soil amendments

Soil Amendments


Healthy soil leads to clean water, drought resistance and restores habitats.  Soil amendments improve the nature of your soil. They reduce compaction, aerating the soil to allow water and nutrients to more easily move through it and reach plant roots. Some soil amendments also add nutrients to the soil and help retain moisture.

Soil is composed of particles of varying sizes. The three basic groups are sand, silt, and clay. Sand is the largest and provides great aeration but no water retention. Clay particles are very small and tend to compact, making for too much water retention and no aeration. Silt particles are medium sized and have properties in between sand and clay. Therefore the loamy soil, loose and water retentive, that most gardeners dream of is composed of the right combination of these three types of particles. By amending the soil, over time you can achieve the loose, crumbly loam that plants will thrive in.

A soil amendment can be any material that is mixed into the existing soil to improve its physical properties such as aeration, water retention, and nutrient holding capacity. The purpose is to provide a healthier environment for plant roots. Just as we eat to replenish our bodies with nutrients soil must be continually be replenished with organic matter. Organic matter decomposes into humus, which is what creates a rich, moisture retentive soil in which nutrients are available to the plants. Mulching is not the same thing as an amendment because mulches are placed on top of the soil surface and not worked into the soil.

There are two types of soil amendments, organic and inorganic. Organic refers to anything that comes from something that is or was alive such as peat, grass clippings, straw, manure, wood chips, compost, bone-meal and earthworm castings. Organic amendments have an added benefit of providing an energy source for bacteria, fungi and earthworms that live in the soil. Inorganic amendments refer to items that are either mined or manufactured such as lime, vermiculite and perlite.

Common organic soil amendments

  • Peat Moss absorbs water, slowly releasing it for use by plant roots. It lightens clay soil, providing aeration, and adds mass to sandy soil, helping prevent the leaching of nutrients. Don’t confuse sphagnum peat moss with decorative sphagnum moss, which is primarily a floral design product.
  • Humus consists of decayed organic matter. It improves fertility and aeration and helps soil hold moisture.
  • Composted or Aged Manure is an odorless farm byproduct. In addition to improving aeration and moisture retention, it enriches the soil. Dehydrated manure is a similar product that contains less moisture.
  • Topsoil is commercially produced compost that’s usually partially decomposed. Because of its rough texture, use topsoil in the yard or mixed with other products, and not as a potting soil.

Common inorganic soil amendments

  • Lime raises soil pH, reducing acidity. For plants that prefer “sweet” soil, this makes the nutrients that are already present much more available to the plants. Besides raising the pH, lime also provides calcium and trace nutrients. The constant addition of manure over time will cause your soil to become more acidic, lime is used to counterbalance this.
  • Sulfur lowers soil pH, increasing acidity.
  • Gypsum improves aeration of compacted soil, helping it drain more efficiently.
  • Perlite, a naturally occurring volcanic glass, improves aeration and drainage. It is sterile and has a nearly neutral pH so it does not alter the soil pH–an excellent amendment for heavily compacted clay soils.
  • Vermiculite improves moisture retention and aeration, therefore it is an excellent amendment for sandy soils. It also absorbs excess nutrients.
  • Builder’s sand is coarse sand that can improve soil drainage.