Fall is a good season to amend your soil; it may even be the best time of the year to do it. Amending soil in fall gives the fertilizers and soil conditioners time to incorporate, so plants are rarin' and ready to go in spring. And it's one less chore you'll have on your to-do list next year.
Smart use of amendments improves the soil's tilth, making it easier to work in and for plant roots to penetrate. It also retains water more effectively. Plants growing in soil amended to their needs will be healthier and better able to fight pests and diseases. Fruits and vegetables will taste better.
If you haven't done a soil test for a few years, do one as soon as you can to determine if your soil needs nutrients (not all do), which ones, and in what amounts. You should get your results rather quickly, since more people test their soil in spring. You can use an at-home test kit, but the results won't be as accurate as those sent to a lab.
Amending your soil in fall also has a cheapskate advantage. Most people amend in spring; so garden centers and big box stores often have fertilizers and amendments on sale at this time.
Amendments to Add
Compost: Make it or buy it, compost is one of the best materials you can add to your soil. Dig it in, or you can spread an inch or two on top, then top that with mulch. Organisms in the soil will incorporate it in over the winter.
Animal manure: Usually I recommend aged manure that's had the ammonia seasoned out of it, but if you're adding it to a bare bed and won't be planting anything, you can use fresh manure. It will break down over the winter, as long as you live in a climate that gets below freezing. Only use manure from non-meat eating animals, such as cows, horses, rabbits, etc. No manure from dogs, cats, or humans, which contain pathogens.
Organic fertilizers: Organic fertilizers release nutrients slowly, over months. When added to the soil in fall the nutrients will be readily available to plants in spring. Organic fertilizers include bone and blood meals, greensand, kelp meal and rock phosphate. Dig them into the soil, or side-dress around plants. You can use organic fertilizers at the same time as compost, manure, and shredded leaves.
Raw organic matter: you can treat your garden beds like one big compost heap, and dig in kitchen scraps, fresh (not dried) leaves, small twigs, spent flowers and vegetable garden detritus, as long as it isn't diseased or pest-ridden. If the pieces are small enough, they'll break down over months. Don't dump them on top of the soil, however, they should be dug in or they won't decompose.
One thing to keep in mind: as it decomposes, organic matter uses up nitrogen in the soil. Even if your soil test didn't call for it, adding additional nitrogen in the form of aged manure or an all-purpose fertilizer to whatever else you are adding should counter any depletion. This is important if you are incorporating fresh rather than aged materials.
In the Ornamental Garden
Mulch around ornamentals with compost or shredded leaves. Keep them an inch or two away from the stems and foliage.
Dig in compost where you've pulled out annuals, when transplanting, and when planting bulbs. There are mixed opinions about adding compost or fertilizers when planting trees or shrubs. Some people swear by it, others believe the richer soil in the planting hole discourages the roots from growing into the surrounding soil.
In the Vegetable Garden
As you pull out dead crops, dig in a few inches of compost, composted manure, or shredded leaves. Still-warm soil will help break down and incorporate the amendments.
Or try cover crops. Sow winter wheat or winter rye about six weeks before your first frost date. Once mature, but before they go to seed, cut them down and incorporate them into the soil, or you can leave the stalks up – which helps prevent soil erosion, if that's a concern – and till them under in the late winter or early spring.
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