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Organic Matter and Soil Amendments

Soil Amendments

Part 2 of 4

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

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 have been heat-dried resulting in a relatively high guaranteed nutrient analysis (5-2-5).


  • 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 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.

ARTICLE Organic Matter and Soil Amendments Soil Amendments Edit

  • 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.


  • 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 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.

Cover crop Clover Edit

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 the 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.

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 the 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).


  • 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.

An excerpt from the article “Organic Matter and Soil Amendments“, courtesy of the University of Maryland. Author: Jon Traunfeld, Director HGIC, Extension Specialist, Fruits and Vegetables.

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Resources and Documents
  • University of Maryland Extension
    University of Maryland Extension (UME) is a statewide, non-formal education system within the College of Agriculture and Natural Resources and the University of Maryland Eastern Shore.

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