No perfect definition of "good soil structure" exists. After all, what's good for the sand-loving cactus would be disastrous for a bog-dwelling fern. Yet when gardeners talk about good soil structure, they usually mean loam, which contains medium-sized soil particles. Boggy, clay soils have tiny particles, while sandy soils contain large ones. To improve soil structure, or tilth, in your garden bed, work in compost or plant a legume-rich cover crop.
Let the Sun Shine in
Good soil structure gets spring growing off to a great start. The sun's rays penetrate the top several inches of soil, waking up roots and bulbs and sprouting newly sown seeds. A garden with good tilth reaches a warm temperature earlier in the spring than a garden with clay soil. That means perennial trees, shrubs and small plants leave the dormant stage and begin to add new growth, both above and below the soil, while annual crops can be planted sooner.
Room to Grow
If a plant's root system can't find room to expand, it will suffer stunted growth, poor foliage and low yield of fruits or flowers. At worst, it will die. A loamy soil allows a plant's roots to expand in the direction they need, whether straight down or horizontally. A plant's roots mine nutrients from the soil and grab water. In addition, a good root system anchors the plant into the soil, protecting its above-ground portion from extreme weather. Finally, tiny hairs on plant roots perform the vital function of retrieving oxygen from the soil and releasing carbon monoxide.
Food and Water Availability
When a soil's particles are too large--as with sandy soils--water, oxygen and nutrients drain away, leaving plants starved for all three. On the other hand, though the small particles making up clay soils tend to be fertile, they also hold water too well, leading to waterlogged conditions that kill most plants. Loam offers plants water that sticks around long enough to benefit the plant but not long enough to cause rotting, nutrients that can be taken in by the plant's roots, and enough air for the plant to function.
Loamy soil attracts helpful guests, such as plant-friendly microorganisms and earthworms. Good soil structure also makes it easy to remove unwanted guests, such as weeds.