Difference Between Fertilizers and Soil Amendments


The word “fertilizer” is often used liberally and many times incorrectly these days to describe manure, soil conditioners and pretty much anything dumped onto the surface of a garden to make something grow.

There is an important difference between fertilizers and soil amendments. This matters because you need to know why you are using a certain material and what effect it will have on your soil and plants, short and long-term.


Fertilizers impact plants soon after application. Most of the nutrients are in water soluble form, making them easier for plants to absorb. Fertilizers last for hours, days or weeks and don’t do much to improve the quality of your soil (especially synthetic fertilizers). Plants in containers usually need a constant supply of fertilizers to stay healthy.

Commercial fertilizers must be tested and their nutrient content printed on the package. I know from personal experience that to register a fertilizer for sale in the State of California, it must be analyzed first to arrive at its N-P-K value: the relative percentage of nitrogen (N), phosphorus (P) and potassium (K) contained in the packaged material.

In this book I’ll be referring to some of the natural materials as fertilizers even though they are not commercially branded products with published N-P-K.

Soil Amendments

Soil amendments improve soil structure over time and their nutrients become available over weeks, months or even years.

In California, soil amendments can be sold legally without N-P-K information because their primary role is not nutrient-based. Their claim to fame is that they change the condition of the soil, improve water retention, increase porosity and aid nutrient processing.

Without a good soil that drains well and provides organic matter for microorganisms, fertilizers won’t do much good.