Using Compost in the Home Garden
- Compost products
- Application rates and salt problems
- Nitrogen release is slow
- Beware of unfinished compost
- Weed seeds and diseased plants
- Pet manure
For information on home composting see CSU Extension fact sheet #7.212, Composting Yard Waste.
Home made or commercial compost is a great source of organic matter for the garden. Compost provides a food source for beneficial soil organisms, enhancing the soil food web and releasing nutrients over the long term.
A home compost bin is an environmentally sound method to convert yard wastes into a valuable soil-building resource. Using compost has also been found to suppress some soil borne plant disease pathogens in special situations.
Home compost has the advantage that the gardener controls what goes into the compost pile and can avoid weed seeds, diseased plants, and salt problems.
There are many bagged compost based products available in the retail trade. They could be any combination of plant residues, manure, and/or biosolids. Some products also have added rock minerals or animal by-products. These bagged products will often be three times as expensive as manure and compost supplies available in bulk. They provide a long-term release of nutrients and add organic matter to soils, improving soil physical properties.
In Colorado, compost is unregulated. Materials sold as “compost” could be anything (plant materials, manure, biosolids, animal by-products, etc.) and could be at any stage of decomposition. Not all “composts” are good for the soil.
General application rates for compost are based on the salt content of the compost and soil and on the depth to which it is cultivated into the soil. Ideally, cultivate the compost into the top six to eight inches of the soil. On compacted/clayey soils, anything less can lead to a shallow rooting system with reduced plant growth, lower vigor, and lower stress tolerance.
Table 1 gives standard application rates for compost. Compost made solely from plant residues (leaves and other yard wastes) is basically free of salt problems, and higher application rates are safe.
Routine Application Rate for Compost
Incorporation Depth 2
Depth of Compost Before Incorporation 1
and other compost know to be low in salts 3
Compost Made with Manure or Biosolids for which the salt content in unknown 4
|One-time application - such as lawn area to be planted||
|Annual applications to vegetable and flower gardens - first three years||
|Annual applications to vegetable and flower gardens - fourth year and beyond||
1. Three cubic yards (67 bushels) covers 1,000 square feet approximately 1 inch deep.
Compost, that includes manure or biosolids as a component, has a potential for high salts. Excessive salt levels are common in many commercially available products sold in Colorado. On compost made with manure or biosolids, application rate is limited unless a soil test on that batch of product shows a low salt level. An amendment with up to 10 dS/m (10 mmhos/cm) total salt is acceptable if incorporated six to eight inches deep in a low-salt garden soil (less than 1 dS/m or 1 mmhos/cm). Any amendment with a salt level above 10 dS/m (10 mmhos/cm) is questionable.
Note: dS/m or mmhos/cm is the unit used to measure salt content. It measures the electrical conductivity of the soil.
Compost needs to be thoroughly mixed into the upper six to eight inches of the soil profile. Do not leave compost in chunks, as this will interfere with root growth and soil water movement.
As the soil organic content builds in a garden soil, the application rate should be reduced to prevent ground water contamination issues. A soil test is suggested every four to six years to establish a base line on soil organic matter content.
Typical nutrient content includes 1.5% to 3.5% nitrogen, 0.5% to 1% phosphate, and 1% to 2% potash, plus micronutrients. Thus compost is more of a soil conditioner than a fertilizer. In gardens where compost is routinely added, phosphorus and potassium levels are likely to be adequate.
Like in other organic soil amendments, the nitrogen release rate from compost will be very slow, (i.e., over a period of years). When the organic content is below 5%, additional supplemental organic or manufactured nitrogen fertilizer may be needed.
- 5% Organic Matter – Soils with 5% organic matter from compost will mineralize (release to plants) about 0.2 pound of nitrogen per 100 square feet per year. This should be sufficient for plant nitrogen needs.
- 3% Organic Matter – Soils with 3% organic matter from compost will mineralize about 0.1 pound of nitrogen per 100 square feet per year. Additional nitrogen fertilizer will be needed for high nitrogen crops like broccoli, cauliflower, cabbage, potatoes, and corn.
- 1% Organic Matter – In soils with 1% organic matter or less, the release rate for nitrogen will be too low to adequately provide the nitrogen needed for crop growth. A supplemental organic or manufactured nitrogen fertilizer may be needed.
Finished compost is dark and crumbly, does not resemble the original contents, and has an earthy smell. Compost that has not thoroughly processed could be “hot” with high ammonia content. This could burn plant roots (when applied to the soil) or plant leaves (when applied as a mulch). If the compost smells like ammonia, it should be processed longer or be worked into the soil at least one month prior to seeding or transplanting in the area.
Compost maturity can be assessed in a laboratory by measuring the carbon dioxide production by the microorganisms living in the material. Lower levels of carbon dioxide indicate more mature compost (i.e. microbial activity is low because they have used the available nitrogen to decompose the carbon in the compost). Conversely, if microbes are producing carbon dioxide, they are consuming oxygen. Unfinished compost can consume all of the oxygen from the root zone and greatly inhibit root growth. Finished compost should smell earthy, like healthy soil, not like ammonia.
When making compost at home, it is advisable to turn the pile when temperatures drop below 120°F and when temperatures exceed 145°F. To encourage active microorganism processing, moisten the pile so that it feels like a wrung-out sponge. When temperatures do not rise above 120°F after turning to reheat, compost has entered its curing stage. It should cure for 45 days before being considered finished. This curing period allows nitrogen and other chemical constituents to stabilize into forms suitable for placement around plants.
It is advisable not to compost diseased plants or weeds loaded with seeds. If the compost pile did not heat adequately or was not turned, the compost could be a source of weed seeds or plant disease pathogens. All parts of the compost should reach 145°F to kill weed seeds and plant disease pathogens. Because only the inner layers of the pile will reach this temperature, it is important that the outer layers are folded into the inner layers and the pile is allowed to reheat to 145°F. These temperatures must be maintained for at least 3 days. Temperatures of 130°F will somewhat minimize weed seeds and pathogens.
Livestock manure (horse, sheep, cow, swine, etc.) can also be a source of weed seeds in compost if the animals were fed hay with weed seeds or if seeds blew into a pile of manure.
Do not add companion animal (cat, dog, etc.) feces to compost as this increases the incidence of nuisance animals rummaging through the compost pile and disease transmission to humans.
CMG GardensNotes on fertilizers
CMG GardenNotes on soil amendments
- Cover Crops and Green Manure Crops, #244
- Making Compost, #246
- Soil Amendments, #241
- Using Compost in the Home Garden, #243
- Using Manure in the Home Garden, #242
CMG GardenNotes on soil management
- Asking Effective Questions about Soils, #251
- Earthworms, #218
- Estimating Soil Texture, #214
- Introduction to Soils, #211
- Iron Chlorosis, #223
- Managing Soil Tilth, #213
- Mulches for the Vegetable Garden, #715
- Mulching with Wood/Bark Chips, Grass Clippings and Rock, #245
- Soil Compaction, #215
- Soil Drainage, #219
- Soil pH, #222
- Soil Tests, #221
- The Living Soil, #212
- References and Review Questions: Soils, Fertilizers and Soil Amendments, #210
- Homework: Soil, Fertilizers, and Soil Amendments, #253
- Worksheet: Soil Texture and Structure Lab, #252
- CMG GardenNotes are available online at www.cmg.colostate.edu
- Colorado Master Gardener/Colorado Gardener Certificate Training is made possible by a grant from the Colorado Garden Show, Inc.
- Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating
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- No endorsements of products mentioned is intended nor is criticism implied of products not mentioned.
- Copyright. 2011-2014. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.
Revised October 2014