At a glance
- Best For
- Improving soil structure and feeding soil life
- Time To Results
- Structure in clay builds over several years
- Not A Substitute For
- Fertilizer for a crop that is hungry now
- Typical Rate
- 1 to 2 inches a year on established plant-based beds
What does compost do to the soil?
Compost works on soil structure first. Organic matter feeds soil bacteria, and the sticky substances those bacteria release glue mineral particles into small crumbs. That gives the soil structure and pores, which is why a soil with stable crumbs is easy to dig, takes water readily and makes a good seedbed.
In clay, which holds water that roots struggle to extract and passes air and water slowly, the crumbs open the soil up. Over a period of years that improves drainage and aeration and lets roots reach deeper, and it makes the soil easier to work and plant.
Sandy soil drains fast and needs frequent watering. Organic matter holds over ten times more water and nutrients than sand does, so compost improves how much water a sandy bed holds.
The same material is food for soil organisms. As their numbers grow they convert more of the soil's nutrients into forms plants can use.
What does compost save you?
Soil that takes up and stores water better may cut the water a newly planted bed needs. Oregon State adds that amended soil acts as a better sponge, so more water goes in and less runs off the surface, which keeps fertilizer and pesticides in the soil instead of washing into nearby waterways.
Over time a well-amended soil also supplies more of the nutrients plants need, and that reduces fertilizer requirements. Amended gardens will probably need water less often, but in most areas they still need it in summer.
Composting leaves at home means you do not pay someone to haul them away, your own compost replaces bought peat moss in a potting mix, and food scraps and yard waste stay out of the landfill.
What will compost not do?
Compost is a soil amendment rather than a fertilizer. Its nitrogen content is typically only 2 to 4%, and just 5 to 25% of that nitrogen is released in the first year, so a crop that is hungry in midseason needs a fertilizer that works now, as in the best fertilizer for vegetables.
Batches also differ in what they contain, which is why a lab test lets you apply compost more accurately.
Compost does not fix a drainage fault. Where water ponds in a low spot with nowhere to drain, the soil stays saturated, and organic matter added to saturated soil can decompose without oxygen and produce organic acids that are toxic to plants.
Compost does not clear soilborne disease either. Better drainage lowers the chance of some soilborne diseases, but Oregon State says research does not show that organic matter eliminates or reduces them otherwise.
A home pile also passes along what went into it. It is common for weed seeds, vegetable seeds and plant disease organisms to survive home composting, so diseased plants and seedy weeds stay out of the pile. How long compost takes to be ready covers the temperature that destroys most of them.
How much compost goes where, and when?
Compost made only from plant material (leaves, grass clippings, wood chips and other yard waste) can go on at higher rates than compost made with manure, because manure compost may carry more salt.
| Where | How much | How to put it down |
|---|---|---|
| Vegetable and flower beds | 2 to 3 inches of plant-based compost a year for the first three years, then 1 to 2 inches | Mixed into the top 6 to 8 inches of soil |
| Beds with manure or biosolid compost | 1 inch in new beds and ½ inch in established beds, unless a soil test shows low salt | Mixed in to the same depth |
| A new lawn | ½ to 1 inch of screened compost in Oregon State's guide, with particles under ½ inch, and 2 to 3 inches of plant-based compost in Colorado State's | Tilled into the top 6 inches before seeding or sodding (Oregon State), or 6 to 8 inches deep (Colorado State) |
| Trees and shrubs | A 2 to 4 inch mulch layer, and coarse compost counts | Spread under the plant and kept off the trunk |
| Containers | 50 to 100% of the potting mix | Water more often than with peat moss or coir, and halve the fertilizer if half the mix is compost |
Amending only the planting hole of a new tree or shrub turns it into a large pot, because roots grow quickly through the loose soil and then struggle to enter the hard soil around it. The amendment therefore goes into as wide an area as possible, with at most 25% by volume mixed into the loosened soil if the soil is poor.
Compost goes into the soil as you prepare it in spring. Yard trimmings compost usually raises the nitrogen a crop needs for the first two months after application and has little effect after that, so seedlings planted right after amending may need a nitrogen feed early.
What should you check before using compost on edibles or in pots?
Home compost has an advantage over bought compost, because you control what goes in, and that reduces problems with salts, weed seeds and plant diseases. Three risks still apply before it goes near food.
- Dog and cat manures can contain harmful pathogens and stay out of the pile.
- Composted manure is preferred over fresh manure for food crops because of pathogens such as E. coli. Fresh manure that is used anyway goes on at least four months before harvest, so Colorado State applies it in fall, after the crops are out.
- Compost can carry herbicide residues from treated turf, farm fields or animal manure. They survive composting and injure the next planting.
A pot test shows herbicide residue before the compost goes into a bed.
- MixCombine equal parts potting soil and the compost you want to test.
- FillFill four pots with that mix and four pots with potting soil alone.
- PlantPlant each pot with the plant you want to grow, and give it light and regular water.
- CompareCompare the two sets of pots, since a difference between them points to herbicide in the compost.
In pots, compost can be a substitute for peat moss, but it still needs fertilizer because its nutrients release slowly and usually do not last a whole season. An aging compost mix also compresses and loses porosity, so Maryland Extension mixes last year's medium half and half with fresh ingredients or compost before reusing it.
Pro Tips
- Chop or shred leaves and stems before they go in the pile, because small pieces break down faster.
- Keep the pile as wet as a wrung-out sponge.
- Cover kitchen scraps with a layer of dry browns to keep flies away.
- Keep diseased plants and seedy weeds out of a home pile.
- Take a soil test before adding more compost each year, since over-application can create nutrient imbalances.
- Keep mulch and compost off the main stem or trunk of a plant.
- Treat compost as a soil amendment, and give a crop that is hungry now a fertilizer that works now.
Frequently Asked Questions
How long does it take to see benefits from composting?
In clay, the change in structure builds over a period of years. Because about half of the organic matter in yard trimmings compost is still in the soil after two growing seasons, regular additions keep the benefit going.
Can I use compost instead of fertilizer?
Not for a crop that needs feeding now. Compost is typically only 2 to 4% nitrogen, only 5 to 25% of it is released in the first year, and the amount varies between batches.
Does composting attract pests or rodents?
It can when the wrong materials go in. Meat, dairy and fatty foods attract pests, so bury food waste in the pile, cover it with browns to keep flies away, and consider a rodent-proof bin if rodents are a problem.
Should I add sand to clay soil instead of compost?
No. Sand worked into clay can form a concrete-like mixture that is harder to manage than the original soil, and no amount of sand turns a clay loam into a sandy loam. Organic matter is what opens clay up over time.
Should I fill a tree's planting hole with compost?
No. A heavily amended hole works like a large pot, because roots grow quickly through it and then struggle to enter the hard soil around it, and water can pool in it. Loosen a wide area and mix in at most 25% organic amendment by volume if the soil is poor.
Sources & References
- Improving Garden Soils With Organic Matter (EC 1561), Oregon State University Extension
- Vegetable Gardens: Soil Management and Fertilization, Colorado State University Extension
- Compost: How to Make It and How Much to Use, Penn State Extension
- Getting Started with Home Composting, Illinois Extension
- Composting (HGIC 1600), Clemson Cooperative Extension
- Growing Vegetables in Containers and Salad Tables, University of Maryland Extension
- Small Scale or Backyard Composting, Cornell Waste Management Institute
- Do the Rot Thing: Choosing and Using a Composting System (EM 9475), Oregon State University Extension
- How It Works, Illinois Extension Composting
