Some people won’t use synthetic fertilizer because it contains “salts”. For years I was told not to use mushroom compost because it contains “too many salts”. Salt is also recommended to kill weeds and just today I saw a post which suggested pink Himalayan salt makes plants grow better. You can buy commercial sea salt to fertilize your plants. This doesn’t make sense. Salt is a weed killer and it’s a good fertilizer?
As a gardener it is crucial that you understand which salt is being discussed so you don’t use the wrong one and harm your plants. This blog will help you through the maze of salt crystals.
What Is Salt?
A big part of the confusion stems from the use of this word. It has two very different meanings. The general public normally uses the term to refer to table salt while chemists use a completely different definition.
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 road 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.
Table salt is the most common salt used in an ion exchange water softener. I’ve discussed the effect of sodium on plants from such softened water in, Chlorine, Chloramine and Plants – Everything You Need to Know.
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. Native plants along the coast can tolerate higher doses.
Although sodium chloride will harm plants, each type of plant has a certain tolerance level to salt. for example, asparagus can tolerate high levels of sodium chloride. It is the dose (i.e. the amount) of salt that is important. A small amount will not harm a plant. A bit more may have an effect, 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 important to properly understanding the effect salts have on soil and plants.
Once dry salt hits the soil, it dissolves in the soil water fairly quickly – almost instantly if the soil is wet. Because of this, most soil does not contain salt – it only contains 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.
Salt does not harm plants, but the ions can if they exist at toxic levels.
To understand fertilizer better have a look at this: Fertilizer NPK Ratios – What Do They Really Mean?
What Are Ions?
Ions are molecules that either have extra electrons or have lost electrons. 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 and has a negative charge. Salts are formed when negative ions (anions) and positive ions (cations) join together and neutralize the charge. Therefore dry salt has a zero charge.
The following video shows how water is able to convert a salt into ions.
Nutrients vs Ions
What is the difference 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).
Gardeners talk about plants using iron, magnesium, calcium, and manganese, to name a few nutrients, but the reality is that plants can’t use any of these. Plants can only absorb the ions of these minerals. When someone suggests putting an iron nail or a copper penny in soil to provide iron or copper for the plants, you know this won’t work. Plants cannot use the metals. They can only use the ions of the metals.
Are The Salts In Fertilizer Good For Plants?
Most synthetic fertilizers are made up of salts. When these salts dissolve in water they release ions and it is these ions that plants use.
When people say that fertilizer contains salts – they are right. But, plants need the ions from these salts. Fertilizer does NOT contain table salt, although some fertilizer may contain very small amounts of sodium because plants actually need a little bit. The idea that the “salts” in fertilizer harm plants or microbes is ridiculous. Neither plants nor microbes can live without them.
What about organic fertilizer?
Organic fertilizer contains very few salts and very few ions. That is why they are not a good good food source for plants in the short term. They do contain molecules like nitrogen, magnesium, calcium etc., but these molecules are tied up in larger “organic” molecules. For example, a lot of the nitrogen is tied up in large protein molecules. Once the organic matter completely decomposes it releases these nutrients as ions but that takes months or even years to happen. The ions from organic fertilizer are identical to those of synthetic fertilizer. There is zero difference. Neither a lab, nor a plant, nor microbes can tell where the ions came from.
The idea that plant nutrient ions from organic sources are better than synthetic sources is completely wrong.
Salt In Mushroom Compost
The high level of “salt” in mushroom compost is due to a high level of nutrients that plants need. The salts are not sodium chloride. I have discussed this in more detail in Is Salt In Mushroom Compost Harmful To Gardens?
Salt As A Weed Killer
Too much nitrogen from fertilizer will kill plants, but when people talk about using salt to kill weeds they are talking about table salt – sodium chloride. Sodium is toxic to plants at even low levels and so it will kill plants. Unfortunately it also kills the good plants. This is a terrible solution for dealing with weeds.
Pink Himalayan Salt
There is a lot of mystery around this salt and all kinds of powers have been ascribed to it. They are all myths.
This is 98% sodium chloride. It kills plants and has no place in the garden.
Sea Salt In The Garden
When I first saw these products for sale I thought they were a scam. Why would anyone put sea salt on their plants? It is not a scam and some people believe that the micronutrients in sea salt are good for plants.
Some products claim that they have 80 or more different minerals that make them ideal for plant growth. These marketers seem to have forgotten some basic biology. Plants don’t use 80 minerals! At most they use 24 nutrients.
Sea salt does contain a lot of the micronutrients. The problem however is that it is mostly sodium chloride. This means you can only put a small amount on soil to ensure the sodium does not reach toxic levels. At such low levels, the amount of micronutrients you apply are insignificant. Sea salt is a poor choice for a fertilizer.
Some sea salt products remove the sodium and then it might be suitable if your soil needs the micronutrients.
Epsom salt is also used in the garden and is magnesium sulfate. It will add both magnesium and sulfur to soil, and so it can act like a special fertilizer, but it is incorrect to call it a fertilizer, at least in the legal sense, since it does not contain nitrogen, phosphate or potassium.
There are all kinds of claims for Epsom salt, but except for adding magnesium and sulfur, they are all false. It doesn’t even do much for you in the bath, except maybe make you feel good.
If the above video does not play, try this link: https://youtu.be/8tH7Q945gKU
Do Fertilizer Salts Kill Soil Microbes?
No. Microbes use the same nutrients as plants, as do you and I. We all need nitrate, phosphate and potassium. Salts Don’t Kill Plants or Microbes.
Saline vs Sodic Soil
Soil can be saline or sodic and these two terms can be confusing. Saline means “salty” or “containing salt” and medically speaking, it is used to describe things that contain sodium chloride. The average gardener would also use the term this way.
When scientists talk about soil they use a different definition. Saline soil contains high levels of soluble ions of mostly sodium, calcium and magnesium. If a soil contains mostly sodium, and very little calcium and magnesium it is called sodic.
Both conditions cause problems for plant growth. The issues and solutions to these problems are more fully described in my book, Soil Science for Gardeners.
The Bottom Line
Table salt has no place in the garden. Sodium harms your plants and runoff containing sodium is bad for the environment. Save it for eating tomatoes.
Synthetic fertilizer does contain salts as defined by chemists and these are essential for plant growth. A plant can get them naturally from soil or you can provide them from either organic sources or synthetic sources – your plant does NOT care where they come from.