Soil amendments, defined as material added to soil to improve its structure or add nutrients, fall into two groups: organic and inorganic. Organic soil amendments, defined as soil amendments derived from living organisms, add organic matter to the soil. Inorganic soil amendments are either mined from the earth or manufactured. Organic soil amendments improve drainage, increase aeration and provide slow-release nutrients.
Well-rotted animal manure adds organic matter to soil that acts as both to fertilize and to improve the texture of the soil. Composted manure comes in bags but is commonly purchased by the truckload. Spreading the manure over the soil and working it into the top 6 inches increases aeration and improves drainage. It is also typically high in nutrient value, but nutrient value does depend on the type of manure used. Manure requires at least a year to decompose. Amending soil with fresh manure carries the risk of burning young roots or seedlings.
Composted yard and garden wastes create an effective organic soil amendment and may be the best organic soil amendment for your garden, according to the University of Colorado Extension. Added to the entire garden area or applied to the planting holes for seedlings, compost improves drainage, increases aeration and provides a good source of nutrients for growing plants. It also encourages beneficial microorganisms in the soil. Allow organic matter to fully decompose in your compost bin before you use it. Compost is ready when it is dark and crumbly and smells like fresh earth.
Peat moss, sold in bags at hardware stores or home improvement centers, loosens soil and increases it water-holding capacity. It is especially useful in sandy soils. When mixed into the top 6 inches of soil, peat moss improves texture and increases aeration. Peat moss is acidic and may lower the pH of your garden. When using peat moss, test your soil periodically to determine the pH, and adjust as necessary.
Decomposed leaves are referred to as leaf mold. Although many gardeners prefer to add fallen leaves to the compost bin along with other organic wastes, leaves can be composted separately and added to the garden as an organic soil amendment. Like compost, leaf mold improves texture and encourages beneficial microorganisms.