Plant gardening in clay soil has it own set of challenges. The fine mineral particles in clay soil pack together, which makes it drain slowly. Roots have difficulty penetrating clay, so they become stunted. Freezing and thawing can cause clay to buckle, while heat can cause it to harden and crack. Wet clay becomes sticky and is difficult to work with. However, there ways to improve clay soil to make it better for gardening.
Fill a quart jar half full of soil and half full of water. Put the lid on the jar and shake it vigorously. Set it aside. Sand is heaviest and will settle in a few minutes. The silt will settle next. Clay particles will settle in one or two days. To be classified as a clay soil, 50 percent of the particles should be clay.
Fill a hole that is 1 foot wide and 2 feet deep with water. Drain and refill. If it now drains in less than 12 hours, the soil drains well. Clay soil takes from 12 to 24 hours to drain. If the soil takes more than 24 hours to drain, you can grow trees in it but not garden plants.
Breaking Up Clay
Use a spading fork rather than a shovel to dig up clay. Break up the clods with the side of the fork and let them dry in the sun. When the clods are dry, soften them with a fine spray and rake them into smaller particles.
Incorporating organic matter into the clay will improve its structure. Compost, green plant material, decomposed leaves and manure make good amendments. Organic amendments should be coarse, not fine. Less desirable are amendments that decay slowly including peat moss, sawdust, shredded bark, straw and rice hulls; they compete with plants for nitrogen. If they are used, extra nitrogen should be applied. Gypsum can be used to amend soils that are alkaline, having a pH of more than 7. Sand can be used to amend clay soil but it should be coarse sand; fine sand can make clay pack even harder.
Leach manure compost with water to remove excessive salts that can injure plants. Spread the amendments 4 inches deep on top the clay and work it in 8 inches deep. Amend large areas so plant roots won't suddenly run into impenetrable clay barriers.
Find the flare of the roots where they grow from the stem or trunk of the plant; this is 1 or 2 inches below the top of the container or root ball. Dig a bowl-shaped hole as deep as the distance from the root flare to the bottom of the roots. It should be twice as wide as the root ball. Refill the hole adding soil amendments to the top 6 inches. Adding gravel to the bottom of the hole will make the clay above the gravel hold even more water. Plant large plants with the flares 2 to 3 inches above the soil; flares of container plants should be 1 to 2 inches above the top.
Automatic watering systems can drown landscape plants.The top few inches of clay soil may be dry, but it could still be wet where the roots are. Water your plants carefully to avoid moisture collecting on the clay beneath amended soil. Use organic fertilizers in clay soil.
Raised Beds and Mulch
You can improve the drainage of clay soils by building raised beds. If you can reach the middle of a raised bed without stepping on it, the clay soil won't compact as much. Winter mulch will soak up the water and prevent the clay from packing down. It will also reduce erosion and keep weeds from growing.