citric acid is a white crystal and is used as a food additive to provide a lemony flavor. It is a weak acid and it is safe to handle with precautions. We can buy it in 50 lb bags for US$62 in 2020 prices since it is used by the local wine industry. This may be too expensive for use in a large scale operation, but it works for our large garden. One bag will last about 3 months in the summer.
2023 Update
Unfortunately the price of citric acid has tripled since this article was originally written, and is now over $3 / lb. We no longer see this as a viable means of lowering irrigation water pH. We are investigating humic acid as an alternative, not to lower the pH but because of its claimed ability to complex minerals and keep them from tying up due to high pH. More later!
Small scale injection systems work by the Venturi effect, which is a fun physics effect. The effect is created when there is a liquid flowing at sufficient speed past a small opening; a partial vacuum is created at the opening which provides suction. The suction can be used to inject air or liquid into the flow.
The simplest injection gadget, the Dramm 22625 Siphonject Siphon Mixer, goes on the outdoor faucets and is used when we water with the hose. A bucket of water with the acid mixture sits by the faucet and is sucked into the injector whenever the water is on. The injectors are <$15 on Amazon in 2020 so it is a low cost system. The drawback is that it only works with a full flow hose, so there is no option to use it with a low-flow sprinkler or drip system. The other drawback is that it only works with a 50 foot or shorter hose. It needs 3 – 5 gallons per minute of flow to work and it reduces the water pressure at the hose end. Fortunately, we have found that it does work with a small passive sprinkler. A 5 gallon bucket of acid/water mix is emptied in about 15 to 20 minutes.
There are several different types of plumbed-in injectors for irrigation water (usually called fertigation injectors). The Dosatron or MixRite injectors use a pump to create the Venturi effect. These are more expensive and have more maintenance costs than the Mazzei injector, which has no moving parts. The Mazzei takes some time to set up and calibrate, and if plumbed in line into an irrigation manifold, the irrigation branches must be fairly well balanced in term of flow in order to produce the same amount of acid injection in each. By contrast, the Dosatron should produce a metered output over a wider range of flow rates. We were put off by the annual maintenance requirement and cost of the Dosatron so we went with the Mazzei.
We use a Mazzai injector (see ref 1) which we plumbed into our irrigation manifold. Next to it sits a plastic garbage can full of acidified water which is drawn into the irrigation system whenever it is running. We have several different irrigation circuits and the Mazzei system feeds all of the ones with sufficient flow. It too is fairly inexpensive, <$30 for the injector, another say $50 for plumbing parts and the garbage can. We also spent about $40 on pressure gauges – these proved to be extremely valuable! The main drawback of this system is that it required a lot of time and plumbing for us to get right since everything is confounded by our garden being on a hill.
Citric acid seems to be a good choice for growers who cannot afford an RO system (which runs into the $1000’s) and who may benefit from reduced alkalinity of their irrigation water. It has a number of benefits:
- Citric acid, like other acids, turns bicarbonates into carbon dioxide and water
- It is a white crystalline powder that is used in food processing to provide a lemony taste
- It is safe to handle with protection and dissolves easily in water. Be sure to read the safety data sheet before use.
- It is approved for organic use
- It is relatively inexpensive compared other water acidifiers approved for organic use
- It is a biostimulant and it is completely metabolized as an energy source by soil micro-organisms (see ref 4)
- It is a chelating agent and a complexing agent and it therefore makes micronutrients more available (see ref 5)
These last two benefits sound really interesting!
*Note* There can be problems with citric acid use if you have heavy metal contamination in the soil; due to it’s chelating and complexing properties, citric acid may be used to extract heavy metals such as lead or cadmium from the soil and concentrate them in the plant. If you have heavy metal contamination in the soil, we recommended that you not acidify your water until you understand how it will impact the metal content of your plants.
The first and most important safety tip is to make sure that there is a working backflow preventer plumbed in between the incoming water and the Mazzei system. The last thing you want to have happen is to have acidified water or fertilizer contaminate the rest of your water, especially if the water is also used for drinking. Yuk!
The Mazzei injector requires a significant pressure differential across it in order to generate the suction required for it to work, so it is plumbed in parallel across either a pressure reducing valve or a pressure reducer.
Two suggested installation configurations from the Mazzei website here.
There are different sized Mazzei injectors depending the size of the plumbing and the flow rate. To select which injector to use, Mazzei provides a handy table here. To use the table you find your inlet operating pressure, and the outlet pressure you hope to have. In our case we have in inlet pressure of about 35 psi when the irrigation is running. We need an outlet pressure of at least 20 psi when the irrigation is running. Based on these two values you would choose an injector from the list of Motive Flow and Water Suction combinations for each injector. The Motive Flow is the flow through the Mazzei injector, not the total amount of irrigation water that you want to deliver. The trouble is that we don’t know how much water will be diverted to the Mazzei versus the main flow. So we called the company and they recommended we use the lowest flow injector, the Model 283.
Our garden is on a hill and unfortunately the irrigation manifold is located at the bottom of the hill near the electricity that powers the irrigation controller and solenoids. This really confounds things. As water is pumped up hill it loses 0.433 psi for every foot of elevation change. There are additional pressure losses due to the length of poly pipe used to get the water to the top of the hill above the garden. The garden is irrigated through drip tape and this produces back pressure when all the tape is full of water and dripping. As a result this meant that we could not get enough pressure differential to get the Mazzei to produce suction at the top of the hill when the drip tape was dripping. We figured this out by installing it there and having it not work (although it did work while the drip tape was filling). It also meant that installing the Mazzei across a pressure reducer at the bottom of the hill also would not work since the back pressure from the confounded hill was higher than the outlet pressure of the pressure reducer. Sadly, we figured this out the hard way too. Remember when I said that pressure gauges turned out to be very important to getting the whole thing to work? Sheesh. If only we had started by installing pressure gauges on our little setup (See Typical Installation Methods above) …
Finally we installed pressure gauges on both the input and output sides of the Mazzei injector. It is important to measure the pressure while the irrigation is running. The pressure drops considerably when the water is moving. According to Mazzei, it only requires a 20% drop in pressure in order to produce suction, however we found that more than that was necessary in our case, probably due to the confounded hill.
In the end we removed the pressure reducer and installed a ball valve in its place, that is, we moved from a figure 2 configuration to a figure 1 configuration in the illustration above. The ball valve allowed us to reduce the outlet pressure enough for the Mazzei to produce suction. It was a bit tricky to set the valve in a spot where there was sufficient suction but also also enough outlet pressure to get the irrigation water to the top of the confounded hill. Fortunately we were able to find the sweet spot and did not have to run power up the hill, move the irrigation manifold, and re-plumb all the irrigation branches. That would of course be the correct way to do irrigation. We know that and respect anyone who has properly installed their irrigation plumbing.
Once the Mazzei injector is installed and producing suction it’s time to figure out how much acid to add the water being injected.
The first step in determining the amount of citric acid to add to the water is to find a reliable method of measuring the water’s alkalinity and pH. To get the alkalinity right is a matter of trial and error, so it is important to have the proper tools on hand. We use test strips from SenSafe which provide both alkalinity and pH, in conjunction with a liquid pH indicator solution which has a wider range of pH values. Once we have our water acidified to the proper alkalinity we can measure the pH and use this value for later measurements.
Note that the type of citric acid to use is the anhydrous form. It is available as food grade but that is not necessary. The price per pound decreases with the amount purchased.
Next, get an idea of how much acid is needed to bring the alkalinity of your water to somewhere between 40 and 80 ppm. To do this, fill a gallon container with water and add some acid, starting with less than 1/8 tsp per gallon (1.3 ml per liter). Stir to dissolve the acid. Measure the pH and alkalinity of the mixture. Keep adding small amounts of acid until you have reached the desired alkalinity and pH, usually at less than 80 ppm alkalinity and with pH in the 6.0 to 6.5 range. Add up the amount of citric acid you have added and you will then have an idea of how much citric acid will be needed in the final irrigation water mix. If you have an idea of how much water you are applying during each irrigation session, this will also give you an idea of much citric acid you will be using overall.
We developed calculator to help you determine the flow rate of each of your irrigation branches and the amount of citric acid that will be needed for that branch.
We live in California where the rains only come in winter. For years we had noticed how all of our citrus and perennial plants really perked up with the rain. We put this down to “rain is the best fertilizer”, but that did not actually make sense. Sure, rain picks up nitrogen from the air, but nitrogen in the winter is not so much of a factor. We noticed that when the rain lasted later into the spring, everything did better in early summer. Finally it dawned on us; perhaps the water we were irrigating with was a problem. So we got an irrigation test from Logan Labs and sure enough, it was worse than we thought.
It took a fair amount of research to determine how we might improve our water, using organic methods. I’ve added some links to resources below. In the end, despite the confounded hill, we now have a system we think may work for years to come.
So, if you need to, put your soil on a low carb diet!
Happy Growing!
(1) Mazzei injector brochure and installation details
(2) Soil Acidification: How to Lower Soil pH – Ohio State University Extension
(4) Rhizospheric Organic Acids as Biostimulants – Frontiers in Plant Science
(5) Origins, Roles and Fates of Organic Acids in Soil – A Review
(6) Irrigation water and saline and alkali soils
(7) Soil pH and Organic Matter – Montana State University Extension
(8) Lowering the Soil pH with Sulfur – Michigan State Extension
