Under every beautiful garden is something you can't see: a living soil, full of roots, fungi, worms and billions of tiny creatures doing the work of feeding your plants. This lesson is about building that soil, and about the simple truth that you feed the soil, and the soil feeds the plants.
Most worn-out ground isn't missing chemicals so much as it's missing life. Bare soil, heavy tilling and compaction starve it. Cover, compost and living roots bring it back, sometimes faster than you'd expect.
- 1Describe what soil is made of and why its empty spaces matter.
- 2Explain how the soil food web feeds plants.
- 3Build a compost pile with the right size and mix to heat up.
- 4Build soil without digging, using mulch, sheet mulching and cover crops.
- 5Test your soil at home and know when to send a sample to a lab.
What soil is made of
Pick up a handful of good garden soil. It crumbles, smells sweet and earthy, and holds together loosely when squeezed. That texture comes from a balance of four things.
The mineral part (sand, silt and clay) is mostly set by your land; you'll work with what you have. Organic matter is the part you can change. Raising it even a little helps the soil hold far more water, feeds the soil life, and makes heavy clay looser and sandy soil richer.
The soil food web
Plants aren't just taking from the soil. They pay for what they get. Roots release sugars made from sunlight, and those sugars feed bacteria and fungi gathered around them. When tiny predators eat those microbes, the nutrients come out in a form plants can use, right at the roots.
The four principles of soil health
The USDA's Natural Resources Conservation Service sums up how to care for this web in four principles. They apply to a farm, an orchard or a single garden bed.
Disturb it less
Dig and till as little as you can. Tilling tears fungal threads and burns up organic matter.
Keep it covered
Mulch, leaves or plants on the surface, all year. Bare soil bakes, crusts and washes away.
Grow variety
Many kinds of plants feed many kinds of soil life. Mix them, and rotate crops.
Keep roots growing
Living roots in the ground as many months as possible keep the web fed.
Compost: turning scraps into soil
Compost is organic matter that soil life has already broken down. You make it by giving microbes the right mix of food, air and water. The two kinds of food are called browns and greens.
Dry and woody
Fall leaves, straw, wood chips, shredded cardboard and paper, dry plant stalks.
Fresh and moist
Kitchen scraps, fresh grass clippings, garden trimmings, coffee grounds, chicken or horse manure.
Will your compost heat up?
A hot pile needs at least 1 cubic yard (27 cubic feet, about 3 × 3 × 3 ft) to hold its own heat. Most home guides aim for roughly 2 parts browns to 1 part greens by volume; very fluffy, dry browns can go closer to equal parts.
Hot or cool?
A big, well-mixed pile turned often heats to 130–160°F and can be ready in one to three months. Holding 131°F or more for at least three days kills most weed seeds and disease organisms. Check it with a long compost thermometer.
A pile you add to over time and rarely turn still becomes compost, in six months to a year. It's less work but won't reliably kill weed seeds, so leave seeding weeds and diseased plants out.
- Meat, fish, bones, dairy and oils (smells and pests)
- Dog and cat waste (can carry diseases people catch)
- Diseased plants and seeding weeds, unless you compost hot
- Wood or sawdust from treated or painted lumber
Build soil without digging
You don't need a tiller to make a new garden. Layering does the work, and the soil life does the digging for you.
Sheet mulching, step by step
- Cut it down. Mow or flatten the grass and weeds where the bed will go. Leave the roots in place.
- Soak it. Water the area well.
- Lay cardboard. Plain brown cardboard with tape and labels removed, overlapping 6 inches so no light gets through. Wet it again.
- Add compost. 2 to 4 inches on top. This is where you'll plant.
- Top with mulch. 3 to 4 inches of straw, leaves or wood chips. Pull it back from plant stems.
- Plant through it. Cut a hole in the cardboard for transplants, or wait a few months for it to soften before sowing seeds.
Cover crops: green manure
A cover crop is planted to feed the soil rather than the table. Sow it in fall on empty beds, then cut it down in spring and leave it on the surface as mulch.
| Cover crop | What it does |
|---|---|
| Peas, vetch, clover, fava beans | Legumes: partner with bacteria to pull nitrogen from the air into the soil |
| Cereal rye, oats | Big roots and lots of mulch; holds soil over winter and smothers weeds |
| Daikon radish | Deep roots punch through compacted soil, then rot and leave channels |
| Buckwheat | Fast summer cover; flowers feed bees in weeks |
A bed under thick straw mulch, edged with flowering cover crops, looks tended and calm even in the off season. Soil building can be one of the loveliest parts of the garden: a meadow of clover and peas between the vegetables, humming with bees.
Testing your soil
Two kinds of tests tell you different things. Simple home tests show whether the soil is alive and working. A lab test shows the chemistry: pH and nutrients.
| Test | How | What it tells you |
|---|---|---|
| Worm count | Dig out a block about 1 ft square and 1 ft deep in spring or fall, and count the earthworms | Many guides treat 10 or more as a sign of active, healthy soil. Dry western soils often have fewer |
| Squeeze and smell | Squeeze a moist handful, then sniff | Sweet and earthy is good; sour or rotten means too wet and airless |
| Water soak | Push a can with both ends removed 3 inches into the soil, pour in 1 inch of water, and time it | Healthy soil drinks it in minutes; slow soaking means compaction or crusting |
| Cotton test | Bury a pair of 100% white cotton underwear about 6 inches deep and dig it up after two months | The more it has been eaten away, the more life your soil holds |
| Lab test | Mail a sample to your state extension service or a soil lab | pH, major nutrients and organic matter, with advice for what you're growing |
Most vegetables grow best with a pH between about 6 and 7. A lab test every two or three years is plenty for a home garden; add amendments based on the results, not guesses.
Start a compost pile and test your soil
Build a real compost pile together, track its temperature like scientists, and run the home soil tests in two different spots, such as a lawn and a garden bed, to compare them.
You'll need
- Browns: leaves, straw or shredded cardboard
- Greens: kitchen scraps, fresh grass or manure
- A pitchfork and a hose
- A compost thermometer (a long-stem kitchen thermometer works)
- A shovel, a ruler and a can with both ends cut off
- Two pairs of 100% white cotton underwear (or cotton cloths)
- Your land journal
Do it together
- Pick the spot in Zone 1 or 2, between the kitchen and the garden, on bare ground.
- Build the pile about 3 × 3 × 3 ft, layering browns and greens and watering each layer until it's as damp as a wrung-out sponge.
- Take its temperature every day for two weeks and make a line graph. Turn it when the temperature starts to drop.
- Run the soil tests in two spots: worm count, squeeze and smell, and water soak. Record the results.
- Bury the cotton in both spots, mark them, and set a reminder for two months.
- Mail a lab sample from your future garden bed, if you can.
Little soil scientists
- Sort scraps into brown and green buckets
- Count and gently return the worms
- Draw what the cotton looks like when you dig it up
Older soil scientists
- Graph the pile's temperature and mark each turning
- Compare the two spots and explain the differences
- Read the lab report and list what the soil needs
Mark your compost pile and test spots on your base map. The finished compost goes into the kitchen garden in Lesson 5.
Feed the soil
Building soil is slow, quiet work. You won't see it happen day to day. But each bucket of scraps, each layer of mulch and each season of cover crops adds up, until one spring you push a shovel in and it slides through dark, crumbly earth full of worms. That soil is an inheritance you're building for whoever tends this land after you.
Next, we work with the other great force on the land: water, and how to slow it, spread it and sink it in.
Your Lesson 3 worksheet
Work through these together. Your answers save on this device as you type, and print cleanly for your records.
Progress record
What this lesson practices:
- ScienceMovement of matter among plants, animals, decomposers and the environment (NGSS 5-LS2-1, MS-LS2-3).
- MathRatios and volume (Common Core 6.RP.A.1, 5.MD.C.5); graphing data over time.
- InvestigationPlanning and carrying out a fair comparison between two sites; recording and explaining results.
- Life skillsRecycling food and yard waste; caring for a living system over time.
Standards mapping is a draft. If you use charter or ESA funds, confirm it with your education specialist.
Sources for this lesson
- USDA NRCS — principles for high-functioning soils
- USDA NRCS — soil health
- UC Master Gardeners of Sonoma County — hints for composting
- EPA — approaches to composting
- Soils 4 Teachers (Soil Science Society of America) — Soil Your Undies
- Michigan State University Extension — earthworms as an indicator of soil health