Soil Amendments – The Basics

Definition – What does Soil Amendments mean?

Amendments are substances worked into the soil in order to encourage plant growth by improving the soil. Most do this by improving soil structure, but some change the chemical balance of the soil, or add plant nutrients.

Importance of Soil Amendments

Soil amendments help improve soil properties. In fact, most gardens have soils that are often sandy or clay-like. Poor soil may dry into hard clods that small roots cannot penetrate, causing plants to grow slowly. By adding amendments such as compost, peat moss or well-rotted manure, you supply organic materials that will decompose, releasing nutrients and improving drainage. The improved soil texture is easier for fine roots to penetrate and get established. It’s a good idea to amend the soil each season before new crops or flowers are planted.

For best results,garden soil for flowers & vegetables replaces do-it-yourself mixtures of topsoil, peat and manure. To help take the guesswork out of gardening, Some special garden soil was developed for planting all types of annuals and perennials.

How to add amendments

You can mix the amendments with the soil removed from the planting holes before replacing it around the plant roots, or spread the materials over the planting bed and till them into the soil.

For a new bed, add 1 to 4 inches of amendment depending on your soil. Use more for poorer soils, less if the soil has been regularly amended in the past. To add amendments in new beds, spread the material evenly over the soil and work it in with a spade or shovel to a depth of about 8 inches. Around existing plants simply spread amendments like mulch to avoid damaging plant roots. Over time watering, worms and microorganisms will carry it downward, gradually improving the soil.

How much soil amendments do you need

You will usually add about 1 to 4 inches of soil amendment to your garden. The chart below will help you gauge the amount you need.

A cubic foot of amendment will cover an area of 12 square feet to a depth of 1 inch. A cubic yard is 27 cubic feet.

Estimating Amounts of Soil Amendments:

Depth of Amendment To Cover 100 Square Feet
1 inch 8 cubic feet
2 inches 17 cubic feet
3 inches 1 cubic yard
4 inches 1.25 cubic yards

Types of soil amendments

Organic Amendments

Soils benefit most from the addition of organic matter, such as compost, leaf mold, sawdust, manure, and sewage sludge. As a side benefit, these amendments may contain plant nutrients; in most cases, however, they are added to improve the physical characteristics of the soil. Although some soils are naturally better than others, all benefit from regular additions of organic matter.

Gardening with organic matter is not new. Before chemists learned to make fertilizers and soil scientists discovered chemical means of making soils more hospitable to plant growth, farmers and home gardeners were working animal manures and cover crops into their soils.

Unless a garden site has been well maintained over a number of years, soil amendments will be a help in growing most vegetables, fruits, and ornamental plants. In all gardens, in order to maintain a high quality of plant growth, the soil should be amended regularly with organic matter to maintain and further improve its structure.

Inorganic Amendments

To improve garden soil and keep it in good condition, inorganic amendments may also be required. Lime, a mineral containing calcium, is used to sweeten sour soils. That’s another way of saying that it raises the pH( see Raising pH with Limestone ) by increasing the alkalinity of soils that are too acid. Conversely, sulfur in a variety of forms lowers soil pH(see Lowering Ph with Sulfur ), increasing acidity in alkaline soils.

Gypsum, a soft, slightly soluble mineral containing calcium and sulfur, is a valuable amendment for improving sodic soils (those containing sodium). The calcium in gypsum displaces the sodium in the soil and improves the structure and drainage of sodic soils.

Fertilizers as Amendments

Although not thought of as soil amendments, fertilizers should be considered whenever soil improvement is contemplated. In general, fertilizers are soil amendments with more than a few percent of plant nutrients. However, they do more than just supply plant nutrients.

Organic fertilizers like blood meal and fish emulsion add organic matter as well as nutrients to soil. Most fertilizers contain calcium, so tend to raise the pH of the soil. Some, called acid-reaction fertilizers, make the pH lower, or more acid.

Many amendments are also fertilizers. Compost and manure usually contain substantial amounts of nutrients, and may be all the fertilizer you need. For more information about fertilizers, see Feeding Plants.

Nitrogen Draft

Amendments that are deficient in nitrogen not only don’t add nutrients, they can temporarily rob the soil of nutrients while they decompose to humus. This effect is called.