Dirt 101: What is Loam Soil?

Know Your Soil Profile to Grow the Best Crops

Dirt 101: What is Loam Soil?

By Miriah Reynolds, Montana

What is loam soil and how does it differ from silt and sand? What is the best mix for the best farming?

Soil, dirt, earth, dust, or filth, whatever you prefer to call it—we all depend on it. To people who do not work the land, dirt is filth that should stay outside, but to the farmer, soil is the heart of survival. I am taking a class in college about conservation management, and we are studying “the nature of soils.” Yeah, I thought it was pretty interesting—for the first week. Week two of the same topic and I did not want to attend class. Now here I am, in week three of more soil studies, and I have decided that even though studying dirt and erosion may not be too interesting, it does have a huge impact on everything around us. From the cost of heirloom tomatoes in the grocery store to the cotton grown to make our underwear, soil is a critical aspect of farming and living. I’d like to share with you the different types, what makes good soil, and a glimpse of the growing qualities of each, and I promise not to take three weeks!

Soils are divided into two categories: fine earth and coarse fraction. Fine earth soils include clay, silt, and sand. Coarse fractions would be any particle larger than two millimeters, such as gravel, cobble, stones, and boulders. Fine earth soils are the most ideal for growing crops.

Clay has the finest particles of any soil and believe it or not, they are negatively charged. These negatively charged surfaces attract positive ions such as zinc, magnesium, calcium, iron, and potassium. Being that clay’s particles are less than .002 millimeters in size, they bind tightly to one another, holding these great nutrients, making them readily available for crops.

Good soils have good permeability, meaning that water and air are moved through the particles more easily. Since clay’s particles fit close to one another, the permeability is limited. Clay holds water on the surface and drains extremely slowly. This is why when you have an area that is mostly clay it is super slick after it rains. Clay is also harder to till, because the particles are difficult to separate. Normally, land that has high clay content will need to be irrigated and fertilized less than an area with sandier soil. Also, due to the tight spaces, aeration is limited, inhibiting root growth. Mixing clay with a larger particle soil will increase permeability and root growth. However, be careful adding sand to clay for permeability because often the large particles of the sand embed themselves into the clay and almost form concrete.

Silt: Silt falls between clay and sand when it comes to the particle size. It is just a tad grittier than clay. Areas near a river, or that have once been flooded, are where silt can be found. Soils with a high silt content make for fertile land because silt originates from quartz and feldspar minerals. One of the downsides to silt is that it erodes quickly from wind and water. Silt is better at holding water and nutrients than sandy soil, and drains more quickly than clay. You’ll need to use moderate watering and fertilizing (if any fertilizing at all) for silty soils.

You’ll find silt soils near river beds.

Sand: Sand has the largest particles in the fine earth category. Unlike clay, sand has fast drainage. This is why sand is generally used in playgrounds; to avoid mud. Normally plants that grow well in sandy soils have deep root systems that can find water and nutrients in another layer of ground. Be aware that with sandier soils, plants can dehydrate quickly, so you’ll need to irrigate and fertilize more than with a clay soil.

What is loam soil? The best soil for crops, loam combines clay, silt, and sand to make the perfect soil for growing crops. The best loam soils have an equal amount of each, for the optimum permeability. Loam retains moisture and nutrients, but also allows excess water to drain from the soil. Loam is also easy to work with and can be manipulated for certain climates. For example, you could add clay to hold water if you live in a hot climate or sand to increase drainage if you get a lot of precipitation.

Borage (also known as starflower) grows in front of a greenhouse in Idaho.

So what is loam soil? It’s a huge part of our life as farmers. I have decided that there is more to the dirt on my boots than what meets the eye!

Originally published in the January/February 2013 issue of Countryside & Small Stock Journal and regularly vetted for accuracy.

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