Raised Bed Soil Calculator

A raised bed is not a decoration. It is a controlled growing environment where you get to decide what the soil is made of, how it drains, and what nutrients are in it. That control is the entire point. The soil you put in a raised bed is the single biggest factor in what that bed produces, and it is the one decision you get to make before anything else happens.

Most raised beds are filled with some combination of topsoil, compost, and a drainage agent like perlite or vermiculite. The ratio depends on the depth of the bed, what you are growing, and how much you want to spend. There is no single correct recipe, but there are recipes that work well and recipes that cause problems.

Mel’s Mix is probably the most widely known raised bed recipe. It comes from the Square Foot Gardening method: one-third blended compost, one-third peat moss, one-third coarse vermiculite. It works. It drains well, holds moisture, and provides a good nutrient base. The downside is cost. Coarse vermiculite is expensive, and at scale the price per cubic yard can get unreasonable. For a single 4×4 bed it is manageable. For six beds it is a conversation about the budget.

The budget approach is three parts topsoil to one part compost. You lose the lightweight feel and the engineered drainage, but you gain affordability and weight. Heavy soil is not always a disadvantage in a raised bed. It holds temperature better, it resists wind erosion, and it settles into a stable structure that does not compact the way pure compost does when it dries out. If you go this route, order screened topsoil from a landscape supply yard. Unscreened topsoil is a gamble on rocks, roots, and clay clods.

The premium mix in the calculator is a middle ground. Topsoil for body, compost for nutrients and biology, peat or coir for moisture retention, and perlite for drainage. It is what most people end up mixing after they have tried the extremes and want something that balances cost, weight, and performance.

Used coffee grounds are roughly 2% nitrogen by weight, which makes them a useful addition to compost piles and garden soil. The nitrogen in spent grounds is mostly in organic form, meaning it releases slowly as microbes break the grounds down. This is a good thing. It means you are feeding the soil biology, not dumping a hit of synthetic nitrogen on the surface.

The mistake people make is applying coffee grounds as a thick layer directly on the soil. Grounds are fine-textured and compact easily. A thick layer becomes a mat that repels water instead of absorbing it. The correct approach is to mix grounds into compost at 10-20% of the pile volume, or work them lightly into the top few inches of soil. In a raised bed, they are best mixed into the initial soil recipe, not top-dressed.

The pH question comes up often. Fresh coffee grounds are mildly acidic. Used grounds are close to neutral, around 6.5 to 6.8. The brewing process extracts most of the acid. If you are worried about acidifying your soil, you are probably overestimating the effect. A soil test will tell you the truth faster than any amount of guessing.

Wood ash from a fireplace or fire pit is a legitimate soil amendment. It is alkaline, which means it raises soil pH. It contains meaningful amounts of potassium and calcium, both of which potatoes, tomatoes, and brassicas want. In acidic soils, which cover most of the Southeast, a light application of wood ash can do the same job as garden lime while also adding potassium that lime does not provide.

The rate matters. Five pounds per 100 square feet is a reasonable starting point. More than that and you risk pushing the pH too high, which locks out micronutrients like iron and manganese. Never apply wood ash around blueberries, azaleas, or anything that wants acidic soil. Never use ash from charcoal briquettes, painted wood, or pressure-treated lumber. Hardwood ash from untreated firewood is the only kind that belongs in a garden.

Earthworms are not a garden accessory. They are the primary engineers of soil structure. A healthy worm population aerates the soil, breaks down organic matter into plant-available nutrients, and creates channels that allow water and roots to penetrate. Worm castings, which is the polite way of saying worm manure, are one of the most nutrient-dense and biologically active amendments you can add to a garden bed.

You do not need to buy worms for a raised bed. If the bed is sitting on native soil and you have added compost, worms will find it. They move through the ground and migrate into environments with organic matter. If your bed is on concrete, pavers, or has a hardware cloth bottom, you may need to introduce red wigglers or nightcrawlers manually.

Worm castings as a purchased amendment are expensive. A 15-pound bag is $20 to $30. At scale, a worm bin is a better investment. A simple bin with red wigglers will process kitchen scraps into castings continuously and give you a supply that would cost hundreds of dollars a year to buy in bags. The worms do not require much attention. They require food scraps, moisture, and darkness, which is a shorter list than most pets.

New soil in a raised bed will settle. There is no way around this. Watering compresses air pockets, organic matter decomposes, and gravity does what gravity does. In the first season, expect to lose 10-15% of the volume. This is not a failure. This is physics. The 10% extra slider in the calculator exists specifically for this reason.

At the end of the first growing season, top-dress the bed with an inch or two of compost. This replaces what settled, feeds the soil biology for next year, and is a better approach than trying to overfill the bed at the start and dealing with soil spilling over the sides every time you water.

Peat moss and coco coir do the same job in a soil recipe: they hold moisture and keep the mix light. Peat is harvested from bogs, primarily in Canada. It is a finite resource that regenerates extremely slowly. Coco coir is a byproduct of coconut processing and is considered renewable on a human timescale. Both work. Coir has a more neutral pH and rehydrates more easily after drying out. Peat is slightly more acidic and can become hydrophobic when fully dry, at which point water runs across the surface instead of soaking in.

If you are choosing between them, coir is the more forgiving material to work with. If you already have peat, use it. Neither one is wrong. The environmental argument favors coir, and the practical argument is roughly a tie.

A soil test costs $10 to $25 through your county extension office and tells you exactly what your soil has and what it needs. It measures pH, nitrogen, phosphorus, potassium, calcium, magnesium, and organic matter percentage. Without a soil test, every amendment you add is a guess. With a soil test, every amendment you add is a response to data.

This matters because most garden problems that get blamed on pests, weather, or bad luck are actually nutrient deficiencies or pH problems. A tomato with blossom end rot does not have a calcium problem because the soil lacks calcium. It has a calcium problem because the soil pH is too low or too high for the plant to access the calcium that is already there. A soil test catches this. A guess does not.