Q.What is the best way to amend clay soil?
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Clay soil has its benefits: it holds water well and it is fertile. But, it is hard to dig and forms a hard crust when it dries out. When dried, water tends to run off clay instead of penetrate. How and whether to amend it to make it more like loam depends on what you are growing. Generally speaking, improving clay soil is not a one-and-done affair. If you are trying to grow a plant that requires fast-draining or sandy soil, like lavender, add several inches of both perlite and compost worked 6-8 inches down. Pea gravel can also be used if perlite isn't practical. (or choose a plant that tolerates clay soil) For most plants, adding organic matter is crucial to improving heavy clay. Add several inches of sphagnum peat moss or coir plus several inches of plant-based compost and work it in the top 6-8 inches before planting. Thereafter, add compost or composted manure annually. You need not dig it in. Just tuck it under/around perennials and let the worms pull it into the soil. In doing this the worms help break up the clay. Several inches of wood or other organic mulch also helps to prevent the clay from drying out and servers as additional compost as it breaks down. Avoid walking on wet clay soil; by compacting the soil, you go back to square one. Use stepping stones as needed. There are other products on the shelf that claim to improve clay soil. Do some research before buying as they may sound good but not be appropriate for northern Illinois soils.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm