Whether your home was built a year ago or a century ago, the soil around it may be compacted, said University of Illinois Extension horticulture educator Nancy Pollard.
"Soil compaction is the result of construction equipment, vehicles, and trampling by animals and people," she explained. "While squashed soil is a pain in the back for gardeners, it causes plants even more distress."
Seeds and roots need to be in good contact with the soil, but too much compaction presses the mineral grains together, reducing the air and water content. There is little room for roots to grow, and they cannot access enough nutrients and water. Most roots grow best in soil that is about 25 percent air and 25 percent water. Plants grown in compressed soils look stunted and drought-stressed, and they are more susceptible to disease.
It cannot be over-emphasized that soil preparation and improvement needs to be done before you plant. This is especially true for perennial plantings.
"Be sure the soil is dry enough to crumble when worked or you will make the problem worse," Pollard cautioned. "Some folks create new beds in late summer and then plant a cover crop because the soil is usually drier, and it may be easier to work, than in spring.
"At any time of year, here's how you tell if the soil is dry enough. Pick up a handful of earth and squeeze it. If the soil remains in a firm ball when pressure is released, it is too wet. Wait several days (without rainfall) before digging or tilling. If the soil ball crumbles when pressure is released, it is ready for working," she said.
If the drainage is acceptable, you may decide to break up compaction. To prepare a new bed, till as deeply as possible once the soil is dry enough, turning in organic matter such as shredded or composted leaves, peat moss, or well-rotted manure. Sawdust in moderate amounts may be added along with about a pound of nitrogen for every 100 pounds of sawdust.
"Why is organic matter so good? It improves the condition of the soil," said Pollard. "Water can infiltrate better and be held available for plant roots. The soil drains better with organic matter. It can keep nutrients from leaching beyond the reach of roots, or running off. Organic matter is not permanent; it needs to be added annually. About eight inches of organic matter will decay into about one-eighth inch of soil in a year or so."
What about adding sand?
Not such a good idea, according to Pollard.
"Research at the University of Illinois showed that homeowners would need to add eight parts of sand to one part clay to improve the soil quality," she said. "Adding less only allows clay particles to fill up between the sand particles, which actually makes the structure worse — more like cement."
If water stands in the garden area, it may be better to build raised beds. When the soil stays wet, clay particles break away and fill up pore spaces, causing the soil to collapse and become even denser.
"A good mix for filling ground-raised beds is one part garden loam and one part compost," Pollard said.