What To Know About Loam Soil for Your Garden
Gardeners love loam soil because it's moist, soft and full of nutrients. Here's how to get more of it in your garden beds.
Over the eons, wind and water gradually weather rocks, breaking them down into particles that help form soil. That silty sand in your yard might have once been a towering granite cliff. That clay? It could have come from a lava flow of basalt, or limestone that formed under an ancient sea.
Wherever it originated, once it becomes soil, we classify it based on particle size. Sand has the largest particles. Clay has the finest. Silt is somewhere in between. Mix this trio together and voilà! — you have loam, the goddess of garden soils.
What Is Loam Soil?
Loam soil is a balanced mixture of sand, silt and clay, ideal for growing garden plants. Loam’s mineral content, and the way the three sizes of particles work together, combine to make it extremely fertile.
The larger sand grains prevent the soil from compacting, so it drains well and helps oxygen reach plant roots. The extra spaces between the sand attract worms and microorganisms. The silt helps the sand and clay mix together, holds moisture and also makes a good home for microorganisms and decaying organic matter (humus).
The clay also holds water in the soil. Plus, it’s negatively charged, which attracts positively charged nutrients like calcium, iron, potassium, phosphorus and magnesium.
“The ideal soil, called loam, contains all three of these different-sized particles in relatively equal amounts,” says Em Shipman, executive director of KidsGardening.org.
What Are the Benefits of Loam Soil?
Loam soil is considered ideal for garden fruits, vegetables and flowers because it:
- Allows water to travel through it slowly enough for the plants to get it, but fast enough to prevent it from getting soggy;
- Retains an abundance of nutrients;
- Keeps plant roots oxygenated;
- Is drought resistant.
What Are the Drawbacks of Loam Soil?
There aren’t any drawbacks when using loam soil for vegetable and flower gardens.
What Plants Do Not Like Loam Soil?
Some gardeners who grow root crops and tubers, or live where there’s a lot of rain, may desire more sandy soil for ease of harvest and better drainage. Also, some native plants that evolved and adapted to local soil conditions may do fine without loam soil.
When in doubt, apps such as NatureID can help you determine the best soil mixture and nutrients for particular plants.
How Do I Know if I Have Loam Soil?
Besides taking a sample to a lab for a soil test, the easiest way is a squeeze test. Take a handful of moist soil and form it into a ball. If it’s loam, it will compress into a loose ball that crumbles easily.
“If no ball is formed, then the sand content is too high,” says Dr. Tony Provin, soil chemist and professor at Texas A&M University. “If the ball formed could be used by the local youth baseball team, the clay content is likely high.”
How To Create Loam Soil
First, let’s clarify the difference between the soil-science definition and common use of the word loam. In science, loam is soil with a balanced mixture of sand, silt and clay. Commonly it refers to nutrient-rich, crumbly garden soils that contain organic matter.
“Technically speaking, you can’t transform your garden soil into loam by adding sand or clay, because that results in a muddy mess, or worse, almost concrete,” says Shipman.
But you can create more loamy soil in the common sense of the word by adding organic matter. That improves soil structure, or how well the soil particles are held together in clumps by humus, the glue-like substance formed when organic matter decomposes.
“A soil with good structure has lots of these clumps, called aggregates, which loosen heavy clay soil and help sandy soils hold moisture and nutrients,” says Shipman.
So the best way to make your soil more loamy is to add organic matter like compost, shredded leaves, grass clippings or composted manure.
Earthworms, microbes and other soil creatures consume and break down this organic matter, transforming it and releasing the nutrients it contains into forms plants can take up. The glue-like humus, which binds soil particles and improves soil structure, forms during decomposition.
To amend your soil:
- Add two or three inches of organic matter. The ideal time is in fall, but you can also do it in early spring.
- Work the organic matter into the top six to eight inches of soil with a shovel or rototiller, then wait to plant until spring rolls around. Don’t overdo the rototilling to avoid harming soil life.
- If you add fresh (uncomposted) organic material in the spring, wait two to four weeks before planting to let the material start breaking down. Otherwise, microbes can “lock up” some nutrients so plants can’t get at them. However, you can add fully composted material any time and plant seeds or seedlings right away.
Can You Buy Loam Soil?
Yes. “Although quality topsoil is more expensive than cheap fill dirt, it’s worth the investment if you hope to grow a bountiful garden,” says Shipman.
For gardens, Shipman recommends a blend of topsoil and compost. For those who want to get more technical, Provin offers this advice:
“Whenever possible during soil purchasing, look for the abundance of soil peds (natural occurring aggregates), as roots do not grow through individual sand, silt or clay particles, but around these soil aggregates. The lack of aggregates, even in a loam-textured soil, is a root-ability issue.”
How To Maintain Loam Soil
Once you’ve created great loam soil, your garden will be gorgeous for a while. But eventually, nutrients need replenshing.
“Soil improvement is not a one-and-done task,” says Shipman. “Rather, it’s an ongoing process that includes adding organic material to the soil each year.”
Here are some methods to maintain your loam soil:
- Organic matter: You can till in organic matter each fall or spring, the same way you first established your loamy soil. You can also add mulch (like leaves) in the fall to protect the soil over winter.
- Don’t over-till: Over-tilling the soil will release the stored nitrogen into the atmosphere and wash away nutrients. It also disrupts the habitat created by billions of helpful microorganisms. “More specifically, many gardeners over-till the soil and subsequently destroy native soil aggregation and undo developing aggregation, thus causing root and water problems,” says Provin.
- Cover crops: Cover crops planted between growing seasons can protect the soil from erosion by enriching it with nutrients. Alfalfa, buckwheat, barley oats, mustard, parsley, radish, rye, red clover, wheat and winter peas are all good cover crops.
Another trick for healthy soil: Planting native plants.
“The root systems of many native plants are complex and large,” says Mary Phillips, senior director at Garden for Wildlife. “They help sequester carbon and their root channels aid in water run-off and absorption in the soil.”
Provin also recommends that you:
- Avoid walking on garden soil, especially when it’s wet. Foot traffic can compact the soil and make it harder for plants to root.
- Test the soil annually. Problems can develop from over-fertilization, which introduces too much phosphorus and soluble salts and may skew the soil pH.