Never Add Clay to Sand or Sand to Clay
Adding sand to clay soil, in any amount, has been proven by the University of California Agricultural Extension and the U.S. Department of Agriculture to be about the worst thing anyone can do for their garden soil. Adding organic soil amendment, such as horse manure, to name just one possible amendment, is a worthy suggestion, especially if the manure has been aged or composted for three months by layering it with rotted alfalfa hay. However, if the manure is salty this can be a negative factor in clay soil since it’s difficult to wash out the salts. In general, the safest method for amending alkaline clay soil is to work in all the pre-nitrolized or fully composted organic amendments one can get one’s hands on or, when impossible to dig them in, then use them as mulch on top of the ground. Plant roots will also eventually help break up soils.
Southwest gardeners need to know that when poor drainage in clay soil is caused by alkalinity, one of the best things one can do is to apply soluble gypsum according to package directions. Organic gardeners prefer using rock gypsum which has not been altered by chemical processing. However, the honest truth is the soluble type works best. Gypsum breaks up clay soils that are alkaline by releasing soluble calcium which replaces some of the sodium on the clay particles and thereby produces a more open soil structure. Work approximately half a coffee can full of gypsum into the earth on the bottom of each planting hole and also broadcast gypsum on the ground surrounding plants in the established landscape so it looks as if a light snow had fallen. Do this at least once every three years and water it into the ground. The action of gypsum does not last forever, so repeated applications are necessary. Gypsum is a relatively inexpensive amendment. It will not hurt your garden soil and may help a great deal.
All soil experts will tell you one of the first rules of amending soil is “never tamper with the structure of your soil.” A soil’s “structure” means the arrangement of the sizes of particles in it and their relationship to one another. All soils are categorized according to their texture and the 4 basic soil types are clay, sand, silt, and loam. Clay is a firm, fine-grained earth containing a large amount of tiny mineral particles less than 0.002 millimeters in size that have negatively charged surfaces. Clay feels slippery and sticky when wet. Sand is a loose earth mainly composed of tiny particles of rock between 0.05 and 2 millimeters in size. It feels scratchy or gritty to the touch. Silt is a fine-grained earth made up of rounded, weathered particles. Loam is a combination of the other three (clay, sand, and silt) in varying ratios, the optimum being equal quantities.
Since loam is a naturally occurring mixture of clay, silt, and sand and is widely considered the best garden or agricultural soil, many people, sadly including some misinformed garden writers, have erroneously supposed that all you need to do to get loam is to mix the other three together, or perhaps only clay and sand, and that—Voila!— you will end up with loam. Unfortunately, this doesn’t work. The fact is that adding sand to clay soil in any amount is an extremely dangerous thing to do. You end up with something akin to concrete. The same is true of adding clay to sand. In either case, the fine clay particles will fill in the larger spaces between the sand particles, thus permanently destroying drainage. In a garden of clay soil, it would be impossible to add sand in sufficient quantities to approximate a natural soil made up of a mixture with a ratio that is more sand than clay.
Gardeners often confuse lack of drainage due to clay soil with the word hardpan, but actually these are different condition. Clay is a type of soil, but hardpan is a condition in which a layer of soil that does not drain lies on top of the soil or is buried under topsoil that drains. Hardpan is a layer of hard, compacted soil of any type cemented together by minerals and almost impenetrable to roots or water. Hardpan is often made of clay but it is not the same as clay soil.
Caliche is yet another type of soil sometimes referred to as hardpan by gardeners. Caliche is a crust of calcium carbonate, usually white or gray in color that forms on the stony soils of arid regions such as found in the Southwest. Hardpan and caliche may be buried under the ground, as in housing developments where it is sometimes covered with a layer of topsoil, or it may lie on top of the ground. In some cases the hardpan may be only be a foot thick in which case one can use a crow bar to break through it to a lower layer that drains, thus creating drainage in the bottom of planting holes. In other cases hardpan may be many feet thick. The best way to combat a clay-based hardpan is to plant in raised beds and terraces filled with amended topsoil, or to make frequent additions of soluble gypsum and a variety of organic soil amendments as explained above.