Today we're going to talk soil science basics, more specifically, soil texture. Soil texture is defined as the proportionate ratio of three recognized components, sand, silt, and clay. The framework of the earth's crust that can hold onto organic matter provides habitat for creatures big and small, and the bones that plant roots thread their way through to create the intricate tapestry of life below our feet.
Every ecosystem has a unique texture. Some are heavier in sand, while others are almost completely clay. But the sweet spot, where water drains, retains moisture, and provides an ideal environment to grow plants is 40% sand, 40% silt, and 20% clay. This magic ratio is called Loam.
Loam soils have three main benefits.
- Aeration: The different particle sizes create a lofty mixture that allows oxygen to reach plant roots and holds on to decaying organic matter.
- Retains Nutrients: Clay is sticky and provides a surface for organic matter and nutrients to cling to and create a habitat ideal for worms and other organisms.
- Holds Moisture: Loamy soil drains well but doesn't dry out quickly, giving plants constant access to water.
Sounds great, right?
I'm going to show you a straightforward test that will help you understand the basics of your own soil texture. Do you have clay soil that doesn't drain? Is your soil sandy causing nutrients to flow through before plant roots can take them up? This test will help you understand what your soil really looks like and what you can do to build it up properly.
For this test, you will need a few household items.
- Sifted garden soil
- Straight-sided, clean glass jar with a lid
- dishwasher detergent
First, dig your soil sample. It's best to get a good cross-section of soil. You want to know what's happening deeper than just the top few inches, so dig a good 10 inches down and try to get all of the layers you dug through. Also, gathering samples from all over your garden will give you a clearer picture.
Next, allow the soil to dry and then sift it to break up clumps and organic bits (i.e., rocks, sticks, roots, etc.). Fill your jar half full of the medium and then top it off with water, leaving a bit of headspace. Add a tablespoon of dishwasher detergent. The detergent is a surfactant that keeps the soil particles separated. You can use dish soap, but the advantage of dishwasher detergent is that it is a non-foaming agent.
Replace the lid and shake vigorously for a good minute or two. Please don't wear your Sunday best while doing this part, as it may leak. Place the jar on a counter and let it settle.
Almost immediately, you'll see the large sand particles settling at the bottom. And after a few hours, the silt layer will sit on top of that. Clay will eventually filter down and settle on top, but those fine particles take much longer. Set it on a counter and circle back the next day to view the results.
Once you're satisfied with the layers you see, get out a ruler. I prefer using metric on this test, so that is what I'm showing today. First, measure the total height of the soil in the jar: 64 mm. Next, measure the height of each layer. This sample has 35mm of sand, 27mm silt, and 2mm clay. Take each of those numbers and divide the total height to calculate the percentage of each. That means this sample is 55% sand / 42% silt / 2% clay.
Take that information and plug it into the Soil Texture Triangle, published by the USDA, to determine what classification of soil you have. Here's how you read the chart. Clay percentages are read from left to right across the triangle. Silt is read from the upper right to lower left. Sand is read from the lower right towards the upper left portion of the triangle. Where those three lines intersect will show you your results.
My test resulted in a sandy loam soil texture. Soil heavy in sand means I will likely have to irrigate and fertilize more often. To help me know what nutrients my soil is lacking, the next step in building my garden is a soil nutrient test. Instead of staring down the aisle of soil fertilizers with all of their different N-P-K ratios and feeling clueless, I'm going to find out what's actually going on so I can feed it what it needs.