Soil & Compost
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All soil & compost
The full section as a record. Every entry answers the same questions in the same order, so two can be read line for line.
Find out what you have before you buy anything
Soil is the only part of a garden that constrains every other decision, and it is the part most often skipped, because testing it feels like a delay before the enjoyable work. It takes ten minutes and a jar of water, and it changes what you should buy.
Texture, and the ten-minute test
Texture is the proportion of sand, silt and clay, and it is fixed. You are never going to change it, so the aim is to work with it rather than against it.
- Fill a straight-sided jar a third full of soil taken from planting depth, not the surface
- Top up with water and a drop of washing-up liquid, which stops the particles clinging together
- Shake hard for two minutes, then leave it undisturbed
- Read the bands after a day: sand settles in a minute, silt over hours, clay over a day or more
- Confirm by hand: wet soil that rolls into a ribbon an inch long is clay-heavy, soil that crumbles is not
Clay is the most fertile texture most gardeners will ever have. The problems with it are drainage, compaction and workability, and all three are structure rather than texture.
What the jar and the ribbon tell you is which of the common garden soil types you are working with. Each has one thing it does well and one thing it does badly, and knowing which is which saves you from treating the wrong problem.
| Soil type | How it behaves | What it does well | What goes wrong |
|---|---|---|---|
| Sandy soil | Gritty, loose, water runs straight through | Warms early in spring, never waterlogs, easy to dig | Dries out in days and loses nutrients with every watering |
| Silty soil | Smooth and soapy when wet, floury when dry | Holds water and nutrients well, and is fertile | Compacts easily and crusts over, so seedlings struggle to break through |
| Clay soil | Sticky when wet, sets hard when dry, rolls into a ribbon | Holds the most water and nutrients of any type | Waterlogs in winter, cracks in summer, and is workable for only a short window |
| Loamy soil | Crumbles into soft granules and holds its shape briefly | The balance every other type is being improved toward | Nothing much, and almost nobody starts with it |
| Chalky soil | Pale, stony, often with visible chalk or limestone | Drains freely and rarely waterlogs | Alkaline pH locks up iron and manganese, so acid-loving plants yellow |
| Peaty soil | Dark, spongy, holds water like a sponge | Very high in organic matter | Acidic and often too wet, and it is not a sustainable material to buy in |
The way to improve garden soil is the same for every one of those types, which is the most useful fact on this page: add organic matter, repeatedly, and let the soil biology do the structural work. Compost opens clay and binds sand at the same time, because it is changing structure rather than texture. What does not work is adding sand to clay soils, which produces something closer to concrete than to loam.
The amendment answer is the same for almost every soil
Organic matter improves clay and sand for opposite reasons. In clay it groups the fine particles into crumbs and opens up the pore space that lets water through. In sand it holds the water and nutrients that otherwise drain straight past the roots. The target is roughly 10 to 20 percent of soil volume, which for a bed eight inches deep means one to two inches of compost worked in.
Do it annually rather than heroically. Extension guidance estimates that yearly additions give even clay-heavy soils noticeably better drainage and more life within about five years, and five ordinary top-ups do more than one enormous double-dig.
Do not add sand to clay. Extension guidance is explicit that no amount of sand changes the texture of a clay soil, and that the mixture can be harder to manage than the original, because the clay particles simply adhere to the sand grains.
pH decides which nutrients the plant can reach
A soil can contain plenty of a nutrient the plant cannot take up, and pH is usually the reason. This is why yellow leaves with green veins on an azalea are a pH problem rather than a shortage, and why adding more fertilizer makes it worse.
Most garden plants sit comfortably between 6.0 and 7.0. The plants that genuinely care are the acid-lovers, azalea, rhododendron and blueberry among them, which want 4.5 to 6.0 and cannot take up iron above roughly 6.0. Test before planting those, because shifting pH in established ground is slow and choosing a different plant for alkaline soil is easier than fighting it every year.
Compaction is a separate problem from texture
Compacted soil has had the pore space crushed out of it, usually by traffic when wet, and amendments spread on top of a compacted pan achieve very little because the problem is below where you are working.
Relieve it first, by deep tilling or subsoiling on a heavily compacted area, then add organic matter. Afterwards keep off it: permanent beds with paths between them, narrow enough to reach across, are the structural fix rather than a tidiness preference.
Mulch does more than it is given credit for
Two to three inches of organic mulch suppresses weeds, holds moisture even so the ground cracks less in summer, stops the surface capping in heavy rain, keeps roots cooler, and feeds organic matter into the top layer without any digging.
It also prevents disease, because splashing soil is how several fungal problems reach the lower leaves of a plant. Keep it a couple of inches clear of stems and trunks, where a permanently damp collar invites rot and hides mower damage.
Raised beds solve one problem and create another
A raised bed sidesteps difficult soil rather than improving it: you garden in imported soil above the problem. For vegetables that is often the right answer, because you get workable ground this season instead of in five years.
It is the wrong answer for trees and shrubs, whose roots reach the native soil regardless, and planting one into a pocket of imported compost surrounded by clay creates a sump that holds water around the root ball.
How to compost, and the one ratio that decides everything
Every soil problem on this page has the same amendment as its answer, and compost is the version of it you can make for nothing out of material you already throw away. Composting is not difficult and it is not precise. It has one rule that matters and a short list of things to keep out, and everything else is a question of how fast you want it.
Browns and greens, which is really carbon and nitrogen
Compost material splits into two groups. Browns are dry and carbon-rich; greens are wet and nitrogen-rich. The organisms doing the work need both, and the mix decides whether you get compost or a smell.
| Browns, the carbon | Greens, the nitrogen | Keep out entirely |
|---|---|---|
| Dry fall leaves, the best and most available of all | Vegetable and fruit scraps from the kitchen | Meat, fish, bones and dairy |
| Straw and hay | Fresh grass clippings | Greasy or oily food, and cooked leftovers in quantity |
| Shredded cardboard and plain paper | Coffee grounds and tea leaves | Dog and cat waste |
| Wood chips and sawdust from untreated wood | Fresh green garden trimmings | Treated or painted wood, and coal ash |
| Dry stalks and spent plants at the end of the season | Aged manure from herbivores | Weeds that have set seed, and diseased plants, unless the pile runs hot |
Aim for roughly two to four parts browns to one part greens by volume. That is the practical version of a carbon to nitrogen ratio of about 25 to 30 parts carbon per part nitrogen, which is what the material wants. You do not need to measure anything. If you add a bucket of kitchen scraps, add two to four buckets of shredded leaves with it and the pile will behave.
Hot pile or cold pile: how fast do you want it
The same materials become compost either way. The difference is time, effort, and whether the process gets hot enough to kill weed seed.
| Method | How it works | Time to finished compost | What the heat kills |
|---|---|---|---|
| Hot, active pile | Built all at once, at least a 3 foot cube, kept damp and turned weekly. Reaches 130 to 150F | Two to three months | Most weed seed and many plant pathogens |
| Cold, passive pile | Material added whenever you have it, and never turned | Six months to two years | Nothing. Weed seed survives and comes back with the compost |
| Tumbler | A sealed drum turned by a handle, filled in one go | Six weeks to three months | Depends on whether it actually heats, and small tumblers usually do not |
| Worm bin | Red wigglers in a shallow bin, fed kitchen scraps, kept indoors or in shade | Two to three months | Nothing, but it produces the richest amendment of any method |
| Trench composting | Scraps buried a foot deep directly in a bed | One to two months in warm soil | Nothing, and nothing needs turning either |
For most home gardens a cold pile is the honest choice. It takes no work, and the only real cost is that weed seed and diseased material have to stay out of it, because nothing in the pile ever gets hot enough to destroy them.
Where to put the pile, and what to put it in
Home composting needs less equipment than the shelves suggest. A heap on bare ground works, and the bin exists to keep it tidy, keep animals out and hold heat, rather than to make the composting happen.
- Site it on soil rather than on paving, so worms and soil organisms reach the materials from below.
- Part shade is ideal. Full sun dries a pile out in summer, and deep shade keeps it cold and wet.
- Within reach of a hose, because the most common reason a pile stalls is that the materials went dry.
- Close enough to the kitchen that you actually carry scraps out in February.
| Bin or method | What it suits | What it costs |
|---|---|---|
| Open heap | Large gardens with plenty of leaves and prunings | Nothing, and it is the easiest to turn |
| Three-bay wooden bin | Serious volume, and composting in batches at different stages | The most to build, and the fastest hot compost |
| Plastic bin with a lid | Small gardens, and keeping animals out of food scraps | Cheap, but awkward to turn and slow to heat |
| Tumbler | Small gardens, and anyone who dislikes handling the pile | Convenient, small in capacity, and rarely genuinely hot |
| Worm bin | Apartments, and anyone composting kitchen materials only | Small, indoor-friendly, and the richest output of any method |
Two seasonal notes are worth having. Composting slows almost to a stop in freezing weather and starts again on its own in spring, so a pile that looks dead in January is not. And fall is when the browns arrive: collecting and stockpiling shredded leaves in autumn is what lets you balance a whole year of kitchen greens without buying anything.
When compost goes wrong, it is telling you which side of the ratio is off
| Problem | What it means | What to do |
|---|---|---|
| Smells of ammonia | Too many greens, too much nitrogen | Add browns and turn the pile |
| Smells sour or rotten | Too wet, with no air reaching the middle | Add browns, turn it, and cover it in prolonged rain |
| Nothing is happening | Too dry, too many browns, or a pile too small to hold heat | Water to the dampness of a wrung-out sponge, add greens, and build to at least a 3 foot cube |
| Animals in the pile | Meat, dairy or oils, or food scraps left on the surface | Keep those materials out, and bury every addition of food scraps in the center |
| Flies on top | Exposed kitchen scraps | Cover each addition with a layer of browns |
Knowing when it is done, and what to do with it
Finished compost is dark, crumbly and uniform, smells of earth rather than of anything it was made from, and holds none of the original material in recognizable form. An eggshell or a twig surviving is normal and does no harm.
- Work two to three inches into a bed before planting, and add more each season rather than trying to fix a soil in one go.
- Topdress a lawn with a quarter of an inch after aeration, once a year.
- Mulch with it around established plants, keeping it off stems and trunks.
- Treat it as an amendment rather than a fertilizer. It improves structure, water holding and biology, and it feeds slowly and mildly. A plant with a specific nutrient deficiency needs a soil test, not more compost.
What to do in the garden, for your zone
One email at the start of each month: what to plant, prune and order in the next four weeks, written for the zone you pick rather than for a national average.
Twelve emails a year, no more. An unsubscribe link in every one, and the zone is used for nothing except deciding which version you get.
When soil & compost go wrong
Three failures account for most of what readers ask about in this section.
Questions about soil & compost
Sandy, silty, clay and loamy, with chalky and peaty soils as regional cases. The jar test above tells you which you have. Loam is the balance the others are improved toward, and almost nobody starts with it.
Organic matter, added repeatedly, rather than sand. Compost changes the structure so water and roots can move through it, while sand mixed into clay produces something closer to concrete. Work in two to three inches a season and stop digging it when it is wet.
Roughly two to four parts browns to one part greens by volume, which corresponds to a carbon to nitrogen ratio around 25 or 30 to 1. Precision is not needed. More browns than greens is the part that matters, and a pile that smells is telling you it has too many greens.
Only if you want it fast. Turning brings air to the middle and keeps a hot pile hot, which produces finished compost in two to three months. An unturned pile still becomes compost; it just takes six months to two years.
Meat, fish, bones, dairy, greasy food, dog and cat waste, treated wood and coal ash. Weed seed and diseased plant material can go into a hot pile that reliably reaches 130F, and should stay out of a cold one.
An ammonia smell means too much nitrogen, so add browns. A sour or rotten smell means it is too wet and airless, so add browns and turn it. Compost that is working properly smells of soil and nothing else.
When it is dark and crumbly, smells earthy, and you cannot identify what went into it. If it still looks like leaves and scraps it needs more time, and using it half-finished can tie up nitrogen in the bed while it finishes breaking down.
No. It improves soil structure, water holding and biological activity, and releases a small amount of nutrients slowly. That is enough for most established plantings, but a deficiency identified by a soil test needs a fertilizer that supplies the missing nutrient.
Wet a handful and try to roll it. Sandy soil will not hold a shape and feels gritty; clay rolls into a ribbon and feels slick; loam holds together loosely and crumbles. That test answers most planting questions. A pH kit answers the rest, and matters most if you plan to grow acid-loving plants.
Temporarily and locally, yes; permanently, not really. Sulfur lowers pH and lime raises it, both slowly, and both are undone over a few years by your local water and parent rock. For a whole bed of acid-loving plants, growing in containers or raised beds is more reliable than trying to hold a whole garden at a pH it does not want.
From a few months for a hot, turned, well-balanced pile to well over a year for a cold heap left alone. The three things that slow it down are too little nitrogen, too little moisture and too little air, and all three are fixed mechanically rather than with an additive.
Usually not much. Most plants establish better in unimproved ground with good structure than in a hole full of rich compost, which encourages roots to circle inside the hole rather than spread into the surrounding soil. Feeding comes later, once the plant is growing.