Some gardeners are downright contentious about the word soil, insisting that it’s not the same thing as dirt. Soil, they insist, is the stuff in your garden; it’s what you grow plants in. Dirt is what you wash off your hands or
sweep under the rug.
Soil. Dirt. Even planting medium. It’s the place roots call home. Call it what you want. The gardener’s secret is never to treat soil like dirt. Savvy gardeners continually improve their soil. It doesn’t matter how long you’ve been growing herbs and other plants: Garden soil is always a work in progress. This chapter is all about soil and what it takes to get it ready for planting.
What Plants Need from Soil
Soil anchors plants to the earth and supplies the oxygen, water, and nutrients that they need to live. Good garden soil, according to the professionals, consists of about 25 percent air, 25 percent water, 45 percent mineral particles,
and 5 percent organic matter.
That’s right — although most people think of soil as a solid, about half the volume of a healthy soil is actually made up of air and water! Picture a glass filled with marbles; the spaces between the marbles are like the spaces between soil particles. Plant roots grow in these spaces — the same passageways through which air, water, dissolved nutrients, and soil organisms travel.
Soil provides plants with much of what they need to survive and grow, including air, water, and nutrients:
Air to breathe: Plants need oxygen, and they absorb some of it through their roots. A few plant species thrive in ground so wet that it contains almost no air. That extra moisture may be okay for watercress, but not for most herbs (or for many of the beneficial macro- and microorganisms that live in your soil). If the roots of most herbs sit for too long in saturated soil — soil in which the spaces between the particles are filled with water — the roots will die, and when the roots die, the plant dies, too. So one of your goals in preparing the soil for an herb garden bed is to make sure that water drains well.
✓ Water to drink: Most plants are about 90 percent water (which is why plant leaves become limp during a drought). And most plants need a fairly constant supply of water, especially during hot, dry weather. So although you want water to drain from the soil after watering or a heavy rain, you don’t want it to drain so quickly that plants are left thirsty. Another one of your goals, then, is to make sure the soil retains some water.
✓ Nutrients for healthy growth: As roots take in the water they need, they also take in the nutrients dissolved in that water — nutrients that the plants need for healthy growth. Some of these nutrients are leached into the water from minerals in the soil; some may be from fertilizer you’ve applied to the soil (more on fertilizing in Chapter 9). Water must be present for plants to take up nutrients.
Particles of rock make up most of the solid portion of garden soils. Soil scientists classify soil separates by their size , beginning with boulders any rock that measures about 10 inches across. That measurement
may sound small to you if you thought a boulder was something big enough to sunbathe on. But those of us with lots of these boulders in our gardens refer to them as “those #%*!! rocks.”
Progressively smaller in size, technically speaking, are stones, pebbles, and gravel, and we hope these items are scant in your garden. Smaller yet are sand, silt, and clay, and these particles constitute the mineral component of garden soil. Although most soils contain a combination of these particle sizes, often one size predominates. Here’s a rundown of the characteristics of these soil particles:
Sand: Sand particles, which can be fine or coarse, are the largest of the three, measuring from 0.5 to 2 millimeters across. You can see them clearly with the naked eye. Gardeners with sandy soil, which feels gritty, often call it light soil because it doesn’t get saturated and soggy and is easy to cultivate whether wet or dry.
Because sand particles are relatively large and angular or round in shape, they don’t cling together closely, leaving space for water and air to move between the individual particles. As a result, sandy soil drains quickly — too quickly for many plants.
✓ Silt: You need a microscope to see silt particles (0.002 to 0.5 millimeters) but you can recognize them by touch: When dry, silty soil feels smooth, like flour or talcum powder. Most silt particles have an irregular shape as sand particles do, but in soils, they’re often thinly coated with clay. Water tends to run off silty soil, but once it penetrates the surface, silt retains moisture better than sand does.
Clay: Clay particles measure less than 0.002 millimeters across. Because of their size and flat shape, clay particles stick together — and feel sticky and slick when wet. (If you’ve ever made pottery, you know what clay soil feels like.) The particles in clay soil are tightly packed, and the spaces between them are small, so water drains poorly, leaving the soil saturated and depriving plants of the air they need.
Clay soil, which may be tinged red, black, gray, or blue, stays wet and cold in spring. Because clay is harder to dig when wet or dry, it’s often referred to as heavy soil.
Your garden soil won’t be all sand or all clay, however, but a mix. If that mix is 40 percent sand, 40 percent silt, and 20 percent clay, you have loam, the ideal soil for gardening.