Products to loosen clay soils Gypsum, soil penetrants, and sand are often recommendedeach for a different reason. Clay soil absorbs water slowly because the clay particles are tiny. They interlock tightly, with small air spaces, so water can only penetrate as quickly as it can fill those spaces. Compaction of the surface by foot traffic, rainfall, or overhead sprinkling smashes these particles even closer together. If you try to put water on faster than it can penetrate the excess will puddle or run off. Loosening clay soils usually involves breaking the crusted, compacted surface, or amending the soil with products that increase the air spaces. Clay has high cation-exchange capacity, meaning that it readily holds on to and exchanges positive ions. Gypsum is calcium sulfate, usually sold in powder or granular form. It is a chemical buffer, meaning that it tends to cause the soil pH to move towards neutral from acid or alkaline (we recommend sulfur in this area instead). Advocates say that it "penetrates clay particles and loosens the soil structure," presumably by means of interaction of the calcium and sulfur ions with the clay particles. They caution that it works slowly, requiring annual applications over at least three years. Soil penetrants are said to work much like detergent, breaking the surface tension, which might slow down water. "Ethoxylated fatty alcohol" is an additive to the Vitamin B product mentioned above. It is hard to imagine that a material sprayed on the surface would make much difference this way. Neither gypsum nor soil penetrants will have as much effect (if any) on clay as will amending it with larger particles that increase the air spaces. Adding sand is NOT helpful. Although sand particles are larger than clay, they are still just little rocks. In fact, the clay and sand particles interlock to form a structure similar to concrete! But mixing in large amounts of organic material will make a big difference. Composted leaves or manure, or shavings (not fresh), or even fine bark all add air spaces. Even spread out on the surface, these will filter in to the soil by means of weather or worms, continuing to naturally amend the soil as they break down. Organic materials improve the soils ability to store and release water and nutrients as well as enhancing penetration. Compaction of the surface of your lawns soil frequently leads to runoff. Here you can break the surface of the soil with a mechanical aerator, and then rake in organic material. In small areas the aeration can be done with a step-on device, while machines for larger lawns can be rented locally. Its important that the device pulls a plug of soil out, rather than just punching a hole. The popular notion of "aerating" with golf shoes is not effective.