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Years ago I saw a bumper sticker that said, "Don't Treat Your Soil Like Dirt" and it struck me as such a perfect introduction to the topic of soil. What makes soil different from dirt? In a word, Life. Soil may look inert but it's teeming with activity, both visible and invisible. It contains a dynamic ecosystem that's constantly changing, and one of the keys to growing healthy plants is to understand and nurture that ecosystem.
A soil scientist might describe healthy soil as containing about 45% mineral particles (clay, sand, silt), 25% each of air and water, and about 5% organic matter. But that doesn't mean much in the real world -- for example, how do you measure how much air is in the soil? I think it's easier to think first about what soil provides to plants:
A healthy soil provides all these things to the plants growing in it.
The size of the mineral particles in the soil determines its texture:
To determine what type of soil you have, pick up a handful of moist soil. Rub a bit between your thumb and finger. If it's gritty, it contains sand. Now try to roll it into a cylinder about an inch in diameter. If it immediately falls apart, it's probably quite sandy. If it holds loosely together but crumbles when you poke it, it's likely silty. If it sticks together (like modeling clay), it contains mostly clay particles.
It's handy to know what type of soil you have in part because different plants prefer different soil types. But most important is what you can do to maximize your soil's potential.
The single best thing you can do for your soil is to add organic matter, and the best organic matter is compost. Compost is simply once-living matter (leaves, kitchen vegetable scraps, garden trimmings) that has decomposed into a dark, crumbly substance. Whether you make your own compost or purchase it in bags or bulk, compost is often called "garden gold" for the miracles it can work in your soil.
Organic matter helps sandy soil retain water better, and also helps clay soils drain better. It contains some plant nutrients, too. But its biggest job is to nurture the soil life in the soil. Beneficial soil organisms, including microscopic bacteria and fungi, beetles and other insects and earthworms consume the organic matter, further breaking it down the material in nutrients that plants can use. In the process, they aerate the soil so plant roots can get the oxygen they need, and they keep pest organisms in check. Organic matter also helps soil particles form small clumps (called soil aggregates) that help prevent it from compacting.
Dark. Moist. Crumbly. I like to compare healthy soil to chocolate cake. It's moist and crumbly, with air pockets throughout. Unhealthy soil is like a cake mix -- dry and packed down with no room for air. Or picture healthy soil as a wrung-out sponge -- it's moist yet contains plenty of air, too.
Soil pH is a measure of its acidity or alkalinity and is measured on a scale of 0 to 14, with 7 being neutral. Most plants prefer a slightly acidic soil with a pH of around 6.8, but some, like blueberries, need a more acidic soil to thrive. Test your soil, and if it's too acidic (the pH is too low) add lime. If it's too alkaline (the pH is too high) add sulfur.
Soil improvement isn't a one-time proposition; it's an ongoing task. The soil organisms break down organic matter, so you need to continually add more -- feed the soil, and the soil will feed the plants.