soil compost

Compost (dark brown material) mixed with soil to create planting mounds for squash. The area is covered with residues of a cover crop that was cut down in late April.

Feb 11, 2021

Key points

  • Organic matter includes plants and animals that are alive, dead, or in some stage of decomposition.
  • Organic matter is a major contributor to soil health. Most garden and landscape plants benefit from increases in soil organic matter.
  • Soil amendments are materials applied to or mixed into the topsoil to change or change soil properties and improve plant growth. For example, compost improves soil structure and lime increases soil pH. Most soil amendments supply some plant nutrients and some compost and manure products double as fertilizers with a guaranteed nutrient analysis.

Organic matter 

  • Soil organic matter (OM) is made up of living, dead, and decomposing plants, small animals, and microorganisms. Materials we think of as dead, like brown, dried up leaves or banana peels, are teeming with microbial life. There can be a billion microorganisms in a teaspoon of compost or soil!
  • Adding organic matter improves soils high in clay or sand.
  • Soils high in OM retain more moisture, have a crumbly structure that resists soil compaction, and contain a reservoir of nutrients that are slowly released over time.
  • OM improves soil aeration, water drainage, root growth, and biological activity.
  • Compost and pine bark fines are good peat moss substitutes (a mined and non-renewable natural resource).
  • Most garden and landscape plants perform best when the soil organic matter level is at least 2% (the goal for vegetable and flower beds should be 5%-10%). These soils are loose, easy to prepare for planting seeds and plants and have a large number of earthworms.
  • Organic matter is measured by weight, not volume. Most soil testing labs include the organic matter test in their basic soil test.

Soil amendments

  • Soil amendments are applied to or mixed into the topsoil to improve soil properties and plant growth.  
  • Practice sustainable gardening by using no-cost or low-cost amendments such as locally available manure and compost and "home-grown" compost, leaves, grass clippings, cover crops, and kitchen scraps.
  • pH adjusters (lime and sulfur) can be found on the Soil Testing page.
  • Listed products are only examples and not endorsements. Read all product label instructions before you open the bag.

Common Soil Amendments and Sources of Organic Matter

Biosolids

U.S. EPA Class A "Exceptional Quality" biosolids (composted sewer sludge) are allowed for use around all types of garden plants. Examples:

  • BLOOM A product of Washington, D.C. biosolids
  • Milorganite A product of Milwaukee biosolids that has been heat-dried resulting in a relatively high guaranteed nutrient analysis (5-2-5). 

Biochar

  • This is a relatively new soil amendment for the U.S. Research studies are attempting to determine biochar’s effects on soil carbon storage, soil reclamation, and improving the nutrient and water retention of soils.
  • Biochar is charred organic matter, made by burning biomass such as wood waste and agricultural residues in the absence of oxygen (pyrolysis).
  • The end product is a fine-grained charcoal that is stable (resists further decomposition), porous, and variable depending on the feedstock and the process used.
  • Although low in nutrients, biochar can hold nutrients that might otherwise be lost to leaching or runoff.
  • Commercial products are available for gardeners and farmers but their value, relative to the many other ways of increasing soil organic matter, has not yet been established.

Compost

  • Home-made compost or purchased compost can be added at any time of year and can be used as a top-dressing or mulch during the growing season.
  • To improve the soil where trees, shrubs, and herbaceous perennials are planted, remove mulch, spread an inch of compost over the area, and move the mulch back in place.
  • Incorporate 2-4 inches of compost into new plant beds that are high clay or have thin topsoil.
  • Organic matter moves downward through the soil profile and is continuously used up through oxidation. It should be replenished each year in flower and vegetable beds. Just 1 inch of compost per year can help maintain garden productivity.
  • 8.33 cubic feet of compost will cover a 100 sq. ft. garden to a depth of one inch; 3 cubic yards of compost will cover a 1,000 sq. ft. garden to a depth of one inch.
  • One cubic ft. of compost weighs about 40 lbs. And one cubic yd. weighs about 1,100 lbs. This will vary depending on moisture content.
  • Plant-based composts have an N-P-K analysis of approximately 1.0-0.5-1.0. Only 5%-10% of the N (nitrogen) is mineralized (plant-available) in the year of application. Most of the K (potassium) and a small percentage of the P (phosphorus) are available in the first year.
  • Manure-based composts are higher in nutrients and more of the N and P is in an inorganic, plant-available form.
  • Make your own backyard compost from leaves, grass clippings, kitchen scraps, and other materials. Every gardener can recycle at least some of their organic waste into compost and keep it out of the landfill.
  • Purchase compost
  • Commercial composts are made from a wide variety of organic materials such as agricultural and food wastes, animal manure, grass clippings, and leaves. Many commercial composts are made using U.S. Composting Council guidelines and are regularly tested for content, quality, and contaminants.
  • The risk from pesticide residues in commercially available compost is extremely low. Herbicides are short-lived in soil and compost and rarely show up as a problem. Producers and sellers have their composts tested regularly and should be able to provide result reports. However, a few long-residual herbicides (aminopyralid and clopyralid) have contaminated manure and commercial compost in recent years.
  • Purchase compost by the bag or cubic yard (pick-up truck size load). LeafGro® is an example of a commercial compost made from yard waste and food scraps. Some county/city solid waste and recycling units make and sell compost for pickup. Check with your local agencies.

Compost tea

  • Made by "steeping" compost in a bucket of water (5 parts water to 1 part compost by volume) for 1-3 days, then straining and applying the liquid to plants. Make compost tea using plant-based compost or vermicompost (worm compost). Do not use compost made from animal manure. Compost tea is low in a wide range of nutrients and good for fertilizing seedlings and transplants.

Cover crops

  • Growing plants year-round is a great way to store carbon in the soil and support the soil food web. Cover crops reduce soil erosion and recycle nutrients which can reduce the need for fertilizers. They can also be used as a mulch after cutting and are an essential part of no-till farming and gardening.

Corn gluten

  • Has a relatively high N organic fertilizer (10-0-0) content and is also labeled as a preemergent herbicide. However, it is not recommended for use on Maryland lawns as an organic preemergent herbicide because the recommended rate for weed control would exceed the amount of nitrogen allowed by Maryland's Lawn Fertilizer Law.

Epsom salt

  • A highly soluble form of magnesium (10%) and sulfur (13% sulfur). It should not be applied to soil unless recommended in a soil test report. Epsom salt will not prevent or reverse blossom end rot of vegetables. A shortage of calcium causes the cell wall breakdown resulting in the sunken, brown/black areas on the bottoms and sides of fruits. Excess magnesium can make the problem worse by making calcium less available for plant uptake.

Gypsum

  • A mineral that does not affect soil pH. It is approximately 23% calcium and 18% sulfur by weight. The calcium is quickly available making it a good choice for mixing into soil to prevent blossom end rot in vegetable crops. Despite the "clay-buster" claim on product bags, gypsum does not improve the structure of clayey soils in Maryland. Gypsum can be applied to soil at a rate of ½ lb. per square foot to prevent salt injury to plants from de-icing salts and salt spray (removes sodium from the soil).

Manure

  • Animal manure has a higher plant-available nutrient content than plant-based or manure-based compost. Poultry, sheep, and rabbit manure are higher in nutrients than cow or horse manure.
  • Horse manure, even if "aged," may contain many viable weed seeds.
  • Lightly incorporate manure into soil to prevent nutrients from washing away or volatilizing.
  • Never add dog or cat manure to your compost pile or vegetable garden soil.
  • Bagged manure products are usually composted or dehydrated (17% moisture) and often carry an N-P-K fertilizer guarantee on their label.

Reduce Human Pathogen Risks

  • Fully composted manures can be applied to the soil at any time of the year. Manures are considered fully composted when a static, aerated pile reaches at least 131ºF. for 3 consecutive days. This kills most plant and human diseases. Temperatures >145 ºF. will kill weed seeds. Not all farmers actively manage and monitor their manure to this standard. So, it's best to treat any local animal manure as un-composted and add it to gardens in the fall.
  • You can safely mix un-composted manures (ranging from fresh through well-decomposed) into the topsoil in fall and then cover the soil with mulched leaves, if possible, to reduce leaching, run-off, and erosion risks.
  • You can also spread and incorporate un-composted manure during the growing season at least 90 days prior to harvesting crops with edible parts off the ground (pepper, tomato, corn, eggplant) and 120 days prior to harvesting crops with edible parts touching the soil (leafy greens, root crops, bush beans). These guidelines reduce the likelihood of un-composted manure contacting your food. And, of course, always wash produce prior to fresh eating or preparation.

Mulch

Organic mulches, including tree leaves, grass clippings, straw, wood bark, and wood chips, decompose and contribute to soil organic matter. Using organic materials available from your yard or neighboring yards keeps them out of landfills and recycles nutrients from plants to soil and back to plants.

  • Mulched or shredded leaves rot faster than whole leaves and are an excellent substitute for wood or bark mulches. They can also be spread on vegetable and flower beds to protect soil over the winter. Move the leaves to plant in spring and then re-apply them as mulch.
  • Grass clippings (no herbicides) can be used as a mulch around plants or added to your compost pile.
  • Leave plant roots in the soil to decompose by cutting off the tops of annual plants when their season is over.

Mushroom compost

  • Compost from mushroom farming is made from manures, wheat straw, corn cobs, feathermeal, peanut meal, peat moss, lime, etc. Once the mushrooms are harvested the compost they grew in is removed and sold. The approximate N-P-K analysis is 2.75-1.5-1.5. It can have high soluble salt levels and should be watered prior to planting to leach excess salts.

Mycorrhizae

  • Beneficial fungi that occur naturally in soil and grow symbiotically on plant roots (ectomycorrhizae) or in plant roots (endomycorrhizae). It's estimated that 80% of all land plants on Earth are colonized by endomycorrhizae.They extend the root system by sending out tiny filaments to forage for water and nutrients used by plants. They have been shown to also help plants fend off pathogens. Some crops, like blueberry, rely heavily on mycorrhizae for nutrient uptake. These fungi are prevalent in Maryland soils. Research does not show any benefit to garden plants when mycorrhizae are purchased and applied to soil. This is especially true when plant nutrients and soil moisture are plentiful, and also because commercial mixes contain a narrow subset of mycorrhizal species. You can increase the populations of beneficial fungi through plant diversity (especially native plants), reduced soil disturbance, and planting cover crops.

Peat moss

  • Is partially composted moss mined from prehistoric non-renewable bogs. Peat moss is light and porous, absorbing 10-20 times its weight in water. Contains little nutrient value, but has a high nutrient-holding capacity. More sustainable options like compost and pine bark fines should be substituted when possible.

Pine bark fines

  • Finely shredded pine bark product that retains moisture. Sometimes a component of soilless growing media. It can be incorporated into annual and perennial beds. Very acidic, so watch soil pH levels if large quantities are used.

Sand

  • Can be used to improve soil for succulents, but a minimum of 50% by volume is necessary. Use only coarse builder's sand, not play sand. Adding organic matter, not sand, on a yearly basis is the key to improving clayey soils.

Sawdust

  • Only well-decayed sawdust should be incorporated into the soil. Fresh sawdust can burn plant roots and "tie up" nitrogen as it decomposes. (Soil microbes that break down the high-carbon sawdust need nitrogen and can access available nitrogen more easily than plant roots.) Good for mulching blueberry beds.

Topsoil

  • There are no quality standards for topsoil and it is not a regulated product in Maryland. If you plan to buy topsoil in bulk, go to a reputable nursery or topsoil dealer and ask for soil test results and information on the origin of the soil, on-site mixing, and storage practices. Examine the soil before purchase or delivery. Topsoil should be dark and crumbly with an earthy smell. Do not purchase soil that is very high in sand or clay, foul-smelling, or has grayish mottling or a chalky, sticky, or rough texture. Some businesses sell a topsoil/compost mix which can make an excellent growing media for raised beds.
  • The bags of "organic soil" you may see for sale typically don't contain topsoil (i.e. mineral soil with clay, silt, and sand particles). These products contain various combinations of wood waste, bark fines, compost, peat moss, and other organic materials.

Wood ashes

  • Ashes from wood and pellet stoves contain large amounts of potash (10%) and calcium carbonate (25%). For liming purposes, two pounds of wood ash is equivalent to approximately one pound of calcitic limestone or dolomitic limestone. Use ashes based on soil test results and don't exceed 20 lbs./1,000 sq. ft. per year. Apply wood ashes in the fall or winter. Dispose of excess ashes in the trash, not the compost bin.

Worm castings

  • Worm 'poop' produced by red wiggler (Eisenia fetida) and other earthworms that is rich in nutrients and microbes. Commercial castings are produced by vermicompost businesses that use worms to convert organic materials into compost in a controlled environment. Vermicompost is a combination of worm castings and partially decomposed organic materials. You can produce your own vermicompost in a homemade or purchased bin.
Author: Jon Traunfeld, Director HGIC, Extension Specialist, Fruits and Vegetables.
11/2020