Question: When should I apply compost so it will be most profitable for both the soil and my plants?
Answer: There are two ways of looking at compost: as a soil amendment or as a fertilizer.
As a Soil Amendment
As a soil amendment, there is no specific season for applying compost. You can add it whenever the soil is not frozen and it will be effective. And unlike fertilizers, it can’t burn roots, so you can apply it generously.
Its benefits as an amendment include:
- Improving soil quality (light soils retain more water and minerals, heavy soils drain better and are better aerated);
- Helping prevent erosion;
- Reducing the effects of drought;
- Making acidic soils less acidic and alkaline soils less alkaline;
- Introducing and maintaining all sorts of soil organisms and microorganisms that are beneficial to plants;
- Helping eliminate plant diseases present in the soil;
- Decomposing pesticide residues and other pollutants and turning them into useful minerals plants can use for their growth;
- Naturally recycling products that would otherwise have been treated as waste.
All these advantages benefit the soil—and secondarily the plants that grow in it!—no matter what season you apply the compost.
As a Fertilizer
Compost normally contains all the minerals that plants need for their growth, but even so, in smaller amounts than most commercial fertilizers. Nevertheless, they are usually in a form that plants can easily absorb.
In order for compost to perform better as a fertilizer, logically it should be applied just before the plant needs it the most. Thus:
- Approximately two weeks before planting or sowing, normally in the spring, although if you replant the vegetable garden with fall vegetables, also in August or early September;
- Failing that, whenever you put in new plants;
- In established beds or in woodland gardens, apply in spring, just before plants wake up;
- For lawns, top dress with up to 1 inch (2 cm) of compost in fall is possible. If not, in spring.
How to Apply Compost
Traditionally, you’re supposed to work compost into the soil to a depth of about 6 inches (15 cm), but … it’s just as effective when you simply leave it on the surface as a mulch or top dressing. Apply a good 3 to 4 inches (7 to 10 cm) of compost for mulching purposes, if you have enough of it that is, but even a lesser amount will be useful. Soil microbes, beneficial insects and earthworms will take care of working it into the soil. You can even apply compost on top of a mulch already in place and the earthworms and their friends will still work it into the soil below.
Personally, as a laidback gardener, I’m more than happy to just toss compost onto the ground, then let soil creatures take care of placing it where it needs to go. It makes feel them happy and useful … and who am I to spoil their fun?