Fertility leaves with the harvest
Plants build themselves from water, air, sunlight and nutrients in the soil, like nitrogen, phosphorus and potassium. When you carry a basket of food out of the garden, some of those nutrients leave with it.
If nothing comes back, the soil slowly gets poorer.
The nutrient loop is the cycle that returns them: food scraps, garden waste and manure become compost, compost feeds the soil, and the soil grows food. The tighter the loop, the less fertilizer you need to buy.
Text description
A cycle diagram with five stages. Soil, holding nutrients such as nitrogen, phosphorus and potassium, feeds the garden crops, which take the nutrients up. Crops become the harvest and food in the kitchen. Food scraps, garden waste and manure from hens go to the compost, where microbes break them down. Compost goes back to the soil. Two side flows: some nutrients leave the loop with the food you eat, and cover crops, leaves and wood-chip mulch add more back to the soil. In the center a note says the tighter the loop, the less fertilizer you need to buy.
What compost is
Compost is organic material, like scraps, leaves and manure, that microbes have broken down into a dark, crumbly soil-like material. It feeds the soil, improves its structure and helps it hold water (EPA).
A pile needs four things:
- Greens: wet, nitrogen-rich material, like food scraps, grass clippings and fresh manure.
- Browns: dry, carbon-rich material, like dead leaves, straw, wood chips and cardboard.
- Water: about as damp as a wrung-out sponge.
- Air: turning the pile, or building it loosely, lets oxygen in.
Balancing browns and greens
Microbes need both carbon and nitrogen. Their balance is the carbon-to-nitrogen ratio (C:N). EPA’s home rule of thumb is about 2 to 3 parts browns to 1 part greens by volume.
Try different mixes of browns and greens. What happens to the pile?
A workable mix: roughly 2 to 3 parts browns to 1 part greens by volume. Keep it as damp as a wrung-out sponge.
Assumptions: Volume ratios are a home rule of thumb for a carbon-to-nitrogen ratio near 25 to 30 to 1 (EPA, extension guides).
Too many greens make a wet, smelly pile. Too many browns make a dry pile that barely changes for months.
Hot and cold piles
Hot composting means building a large enough pile, all at once, that microbes heat it up. Hot piles often reach about 130 to 160 °F (54 to 71 °C) inside (EPA). Heat speeds breakdown and kills many weed seeds and germs. Extension guides suggest a pile of at least 1 cubic yard, about 3 ft (1 m) on each side. Smaller piles may not hold the heat.
Text description
A cross-section of a compost bin about 3 feet (1 meter) wide and 3 feet high, about 1 cubic yard, which is about the smallest pile that holds heat. Inside, thick layers of browns alternate with thin layers of greens, about 2 to 3 parts browns to 1 part greens by volume. Browns are dry, carbon-rich material such as dead leaves, straw, wood chips and cardboard. Greens are wet, nitrogen-rich material such as food scraps, grass clippings and fresh manure. Air enters through gaps in the slatted sides. Water keeps the pile about as damp as a wrung-out sponge. In the middle is a hot core, often about 130 to 160 °F (54 to 71 °C) in a hot pile. A note says too many greens make a wet, smelly pile and too many browns make a dry pile that barely changes.
Cold composting means adding material as you go and letting it break down slowly. It is less work, but it takes a year or more and does not reliably kill weed seeds or germs.
Most homesteads end up with some of each.
In or out?
Is each item a green, a brown, or something to keep out of a simple home compost pile?
- Meat scraps and bones
- Dog or cat waste
- Straw
- Vegetable peels and coffee grounds
- Plants that died of disease
- Dry fall leaves
- Chicken manure with bedding
- Shredded plain cardboard
Manure, with care
Manure from chickens and rabbits, from lesson 37, is a strong boost for the loop. Handle it with care.
- Compost it first, especially chicken manure. Fresh manure can burn plants and can carry germs.
- Time it. If you do use raw manure, leave a long gap before harvest. The USDA organic rule is a useful benchmark. It requires raw manure to be worked into the soil at least 120 days before harvest for crops that touch the soil, and at least 90 days for other food crops. Composted manure is safer.
- Wash your hands after handling it.
Never put human waste, or compost from a composting toilet, into a food garden compost pile. That follows separate rules, as in lesson 40.
More ways to close the loop
- Leaves and wood chips used as mulch break down where they lie and feed the soil.
- Cover crops are plants grown to protect and feed the soil between food crops, then cut and left or turned in. Legume cover crops, like clover or peas, can add nitrogen.
- Worm bins, or vermicomposting, use worms to turn food scraps into rich castings in a small space, even indoors.
- Wood ash from your stove adds potassium and raises soil pH. Use small amounts, and only where a soil test shows the soil is acidic.
Check
Your compost pile is wet, slimy and smells rotten. What should you add?
Summary
- Each harvest removes nutrients. The nutrient loop brings them back.
- A pile needs greens, browns, water and air, about 2 to 3 parts browns to 1 part greens.
- Hot piles work fast and kill many weed seeds. Cold piles are slow and easy.
- Compost manure before using it on food beds, and keep meat, pet waste and diseased plants out.
Next, you will look at trash, recycling and buying less.
Go deeper: what C:N ratio really means
Microbes use carbon for energy and nitrogen to build their bodies. A mix with a C:N ratio of roughly 30 to 1 by weight suits them well. Leaves may be around 35 to 85 to 1, straw about 80 to 1, and sawdust far higher. Fresh grass clippings and manure are much lower, around 20 to 1 or less (University of Alaska Extension). Volume ratios of browns to greens are a home shortcut for this weight ratio. They are rough, because a bucket of wet scraps weighs far more than a bucket of dry leaves.