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Salts Don’t Kill Plants or Microbes

The idea that salts kill plants and microbes seems very prevalent, especially among organic growers. The topic is poorly understood and leads to a number of statements that are either false or mostly false.

“Fertilizer kills plants because it is a salt”

“Farmer fields are devoid of microbes because of the salt in fertilizer”

“The NPK in manufactured fertilizer is made soluble by chemically attaching the NPK to salts”

“They [fertilizers] also make it easier for the chemicals to run off into waterways”

“Organic sources contain fewer salts”

“Organic sources are slow release”

“Fertilizers are designed to be highly soluble”

It’s time for a chemistry lesson to better understand salts, ions and the difference between synthetic and organic fertilizer.

Salts Don't Kill Plants

Salts Don’t Kill Plants

What is Salt?

The general public uses the word salt to refer to table salt which is sodium chloride. Each molecule of table salt contains one atom of sodium and one atom of chlorine.

The idea that salt can harm plants is supported each winter when salt is spread on roads and sidewalks as a deicer. The salt washes into the soil next to the road and plants growing there can be harmed or even killed. Historically this salt was sodium chloride, but some communities now use calcium chloride which is less harmful to plants.

Salt also affects plants growing next to oceans where the wind blows sea salt inland. Sea salt is also sodium chloride and people who garden in such locations need to be careful to use salt-tolerant plants, or else the plants die.  The native plants along the coast are more immune to the salt – they find it less toxic.

That last sentence is very important. Although sodium chloride will harm plants, each type of plant has a certain immunity to salt. It is the dose (ie the amount) of salt that is important. A small amount will not harm a plant. A bit more may start having effects, and once the dose reaches a critical level it will kill the plant.

The Chemists View of Salt

Chemists use the word salt quite differently. For them, a salt is any molecule that is made up of two or more ions. Sodium chloride (NaCl) is made up of two ions; sodium and chlorine. Ammonium nitrate (NH4NO3) is also a salt and is made up of an ammonium ion (NH4) and a nitrate ion (NO3). Potassium chloride (KCl) is common in fertilizer and is made up of a potassium ion (K) and a chlorine ion (Cl). There are hundreds of different salts.

As a solid, the ions join together to form crystals and chemists call these salts. When salts are dissolved in water, the ions in the salt separate and are no longer joined together. They are now properly called ions, not salt. This may seem like silly semantics, but it is an important distinction for properly understanding the effect of salts on soil and plants.

Once salt hits the soil, it dissolves in the soil water fairly quickly – almost instantly if it rains. Because of this, most soil does not contain salt – only ions. Once in solution these ions now acts as separate molecules. The nitrate ion goes off and does its own thing, as does the potassium ion, the calcium ion and so on. Each ion has different chemical and physical properties in soil.

Some ions like phosphate stick tightly to soil and even rain does not move it very far. Nitrate on the other hand is very soluble, does not stick to soil, and quickly moves with the water.

Fertilizer NPK Ratios – What Do They Really Mean?

What Are Ions?

Ions are molecules that either have extra electrons or have lost electronics. Therefore they are charged particles. A sodium ion is more correctly written as Na+, because it has lost an electron and now has a positive charge. Chlorine is Cl- since it has gained an electron. Salts are formed when negative ions (anions) and positive ions (cations) join together and neutralize the charge (ie salt has a zero charge).

The following video shows how water is able to convert a salt into ions.

If you can view the video, try this link.

Nutrients vs Ions

What is the different between nutrients and ions? Nothing really. Gardeners use the term nutrients to refer to the food that plants absorb through the roots and most nutrients are ions.

We talk about plants needing nitrogen, but plants can’t actually use nitrogen. Instead, they absorb nitrate ions (NO3) which contain nitrogen and oxygen. Once inside the plant, they separate the nitrogen from the oxygen and use it. The same goes for sulfur which is absorbed as a sulfate ion (SO4) and phosphorus which is absorbed as a phosphate ion (PO4). Other nutrients like potassium, iron, magnesium are absorbed as simple ions that don’t include the oxygen.

The key point here is that the nutrients which plants use, are exactly the same as the ions resulting from fertilizer salts. They are identical!

What About Organic Nutrients

Organic material consist of whole cells that contain big molecules. DNA contains phosphorus. Proteins contain sulfur and nitrogen. Chlorophyll contains magnesium and nitrogen. Organic material also contains loose ions floating around the cells, waiting to be used to produce larger molecules.

What happens when a plant dies? Initially not much. The cells are still intact and they just sit there. As time goes by microbes move in and start to break down the plant. Fairly quickly, the cell membrane is damaged and the loose ions floating inside cells are released. It is similar to biting into a jelly doughnut – the insides squish out as soon as you bite into it.

These loose ions are exactly the same as they were when the roots absorbed them. They are also the same as the ions that were released from fertilizer.

As decomposition continues, microbes digest the larger molecules and eventually the phosphorus in DNA and the nitrogen in proteins is released into the soil as nitrate and phosphate ions. The important point is that these ions are no different than the ions absorbed by roots or the ions from fertilizer.

The ions from organic sources are exactly the same as the ions from fertilizer.

Do Salts Kill Plants?

Remember that each ion has its own chemical and physical property. Each one affects soil and plants differently and some ions are more toxic to plants than others. It turns out that sodium is very toxic to most plants, even at low levels. It does not take much sodium to kill plants along the roadway where salt is used as a deicer and it quickly kills some plants along the seashore. But sodium is also used as a nutrient by some plants. It all comes down to dose. Small amounts can be beneficial for certain plants and too much kills them.

Nitrates are similar. If you put too much on your lawn you will burn the grass – which is just another way of saying you killed your grass. But grass, just like every other plant, can’t grow without nitrates.

Every ion in fertilizer is both a food source for plants at low levels and a toxin at high levels. Each ion has a different level of toxicity. Calcium, for example, can be quite high and has almost no effect on plants.

When salts (ie fertilizer) are added to soil in reasonable amounts it feeds plants and is not toxic to them. Plants can’t grow without them.

Does Organic Material Contain Salts?

Not in the strict sense of the word. Salts are solids and the loose stuff inside plant cells is dissolved in water – they are ions. So strictly speaking organic matter does not contains salts.

Organic material like manure has a lot of loose ions floating around, and some people incorrectly refer to them as salts. These ions can be in high concentrations and this is why fresh manure needs to be used cautiously in gardens. High levels of ions become toxic to plants.

Soil and Ions

It is also important to understand that soil contains lots of ions. Clay soil usually has a very high level because ions stick to clay. Sand has very low levels since they don’t stick to sand very well and water washes the ions away. This is why clay soil is very nutritious for plants and sandy soil is not.

Ions that are added to soil through fertilizer or organic material, tend to stick to soil and if they are not used by plants or microbes, the amount builds up and can reach toxic levels. This is why you need to be very careful about adding too much fertilizer or organic matter.

Do Salts Kill Microbes?

Plants and animals all require ions as a food source. Microbes are no different. They need phosphates and nitrates to build DNA and proteins. Without ions, microbes can’t live and just like plants, too much of a good thing becomes toxic.

When fertilizer is used in reasonable amounts it actually causes the microbe population in soil to increase – not decrease – contrary to what so many organic followers believe.

This is discussed in more detail in does Fertilizer Kill Soil Bacteria?

Fertilizer Myths

At the beginning of this post I mentioned some quotes.

“The NPK in manufactured fertilizer is made soluble by chemically attaching the NPK to salts”. This one is silly. The NPK are salts, they are not attached to salts. The fertilizer industry does select salts that are more soluble so they enter the soil more easily. That is a good thing – what is the point of having them sit on top of the soil?

“They [fertilizers] also make it easier for the chemicals to run off into waterways” . Since the ions in organic matter and fertilizer are the same, they run off into waterways in exactly the same way. The key here is to add them so that they get used up before they have a chance to run away.

“Organic sources contain fewer salts”. This one is sort of true. If you take 100 g of a 10-10-10 fertilizer and compare it to compost which is 1-1-1, the fertilizer has more salt. But this is a silly comparison. It is like saying a bag of 10 apples contains more apples than a bag of one apple. It is more correct to think of it this way. If both fertilizers contain the same amount of nutrients, they have exactly the same amount of salts/ions.

“Organic sources are slower release”. This one is partially correct. Remember that organic matter contains free ions as well as ions tied up in large molecules. The free ions are released almost instantly – they are not slow release. The ones in large molecules will be released slowly over years.

“Fertilizers are designed to be highly soluble”. They are not designed to be very soluble – most salts are soluble – it is part of their chemical characteristic. Some salts are more soluble than others. For example carbonates and phosphates are much less soluble than chlorides and nitrates. But why is this an issue? The point of using fertilizer is to get ions to plant roots. A more soluble fertilizer does this better. Some people recommend a less soluble fertilizer like rock phosphate but it takes many years to become available to plants.

Is Organic Better

I don’t want to leave you with the idea that organic fertilizers are not better than synthetic – they are better, and I have discussed this in Organic Fertilizer – What Is Its Real Value? The take-away here is that synthetic fertilizers are not the devil some people believe they are. The ions in them are critical for plant growth.

References:

  1. Photo Source: used under license
Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

17 Responses to 'Salts Don’t Kill Plants or Microbes'

  1. James Roberts says:

    As an experiment, I used common rock salt, sodium chloride, and 5% vinegar on 10 acres of my 20 acre property. This was applied last October 2016. Today, April, those treated 10 acres are beautiful. Lush, green grass, tulips are flowering, not a weed to be found.
    The other 10 acres are completely weed infested, some large areas as high as 30 inches. In the past, I’ve tried ortho groundclear and roundup. Neither has worked anywhere near as well.
    As for poisoning my soil with salt?
    Yeah, right. Talk to my beautiful grass and tulips.
    My only mistake with this experiment was not applying the salt and vinegar to all 20 acres.

  2. Roger Brook says:

    Excellent as ever Robert
    I had to turn to your link to learn that in your country bulky organics such as horse manure are regarded as a fertiliser – rather than something different and better. For me a fertiliser is defined as a concentrated source of nutrients.
    I was therefore concerned that you stated organic fertilisers were better than inorganic!
    The inorganic are generally superior in my opinion! For example yaramila (available here at the nurseryman’s analysis) contains every plant nutrient and is ideal for general use. The coated slow release fertilisers are superb for making up growing composts. Inorganic liquid feeds contain a far more complete balance than anything organic.
    Our languages sometime are barrier!

    • I think most people here would not use the term fertilizer with organic. I use the term that way to enforce the idea that they both provide the same nutrients to plants.

      In most provinces and states, the term fertilizer does have a legal definition as well – but they don’t all agree on what that is.

      I agree – there is no issue with synthetic fertilizer.

  3. Steve Webber says:

    Hi Robert,
    I’ve followed your Myth posts for a few months now with much interest. This latest post is up to your usual standard, being factual and easy to follow. It pretty much sums up what I have read about the use of synthetic fertilizers. I go with “organic” myself when I can but every now and then I have to use some quick release fertilizer because of the soil I am dealing with. Its good to know that no harm is being done to the microbes. However, I think it’s a different story with the earth worms. From what I have read the Ammonium produced when some synthetic fertilizers break down can hurt the Earth worm population even at recommended doses. At least I think that’s what I remember reading. (Might have to research that a bit more) Could be a topic for another post maybe?

    • I have not really looked at earthworms – you might be correct. Considering the fact that they are an invasive species, damaging our native woods – maybe this is a good thing?

      I had a quick look at references. One field study showed earth worm populations increased with fertilizer. this summary indicates the same general conclusion: http://extension.psu.edu/plants/crops/soil-management/soil-quality/earthworms. It says “There is good evidence that most inorganic fertilizers favor the buildup of large numbers of earthworms, probably due to the increased amounts of crop residues being returned to the soil. Anhydrous ammonia and ammonium sulfate have been found to have negative effects on earthworms. This may be due to the acidifying effect of this fertilizer, but also due to the toxic effect of ammonia. Liming to neutralize acidity stimulates earthworm activity.”

      • Steve Webber says:

        Invasive species. Really? That’s news to me. I’m pretty sure that my Earth worms are “locals”. A few were in the soil when I started my garden 3 years ago but have increased in numbers since I started composting / mulching the garden. All that I’ve read about earthworms has been good. They are the heavy lifters in the garden, helping in the process of breaking down organic matter in the garden. How are they damaging native woodlands?

        • There are some native earth worms, but most of the ones gardeners talk about are not native. They are good for gardens – but not for the wild areas.

          • Steve Webber says:

            Thanks for your reply. It seems there is more to Earth worms then I thought. I realize I’ve have gone off topic talking about Earth worms in this post so I will finish here. Maybe you could do another post on the benefits of Earth worms in our gardens and if that benefit out weights any harm they are doing to native vegetation.

  4. Andy says:

    Very Interesting and very well described.

    I’m still trying to work out why the large amounts of manure I use aren’t causing problems. I’m thinking that by manure you mean the actual horse poop (bare with me) and not what I call manure. When you say “no more than 1 or 2 inches per year” you are referring to the poop. My manure has a huge amount of wood dust and wood pellets mixed in so if my manure had 50-50 poop and wood pellets (rather than poop & straw as is traditional) it would be ok to put 4 inches on. If there was a higher ratio of wood pellets compared to poop the volume of manure you could spread on the garden each time could also be more?

    Sometimes the manure I get has a lot more wood pellets mixed in so when I use the stuff as a soil conditioner and a mulch (to suppress the grass and weeds) and if I put say 1ft of manure on the surface this could infact be closer to only a couple of inches of traditional poop and straw manure.?

    • The 1-2 inches is just a rough guide, and depends very much on the current soil condition. The current thinking is that too much organic matter causes problems by providing too many nutrients, and there are lots of examples where this is the case. There are other cases where vegetable gardeners have much higher levels of organic matter and they have no problems. I am not sure why there is this discrepancy.

      Use more manure if you like, and periodically, have the soil tested. If you reach high levels of a nutrient, cut back.

      It is best to use all of your material as a mulch.

  5. Bonny says:

    Excellent. Thanks.

  6. Art Thompson says:

    Thanks for this post. Very informative.

  7. Good article. Since you often refer to ‘ions’, some people like me would appreciate a sentence or two to remind us what ions are.

  8. leereich says:

    This article makes no mention of the osmotic effect of salts. Basically, when salt (or ionic, if you wish) concentrations are higher on one side of a biological membrane than on the other side, water can be drawn passively across the membrane to the side with higher salt concentration. If that higher concentration is from recently added fertilizer salt, water can be drawn out of nearby plant cells, dehydrating and possibly killing them.

    Another point is that chemical fertilizers are salts that, as you wrote, instantly dissolve, given moisture, in the soil solution. Problem is that plants might not want all that nutrition all at once. The advantage of most organic fertilizers is that most of their nutrients are tied up in organic compounds, released through the action of microbes in response to moisture and warmth. Plant growth also responds to moisture and warmth, so nutrients from organic fertilizers are released more or less in synch with plant needs. There need not be excess in the form of salts or ions in the soil; that excess might burn plant roots or leach out before plants take them up.

    Chemical fertilizers often need to be added multiple times through the growing season because a full dose at once will burn plant roots or leach out before uptake by plants.

    • These are all good points. Most of the nutrients in fertilizer, nitrogen being an exception, do get absorbed by soil and organic matter in the soil, thereby hanging around for an extended period.

      I certainly agree organic sources are better.