What Is Loam Soil?

Loam soil is a mineral mixture of clay, sand, and silt. In the right proportions, loam is the ideal medium for growing plants. Its mineral content makes up less than half of the soil, while the rest is organic matter and empty space. Without the latter two, little will grow in loam soil. It's that perfect mixture of clay, sand, and silt that will create the empty space which allows minerals, organic matter, water, and air to foster life.

Geology for Gardeners

In the beginning, there were rocks — originally granite and basalt, and later shale, slate, limestone, and sandstone. Soils are the product of the weathering of those rocks, whether through mechanical means (such as erosion from wind or water) or chemical means (such as oxidation or hydrolysis). Granite breaks down into silty sands, basalt into clay-like soil. Shale, slate, and limestone break down into clay and silt. Sandstone, predictably, breaks down into sand.

The difference between clay, sand, and silt is in the size of the grain. Sand is big, clay is little, and silt is in between. William Bryan Logan, the author of Dirt: The Ecstatic Skin of the Earth, provides an example of these distinctions: “If you drop a particle of coarse sand in water, it will fall about four inches in one second. A particle of very fine clay, on the other hand, will take about 860 years to fall the same four inches. Silt will fall the same distance in five minutes.” As separate components that vary so significantly in size, these three soil particles contribute different properties to healthy soil.

Sand, being the largest component, is made of hard minerals, which make it difficult to crush or compact. This creates pockets of air between the grains — necessary for worms and microorganisms to flourish — and improves drainage as water flows easily through them. Silt, made from quartz and feldspar, is slippery when wet, which makes it home to many microorganisms and decaying organic matter (humus). Clay is formed from silicate rocks, composed of silicon and oxygen, and has the convenient property of being negatively charged, which attracts the positively charged elements calcium, iron, magnesium, potassium, and phosphorus — essential for plant growth. Clay particles also hold large amounts of water.

How to Garden With Loam Soil

How do you know if your loam soil has the proper balance of sand, silt, and clay? Do the simple “squeeze test.” Grab a moist handful of your garden soil and squeeze it in your fist. If the clump immediately falls apart, your soil is too sandy. If it doesn't crumble at all, your soil has too much clay. Soil that's too silty will feel slimy when wet and become powdery when dry. The ideal loam soil will hold its shape but crumble if you start poking at it. For a more scientific test, your county extension office or university extension service can determine your soil type and its suitability for gardening.

The way to create the right balance is to add organic matter — such as compost, grass clippings, dried leaves, or composted manure — which will break down over time, add nutrients to your soil, attract worms and microorganisms, create vital air pockets, and help your soil retain moisture. You can jump-start the process by purchasing loam soil from garden centers, which typically costs around $40-50 per cubic yard. (One cubic yard of loam will fill the bed of a pickup truck.)

Plants extract precious nutrients from your soil, so it's important to regularly freshen your soil with organic matter. Do so annually in the early spring or, better yet, late fall, to give the organic matter time to work its way to the root level of your soil. Rather than purchasing compost, however, it's easy enough to make your own. Here are some strategies:

• Grow a cover crop. In the fall, growing a cover crop will return nutrients to your soil. Ideal cover crops include clover, ryegrass, alfalfa, or buckwheat. Once the crop reaches a few inches tall, till it into the soil, let it over-winter, then turn it once again in the spring.
• Spread grass clippings or fallen leaves as a mulch. They will keep the soil in your garden cooler, which helps it retain moisture, invite decomposers like worms, and supply nitrogen, potassium, and phosphorus to your soil.
• Use what you've grown. In the fall, uproot your annual flowers and vegetables but leave some of them on the soil to decompose over the winter. Work them into the soil in the spring. 
• Rotate your crops. Don't plant the same annual flowers or veggies in the same spot each year. Substitute legumes like peas for other plants every few years.
• Make your own compost. A backyard compost bin can be made from four wooden shipping pallets held together with angle brackets.
• Have patience. In nature, loam soil takes centuries to form, so it may take you more than one year to create perfectly balanced loam.