Garden Soil: The Compost Blend, and When It Is Worth It

A gardener in Wisconsin filling beds in May and one in Florida filling them in October are making the same decision from opposite ends of the calendar: which of three soils goes in, and how deep. Garden soil is the enriched one, a soil blended with organic compost and intended for growing in straight away. The other two are built for different parts of a property and wrong for a bed, and the difference is worth five minutes before a truck is booked.

What a compost blend has that screened top soil does not

The three soil products on this site are not grades of the same thing. They are different materials for different parts of a property.

Read that table from the bottom up and the logic is clear. Fill dirt has almost no organic matter, which is the point: organic matter decomposes, and anything that decomposes under a structure settles. Top soil has enough to grow grass and is screened to spread evenly with a rake. Garden soil goes a step further by blending compost in, which is what raises the nutrient content and holds moisture through a dry spell while still letting water through in a wet one.

The practical rule: if you are growing vegetables in a contained bed, buy the blend. If you are growing grass over a wide area, top soil goes further for the same work. If nothing is going to grow there at all, it is a fill job.

Beds where garden soil earns the depth it takes

Four jobs justify the enriched material.

Filling a new raised bed. The whole volume is soil the plants will root into, so the quality of what goes in is the quality of the bed for years.

Starting a new vegetable or herb bed at ground level. Loosen the native soil first and blend the top few inches into the new material, so roots have a path downward instead of hitting a hard line.

Refreshing a tired bed. A couple of inches worked into the top of an established bed replaces what a few seasons of cropping took out.

Flower beds, annual and perennial. The same blend suits both, though perennials care more about drainage than about nutrients.

How deep a new bed has to be filled, and how much an old one needs

Two different numbers, often confused.

For a new bed, the depth is the depth of the box, less an inch or two of freeboard so watering does not float material over the sides. Ten to twelve inches of rooting depth suits most vegetables. A deep box on hard ground can be part filled with coarse material at the bottom, though roots reach further than people expect and the saving is smaller than it looks.

For an established bed, 2 to 3 inches worked into the surface is a top dressing, not a refill. Adding more than that every season raises the soil level until the bed overflows.

Where a shed pad, a splash area and a wall backfill all go wrong

Three requests arrive on soil pages that should not be filled with soil at all, and the reason is the same each time: organic matter breaks down, and anything resting on it drops.

Under a shed or garage pad. A hypothetical 10 by 12 foot pad needs 120 square feet of base. At 4 inches that is 120 x 4 / 12 = 40 cubic feet, 40 / 27 = 1.48 cubic yards. Filled with a compost blend it would settle unevenly within a year and take the pad with it. That is a compacted stone job: Crusher Run or Road Base.

A downspout splash area. Water leaving a downspout will carve a channel through any soil and wash the compost out first. Use 3/4" Crushed Stone over fabric.

Behind a retaining wall. The space behind a wall needs to drain, or winter water pressure pushes the wall out. Organic soil holds that water. Backfill with Drain Rock and keep soil for the planted area above.

Yards for four 4 by 8 raised beds

Soil is ordered by the yard, so work in cubic yards and convert only if you need the weight.

Four new beds. Each bed is 4 x 8 = 32 square feet, so 128 square feet in total. Filled to 11 inches: 128 x 11 / 12 = 117.3 cubic feet. 117.3 / 27 = 4.35 cubic yards. Round to 4.5. Garden soil is figured at about 2,000 lb per cubic yard on this site, so that is roughly 4.4 tons, and this is the one material where a cubic yard and a ton are close enough to be confused.

Top dressing an existing border. A strip 30 feet long and 4 feet wide at 2 inches: 120 square feet, 120 x 2 / 12 = 20 cubic feet, 20 / 27 = 0.74 cubic yards.

Both are hypothetical. Beds that are not rectangles are easier to size in the soil calculator, which takes circles and up to five separate areas at once.

Ordering by the yard and keeping the pile off the lawn

Soil arrives as a single pile and every wheelbarrow load comes out of it by hand, so the drop position decides how the day goes. Pick a spot the truck can reach on firm ground with a clear approach, as close to the beds as the route allows, and lay a tarp under it if it is landing on a driveway. Delivery is arranged at checkout by ZIP code.

Order for the volume you can spread in the time you have. A pile of blended soil left standing in rain compacts and gets heavier to move. Set the yardage and the date here on the product page, and if your beds are an awkward set of shapes, send the dimensions through the contact page and ask for a quote by email.