Home & Garden

Loam sweet loam: It’s not just your garden-variety topsoil

Volunteers fill raised gardens with a mixture of sandy loam and compost in a community garden in Texas.
Volunteers fill raised gardens with a mixture of sandy loam and compost in a community garden in Texas. Star-Telegram

The word “loam” has long confused gardeners, myself included. In rural areas I have often heard it used as a synonym for topsoil, sometimes pronounced “loom,” as in “I can bring you a truckload of loom.”

But topsoil is a much more general term than loam. It’s the soil on the earth’s natural surface — as opposed to subsoil, which lies below. It’s darker and more fertile, because it’s the layer in which plants grow, fertilized by decaying plant and animal matter. Otherwise it would be just like the subsoil, which is only good for filling up an inconvenient swale or pit, as in “I can bring you a load of fill.”

True loam is not just any old topsoil. Technically, it has a certain texture, based on the size of its particles, clay being fine-textured, sand being coarse, and silt in between. Loam is not, as I once thought, a particle size somewhere in the middle of all that, but a balance of the three, all of which have their virtues. You want the nutrient- and water-holding capacity that clay provides, and the good drainage and aeration offered by sand. The ideal loam is said to be 40 percent sand, 40 percent silt and 20 percent clay.

More importantly, loam is not loam unless it contains organic matter as well, from decaying root fiber, microbes, worm castings and all the other ingredients found in a living soil. In fact, organic material is the best remedy to any imbalance in particle size. Rarely would you correct clay soil by adding sand, or sandy soil by adding clay. Organic matter buffers both by making clay drain and sand retain.

Loam is not loam unless it contains organic matter … from decaying root fiber, microbes, worm castings and all the other ingredients found in a living soil.

Barbara Damrosch, writer and farmer

We’re used to adding organic matter in the form of compost, leaf mold, peat moss or some similar amendment to make the soil loamier, but in older times a pasture was often the source.

The most fertile loam you can find is in a pasture of grasses mixed with legumes such as clover, and grazed by livestock. Smart farmers know that if you set arable land aside and put it into such a pasture for a stretch of time, then plant crops again, those crops will thrive mightily.

On a smaller scale, a home gardener might till up a grassy area or section of lawn — or cut it into blocks of turf with a spade and turn them grass-side down — then spread manure over them. The grass could decompose and mellow into the soil. At planting time, plant vegetables in the new garden plot and sow last year’s in grass and clover.

Another old trick is to take those cut blocks of turf and layer them in a stack, grass side down, with manure in between. When it’s rotted down and mature, you’ll have a material perfect for use as a potting mix or as a top-dressing for the garden. And if someone ever says he can bring you a truckload of that, adopt him, marry him, or at the very least take down his cell number, as he is offering the finest loam you’ll ever see.

Damrosch is author of “The Four Season Farm Gardener’s Cookbook”; her website is www.fourseasonfarm.com.

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