If you read much about soil or compost, you’ll soon come across references to salts.
This doesn’t necessarily mean the table salt (aka sodium chloride) you have in the kitchen.
A salt can be any substance that is made up of ions. For example, sodium chloride is made up of sodium ions and chloride ions.
Here’s some example of salts found in compost:
Are salts good or bad for plants?
Many salts are essential for plant growth. That’s because salts break down in water into ions, and some of these ions are nutrients for plants.
Indeed, fertilizer itself is made up of salts which include nitrate, phosphate and magnesium ions.
However, high levels of salts (high salinity) can damage plant growth.
What’s more, when salts levels are too high they interfere with plant roots and stop them from taking up water. That’s especially the case for sodium chloride, but high levels of other salts can have a similar effect.
In short, salts are essential for plants, but too much will kill them. Symptoms of high salt levels can include wilting and yellow or brown leaves.
Is compost high in salt?
Fresh compost and spent mushroom compost can be high in salts. However, as compost matures the levels of salts reduce. (Some of the studies looking at mushroom compost have aged mushroom compost for two years before using it.)
In my research for this post, I’ve struggled to find a great deal of information on how the breakdown happens. However, some studies have mentioned leaching. Compost leaches water, and if it is rained on this process accelerates.
Salts break down in water too, and Garden Myths states that salts break down in the soil – although some salts do so more than others.
Still, it’s a good idea to leave compost to mature to allow salts to leach out before use. How long depends on what you are using it for.
For example, if you are adding half an inch to your soil to improve soil structure, the salts will have a smaller impact as the total amount of salt in the soil will not be greatly increased. That’s especially the case if you apply the compost in the fall or late winter, ahead of planting in the spring.
If you are using the compost for seedlings or tender young plants, though, it is important to let it mature properly.
Gondok et al, 2019, Soluble Salts in Compost and Their Effects on Soil and Plants: A Review
Garden Fundamentals: What Is Salt: It May Not Be What You Think
Garden Myths: Salts Don’t Kill Plants or Microbes