Soil amending

Asked September 22, 2017, 2:00 PM EDT

Hello, is it best to amend soil around existing plants in the fall so there is time for the plant to really absorb it before spring growing season? Or in spring, just before the active growing season? Thanks!

Multnomah County Oregon

1 Response

This question is confusing; depending on what is meant by "amending the soil" the answers could be radically different, and contradictory.

In the most general terms, it is always a bad idea to dig around/damage/disturb plant roots. Feeder roots, especially, are very fragile, and most are located very close to the surface. So while before placing mulch on top the soil, or adding a top dressing of fertilizer, it is a good idea to pull up weeds, it is never is never a good idea to "dig in" nutrients with a shovel, as that is too damaging. Gardeners often "scratch in" fertilizer by gently working it into the top quarter or half an inch of soil with a hand fork, or other light hand tool. That does a several things: it dilutes the active ingredients in the fertilizer, so they are less likely to burn the plant roots, and it distributes the particles so more come in contact with roots as the material is dissolved by rain or irrigation water. Remember, "plants only eat soup"; i.e., all nutrients are absorbed only when they are liquid; or more specifically, when they are charged ions dissolved in water.

On the other hand, "amending the soil" often means working in large quantities of organic matter well down into the soil depth. While this varies somewhat (as always in gardening!) by species, most plants prefer a soil that is about 50% solid, 25% water, and 25% air. Throughout the tri-county area, the majority of our soil is silty loam: rich in all nutrients except nitrogen, but below the top few inches it is heavily compacted and lacks the voids that allow for the air and water that plants need. Thus the common practice of digging in large quantities of compost the full depth that plant roots will penetrate, as that creates the necessary voids. This work should never be done around existing plants; it needs to be completed before plants are put in.

Our native soils are at an excellent pH range for almost all plants. Gardeners here usually only make minor adjustments by top-dressing their established vegetable gardens or lawns with a sprinkling of lime. There are a few rare exceptions--blueberries come to mind, as they require a major acidification of their soil by working in substantially quantities of mineral sulfur--and this work must be done before the plants are put in. (And then that unusual acidity is maintained over time by regular top dressing with mineral sulfur.) But digging in large quantities of sulfur around existing plants would probably kill them by destroying their shallow root system.

Timing also varies with what is meant by "amending the soil".

The most important principle is probably: don't dig in wet soil. To do so just compacts it, even when the intent is to work in large quantities of compost to break up and prevent compaction!

Top dressing with fertilizer is done primarily to provide additional nitrogen, which easily washes out of our local soils. "Chemical" fertilizers provide nitrogen in the form that plants use, and should be placed when the plants are actively growing, and can use it, before it is washed on down through the soil. Keep in mind that plant roots, even lawn roots, start growing several weeks to well over a month before top growth starts. Feeding that root growth is a good thing. "Organic" fertilizers provide nitrogen in many different forms, most not directly usable by plants--but this is a good thing, as these unusable forms are utilized by many of the millions of living organisms that are found in healthy soil, and the by products of those other organisms is usable by plants. So "organic" fertilizers are naturally "time released".

Other nutrients can be placed as a top dressing at any time, as they break down very slowly over time.

Mulch, probably our most important amendment, is always placed on top of the soil, not worked in, and this can be done at any time. Many of us do much of our mulch work in winter: it helps prevent weeds in the spring; we have time to do it because there isn't much else going on; placing it in winter allows tender plants to come up through it, rather than being broken by a load of mulch while they have tender growth above ground. And sometimes mulch is cheaper in winter!