The four main elements of good rose soil are inorganic materials, organic materials, water and air. [Pay attention now... there's gonna be a test when this SRW is over. :-D ] The composition is best described as a "loam." By definition, a loam is about 50% pore space, of which half is water and half is air. The remainder of the loam is about 44 - 46% inorganic material and 4 - 6% organic material. The inorganic material is a 3-1-1 ratio (or 60%-20%-20%) of sand, sediment or silt, and clay respectively. The organic material is composed of humus, materials which have naturally decomposed.
Quite honestly, most of us don't live in a world with perfect loam. We must create it. Here are some pitfalls to watch for when building a quality rose soil.
1) Use caution when incorporating sand into heavy clay soils. Over time, this sand will settle to form a "hard pan" below the soil's surface much as if you were to put a concrete pad under the soil. This will trap water and prevent good drainage.
2) Use caution when adding organic materials to improve existing soil or create a new loam. Too much organic material will drive soil pH into the acid range, and make insoluble pricipitates of many of the nutrients needed for the roses. In acidic soils, limestone may be needed to raise soil pH.
3) Organic matter must first consume nitrogen before it can decompose. When adding undecomposed organic materials, don't forget to add some extra nitrogen into the mix. Depending on the material used, this could be anywhere from 1/4 pound to 1 1/2 pounds of nitrogen per 100 pounds of organic material added.
4) Use caution when using peat moss to the mix. The material is very slow to decompose and can drive soil pH into something akin to a Louisiana bog if used in excess.
5) When you have completed creating your soil blend, check the drainage. To do that, dig a foot-deep (30 cm) hole and fill it with water. If the water drains out in about 15 minutes, the drainage is great. If it takes longer, you need to improve the soil drainage further. If shorter, you may need to add more organic material to help retain it.
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