If you’re trying to shape up the soil in your garden, you’ve likely headed for the garden center to pick up some bags of soil amendments to mix into the existing soil. There are plenty of different soil amendments to choose from, each with its own particular properties. Some soil amendments add nutrients while others add texture, some increase drainage while others help the soil retain water.
Here’s a handy guide to some of the most popular soil amendments to help you decide what’s right for your garden:
Compost: (permeability, water retention, nutrients) Well-rotted compost is one of the best soil amendments you can add to your garden, and it’s impossible to use too much! Compost is highly fertile and lightens heavy soils while providing structure to loose soils. It also possesses the amazing ability to help soil retain much needed water while improving drainage – making compost great for both clay and sandy soils. Compost lasts about six months but has a long-term impact on soil microbes and overall fertility. Continue to add compost, and watch your soil get better and better.
Greensand: (provides permeability, nutrients) This powdered rock material provides slow release nutrients, such as potassium and other trace minerals. It also helps loosen compacted soil.
Gypsum: (permeability for saline soils) While powdered gypsum is considered an amendment for clay soil, it’s actually more important for saline soils where it loosens the clay by reacting chemically with the salts.
Leaf Mold: (water retention, texture, nutrients) Made from shredded, composted leaves, leaf mold holds moisture, loosens soil, provides organic matter, and breaks down slowly to improve overall soil texture. Leaf mold also encourages beneficial microbes that help transfer nutrients to plant roots.
Manures: (texture, nutrients) Composted manures are affordable, readily available soil amendments. Like other forms of compost, manure provides the right balance of drainage and water retention. Manures are high in nitrogen and other nutrients, so they’re particularly useful as slow release fertilizers. Manures can add salt to the soil, so they may not be suitable if your soil is already saline. Use only well rotted manure to prevent burning due to high nitrogen content.
Peat: (water retention) Peat, particularly Sphagnum peat, is a lightweight spongy material that’s great for making sandy soils more water absorbent. Peat will also loosen heavy clay soils, but you need to be careful it doesn’t make the soil too soggy. Peat decomposes slowly and is slightly acidic. Look for peat that’s harvested from sustainable peat bogs.
Perlite: (permeability) This light, volcanic material increases soil permeability and drainage and helps with aeration. Perlite can be crushed under repeated foot traffic or equipment weight, so it’s most commonly used as an additive in potting soils, to make the soil very light for fragile plant roots.
Sand: (permeability) Sand is usually used to loosen heavy clay soils, but you have to add about 50% sand to the clay to achieve good results! Adding smaller amounts of sand will actually make the problem worse by filling in the clay pores and setting up like concrete. Choose the coarsest builder’s or horticultural sand you can find, and avoid play or craft sands. For best results, add a mixture of sand, compost, bark fines, and other organic materials – rather than sand alone – to lighten the clay.
Shredded Bark or Wood Chips: (permeability) Shredded wood or bark chips (sometimes called “fines”) loosen soil to increase permeability and drainage. Bark or wood chips are good for breaking up clay soil, but their slow breakdown rate makes them less helpful in the nutrient department. Wood products use up nitrogen as they decompose, so they’re often accompanied by nitrogen rich compost, manure, or fertilizers.
Vermiculite: (permeability, water retention) Vermiculite lightens soils much like perlite, but it also help soil hold moisture. Vermiculite can make clay soils too soggy, but it can increase the water retention of sandy soils. It’s also great for containers.
While these amendments are good for general soil improvement, there are many other amendments that are used in smaller quantities to add specific nutrients; including bone meal for phosphorus, alfalfa meal for nitrogen, and wood ash for potassium.