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Do salt based nutrients kill bacteria, microbes and mycorrhizae?


Well-Known Member
Does Fertilizer Kill Soil Bacteria?

Read most organic books or blogs and they will tell you that synthetic chemical fertilizers are killing the bacteria, fungi, and the microbes, in soil. Dr. Ingham and her Soil Food Web preach this same message. Stop using fertilizers because they kill the bacteria and fungi. My review of Teaming with microbes found the same message repeated several times.
Does fertilizers really kill bacteria or fungi in soil?
Some people claim that the ‘salts’ in fertilizer do the damage, but anyone making such a claim does not understand what happens to salts in soil. I’ll explain this in more detail below.

Does Fertilizer Kill Soil Bacteria?

Does fertilizer kill bacteria?

Fertilizer Kills Bacteria
Fertilizer provides nutrients like nitrate, ammonium, phosphate, calcium, potassium, sulfur etc. These are all nutrients that plants need to grow.
A lot of organic followers believe that the nutrients from organic sources are some how different from the ones provided by fertilizer. They are NOT! There is no lab in the world that can tell the difference between a nitrate molecule from manure and one from a bag of synthetic fertilizer. Plants can’t tell the difference either, because there is no difference. They don’t care where the nitrate came from.
A lot of people doubt science and in some advanced areas of investigation science may not be 100% correct. This is not one of these situations. All chemists agree on the above fact and have done so for a long time.

Organic material releases the nutrients slowly over many years. Synthetic chemicals release the nutrients as soon as the fertilizer dissolves in water. Is it possible that the quick release of nutrients kills microbes?

Keep in mind that the soil under your fingernail after a day in the garden contains millions if not billions of bacteria. Is it reasonable to think that fertilizer would kill all of them? I don’t think so. Even if the fertilizer killed 99% there would still be billions and billions in every shovel full of soil. And bacteria grow very quickly – as fast as doubling in number every 20 minutes (at least in a lab).

Number of Bacteria After Adding Fertilizer
There have been many studies looking at the number of bacteria in soil after applying fertilizer. In Impact of Organic and Inorganic Fertilizers on Microbial Populations (ref 1), they looked at both bacteria and fungi populations, and compared untreated soil to (a) soil treated with organic material (manure, rock phosphate, neem cake) and (b) soil treated with synthetic fertilizer. Measurements were done at two different depths.
Adding synthetic fertilizer resulted in no change in the number of bacteria and an increase in the number of fungi. Organic treatment increased both fungi and bacteria slightly.
Synthetic fertilizer did not kill bacteria in soil and it increased the number of fungi.
Agriculture Canada looked at the effect of ammonia and urea on the microbes in soil over a 10 year study, ref 2, and concluded that “nitrogen applied according to soil test recommendations had minimal long-term detrimental consequences for soil microbes, soil biochemical properties, or soil structure.”
The science is quite clear. Fertilizer, when used properly, does not kill microbes.
Microbes Eat Synthetic Fertilizer
Why do fertilizers not kill bacteria? The simple fact is that the nutrients in fertilizer, especially the nitrate, is a nutrient required by bacteria. They eat it! They actually absorb it since they have no mouth, but you get the idea. They also eat the other nutrients; phosphate, potassium, sulfate etc. Bacteria and fungi need these nutrients as much as plants do.
Once you understand this, it becomes fairly obvious that adding these nutrients to soil will not kill the microbes, unless they are added in very large amounts that prove toxic.

Think of composting. If you add too many browns the composting process goes slowly because there is not enough nitrogen available for the bacteria to eat. Since the bacteria are starving for nitrogen they don’t multiply and composting is slow. Add some nitrogen, either as a fertilizer, or as ‘greens’ which contain higher levels of nitrogen, and the compost pile suddenly heats up. The bacteria now have enough nitrogen to eat, they are active, and they multiply. All of this activity heats up the compost pile.

Fertilizers are Salts and Salts Kill – Don’t They?
You see comments like this all the time; “fertilizers are made up of salts and salts kill bacteria.”
It is true that fertilizers are salts. This is not sodium chloride or table salt. The term ‘salt’ has a different meaning for a chemist. To them, a salt is a compound made up of two or more ions. Table salt is made up of sodium ions and chlorine ions. Ammonium nitrate fertilizer is made up of ammonium ions and nitrate ions, so it is also called a salt.
In dry form the ions come together to form salts. When the salts dissolve in water, the molecules break apart and form ions. When fertilizer salts are spread on the ground the white and gray balls are salt. When it rains, the water dissolves the salt into ions and washes them into the soil. Once they are in the soil they are no longer salts.
Salt will harm bacteria and plant roots if there is direct contact. Due to the large number of microbes in soil, and the small surface area of the fertilizer crystals, this has no significant effect on the microbe populations in soil. Once the salt is dissolved, the ions quickly become diluted as the water moves through the soil layer.
Diluted ions in water do not harm microbes or plant roots. In fact both of their lives depend on the ions being in the water. It is the ions that they absorb – not the salts.
What happens with organic fertilizers like compost and manure? They contain large molecules like protein and carbohydrates. As these are decomposed, they are converted into ions. These ions are the exact same ions that fertilizer produces.
Once commercial fertilizer dissolves in water it is no different than organic fertilizer. Fertilizer does not kill bacteria or fungi.
  1. Impact of Organic and Inorganic Fertilizers on Microbial Populations; Impact of Organic and Inorganic Fertilizers on Microbial Populations and Biomass Carbon in Paddy Field Soil
  2. Ammonia and Urea fertilization – Facts and Myths; Anhydrous Ammonia and Urea Fertilization: Myths and Facts - Frequently Asked Questions
If anyone has any scientific data disputing this, I would be interested in reading about the experiment conducted to dispute the experiment linked in this thread.

Thank you all.


Grow Journal of the Year: 2017 - Grow Journal of the Month: Sept 2017
I think the key to understanding the impact of salts are phrases such as: “when used responsibly” or “according to the recommendations”.

It’s true that when synthetics are used in small to moderate doses they create little hazard to micro life, but as soon as you start overdoing them and ditch all sane agricultural practices like keeping the ratio of humus at optimum levels and testing your soil periodically, then you’re on your way to endanger the ecosystem.

Now show me how many ag operations use salts moderately? Huge ag ops usually exceed natural limits of their soils especially that many are not very fertile and they have low CEC, which means salts flow freely, usually to the nearest rivers and lakes and from there to seas and oceans.

We wouldn’t have this problem if responsible use of salts was common, but unfortunately it’s not! Humans have a hard time to understand the moderation especially where profits are involved and that’s the core of the problem.

In conclusion I’d say yeah it’s gonna be hard to switch to fully organic agriculture on a large scale, so it’d be much easier to promote a healthy combination of both methods, but that’ll also cut into profits of chemical companies, which firstly wanna keep their sales up and they have little interest in promoting responsible use of their products.

Bush Doctor 77

Well-Known Member
Just about any chemical can be toxic in large amounts, however the interaction of soil bacteria can be also impacted in a positive way by chemicals. For instance: I was involved in a study at CSU involving 2,4D. Range land produced more beef than could be accounted for after weeds were removed by using 2,4D. Turns out that 2,4D actually stimulated the growth of nitrogen fixing bacteria on range land, leading to increased grass production.
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