There is information on specific techniques for improving the drainage of your topsoil below.
On using gypsum to improve soil drainage "What does Gypsum do? It's main purpose is to penetrate the many clay particles in heavy or the layer of hard subsoil type soils and loosen the soil structure. Then this creates air and moisture slots that will loosen and break-up the soil structure."
On improving clay topsoil "in clay soil, use plenty of organic matter and horticultural grit before planting to improve soil structure and drainage"
On using lime to improve soil drainage: "Lime has a tendency to chemically aggregate soil – that’s good. It can help open up the soil structure and make it more permeable. Lime is inexpensive. If lime is added, it can be sprinkled on top of the finished trench and gravel and allowed to leach down and chemically aggregate the bottom of the trench."
On top dressing a lawn with sand: "The final stage needs to be carried out over a number of months. It involves top dressing the lawn with a washed sand over a number of months to build up a carpet of sand at the base of the grass plants to allow the rainwater to move freely on the surface and down the sand slits into the drain."
On using sand in micro drainage trenches: "I'd like to add that sand can add benefits in small areas. For some drains, I've cut "V" shapes slits a few inches deep in areas of water puddles. These slit trenches run from the trench to the perimeter of the puddle. I fill these slits with sand only. Where the slits meet the main trench; the trench is topped with sand, with fabric well-placed to prevent the sand from washing into the French drain. I do this often in lawns, and let the lawn grass spread into the slits. The sandy slits still allow water to move and grass to grow. These are what I refer to as "micro-drainage" or micro-managing a drain line."
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