Plants need a few basic things from soil. Soil that provides them all will grow good plants.
- Suitable environment, including proper temperature and protection of the plant’s roots.
Air in the soil
Unlike animals, plants can’t transport oxygen from one place to another. All parts of a plant, including the roots, need to be able to absorb oxygen from the air.
When water enters the soil, either from rain or irrigation, it fills all the pores, pushing out the air in the soil. As the soil drains, air is pulled back into those pores. Within smaller pores, capillary forces—the same force that draws water from a puddle into a piece of paper—are strong enough to hold the water against the pull of gravity, so they don’t drain. Large pores drain quickly, but don’t hold much water.
Good soil has a mix of pore sizes. Large pores drain quickly to admit air, and small pores hold water until the plant needs them. Soils with only small pores drain poorly and don’t hold enough air for good root growth.
Compaction, such as from foot traffic, crushes the larger pores, sealing the soil surface. Foot traffic usually compacts only the top couple of inches of soil. If your garden has been compacted by heavy equipment, such as when the house was being built, the compaction may extend to two feet deep.
The soil is a reservoir for water. It holds water in the smaller pores until plant roots find and absorb it. After irrigation or rainfall, the reservoir is filled. Plants find a free flow of water to their roots and are able to transpire all the water they need.
As the soil dries, the larger pores empty, leaving water in only very small pores, pores with very strong capillary pull. Plants have a harder and harder time getting enough water, putting them under drought stress.
Capillary action pulls the water in the soil from wetter areas to drier areas. You can see this effect in a damp footprint in the morning. The footprint was dry yesterday, but capillary forces during the night pulled more water to the surface, wetting it. The same forces move water toward plant roots as the roots dry out the soil immediately surrounding them.
The amount of soil water available to plants—that is, the amount held in the soil from the time it finishes draining until the plants growing in it can no longer get any water—is called its field capacity. Irrigation should be timed so that you water when about half the field capacity is used up. At this point, plants are still able to get all the water they need, and are not suffering any stress from drought. Soil still appears moist at half field capacity, but doesn’t wet your finger when you touch it.
Besides water, oxygen, and carbon dioxide, plants need 13 mineral nutrients as raw materials. These nutrients come from the soil.
The soil serves as a reservoir for these nutrients, just as it does for water. The gardener’s job is to ensure that the reservoir is never depleted. Most nutrients are attached more or less firmly to the soil and don’t leach readily. The outstanding exception to this rule is nitrate, the simplest form of nitrogen available to plants. Nitrate is completely soluble, and goes wherever water goes in the soil. For this reason, nitrogen is the nutrient that needs to be added most frequently. Most other nutrients are only slightly soluble in soil water, or attach firmly to the soil, so are depleted only very slowly, over years.
Plants rely on the strength of the soil to hold them upright, and usually this reliance is well-placed. When it fails, the results can be dramatic, as when a large tree falls over in a storm. The combination of heavy rainfall—which weakens the soil’s cohesive strength—and high winds that come with storms can topple otherwise sound trees.
Trees usually fall in storms when their roots are restricted. Shallow soil—less than a couple of feet—or restricted growing areas, as when a tree is trapped between a driveway and a house, can cause otherwise healthy trees to pull their roots out of the soil and fall, sometimes with serious results.
Plants grow in two environments: their branches and leaves grow in the same air-and-light environment that we do, but their roots inhabit the soil. Like the leaves, the roots thrive within a range of temperature and moisture.
Roots “mine” the soil for air, water, and nutrients, and need a suitable medium for mining. They grow more easily and with less effort through a light, fluffy medium than they do a dense, hard one. Some soils are so dense that plant roots can hardly penetrate them at all.
Plant roots grow constantly, sending root tips into new territory all the time. Only the newest, most delicate root tips take in water and nutrients. Older roots provide the strength to anchor the plant in place, but don’t absorb water or food.
Roots need a suitable medium to explore. You can provide this by tilling the soil before planting and by adding organic matter as you plant.
Part of the soil environment is its temperature. If the soil near the surface gets too hot, roots can’t grow there. You can moderate the soil temperature by planting something on the surface to shade the soil, or by adding a mulch—or both.
Mulches act like blankets to keep the soil cooler during the day and warmer at night. The surface of bare soil in direct sun can heat to over 120 degrees during the day, hot enough to kill any plant roots growing in it. A mulch lets roots work the soil right to the surface. Indeed, if the mulch is a compatible one, like compost or rotted manure, roots will grow right into it.