Extension Educator, Horticulture
Over the years I've become somewhat of a soil snob. When I travel to other parts of the country, I have been known to mutter "you call that soil!" I was lucky and moved into a home with great dark rich prairie soil. With a generous pile of compost, I'm ready to garden. However I know many who are not so privileged and must continually add soil conditioners.
Soil amendments can improve water retention and penetration, improve drainage, and improve a soil's ability to retain nutrients. Many fertilizers add nutrients mainly nitrogen, phosphorus and potash to the soil, but do little to change the soil structure. Some fertilizers especially organic fertilizers such as manure may add small amounts of nutrients and serve as a soil conditioner.
I thought it might be helpful to explain a few common, but often misunderstood soil conditioners.
Gypsum - finely powdered rocks or the easier to handle pellets of calcium sulfate. Gypsum is mainly used to improve water penetration and aeration of clay soils. It helps by coaxing the fine particles of clay to stick together, so the soil has better structure. Some gardeners swear by gypsum as a soil conditioner. I'd be more inclined to add lots of compost.
Peat - Also referred to as peat moss, sphagnum moss, sphagnum peat moss and sphagnum peat. Although these terms are often used interchangeably, there are some important differences. For the most part peat is dried plant parts harvested from bogs or wetlands. The material may be partially or fully decomposed and may be light brown to black. It is light-weight, fluffy and is known for its ability to retain moisture. Some peat can be acidic depending on its source.
Sphagnum peat moss can be sold as milled or unmilled. It is the partially decomposed sphagnum moss and other bog plants from mainly Canada, but also Maine and Minnesota. With the Canadian influence most of the bags are also listed in French as Tourbe de spaigne.
Milled sphagnum peat moss is what most people think of when the hear the term peat moss. It is dark to light brown and fluffy from the mechanical milling process of harvesting. The name may not include milled but may just say peat moss or sphagnum peat moss. Milled sphagnum is good as a soil conditioner in the garden or in houseplant soils. If a gardener chooses to use peat moss, they should consider the environmental implications to harvesting non-renewable bogs and wetlands. I'd be more inclined to add lots of compost.
Unmilled peat is also known as floral sphagnum moss. It has a greenish tinge to the light brown color. It is a coarser fibrous material including small twigs and leaves. It is usually sold in small bags and is used for craft projects. Unmilled peat must be hand harvested.
Sphagnum moss is different from sphagnum peat moss. Sphagnum moss is the gray stringy stuff used in craft or floral projects. It is the moss before it decomposes to become peat moss.
Greensand is also known as glauconite. Greensand is the pulverized rock of a sandy clay material called iron potassium silicate. It contains 6-7 percent potash and contains up to 30 trace minerals, magnesium and silica. Potassium is important for disease resistance and winter hardiness.
I've often seen greensand recommended for roses as a soil conditioner and fertilizer.
If you happen to find all this fascinating or at least mildly interesting, you may want to consider the Master Gardener program. Training starts in January so don't delay. For more information, contact you local county University of Illinois Extension office or the Champaign County office at 217-333-7672.