Soil Texture

I grew up listening to talk radio at home.  Specifically KSL News Radio.  On weekdays I didn’t really care either way about it.  But on Saturday mornings?  I loved it.  That’s when they had “The Greenhouse Show, with Larry Sagers”.  People could call in with questions about their yard or garden and Larry Sagers, a horticulture extension agent in the Salt Lake City area, would answer them.  All of them.  I was pretty sure that he knew everything there was to know about plants and gardening. I wanted to be just like him.  Sadly, Larry Sagers passed away from cancer a few years ago.  As far as I know, “The Greenhouse Show” continues with the new horticulture extension agent, but unlike the house I grew up in my kitchen doesn’t have a radio so I don’t listen to it as I do my Saturday morning chores.

I think it was on “The Greenhouse Show” that I first heard about this seemingly magical substance called a “loam” soil.  It seemed that your garden couldn’t go wrong if you had this soil type.  Maybe you’ve heard of this too and wondered whether you have it or not.  Maybe you’ve never heard of a loam and would like to know what the heck I’m talking about.  Maybe you don’t have to wonder because you are a savvy gardener and have had your soil tested.  Whatever the case, that’s what we’re going to talk about today.

It all starts with knowing the relative proportion of sand, silt, and clay particles in your soil.  Remember how we learned that those are the mineral component of what a soil is?  That mineral component is really important because it can determine things that affect the pore space of the soil – things like water holding capacity or nutrient retention.  Each of the three particle types is important, and each is needed to get that desired “loam” soil.

Sand: the particle classes are determined by size, and sand is the largest of them.  It’s size ranges from 2 mm (anything bigger is classified as gravel) to 0.05 mm diameter.  You can see sand with just your eyes.  If you’ve ever been to a beach or played in a sandbox you’ll know it has a gritty texture.  Sand is inert – the particles won’t bind to each other and water or other minerals in the soil won’t bind to sand either.  Because of that, it does a poor job of providing plants with sufficient nutrients.  And because the particles are so large, the pore space is large and water doesn’t hang around either.  It’s one thing to have a well draining soil, but with a sandy soil much more frequent irrigation and fertilization would be required.

Silt: this is the next biggest particle size and ranges from 0.05 to 0.002 mm diameter.  At this size, it’s not visible to the naked eye, but you can feel it.  When it’s wet and you rub it through your fingers it will feel smooth, but not sticky.  Silt particles are only moderately attracted to themselves, so it still remains a well draining soil (like sand) but one with a slightly higher water and nutrient holding capacity.  However, if your soil was dominated by silt, it would benefit from the addition of organic matter.

Clay: the smallest of the three particles measuring less than 0.002 mm diameter.  Like silt, single clay particles are not visible with the naked eye.  But unlike silt, when wet, clay feels sticky.  Clayey soils often get a bad rap for being hard to work with, but as you can see from the  texture triangle below, some clay is needed to get the desired “loam”.  Clay particles are charged, which mean they bind easily to each other, but also they have high water and nutrient retention properties.  Like silt, clay soils can be ammended with organic matter over time to improve the structure.

soil texture triangle

A loam is ideal because it contains sand, silt, and clay in the right quantities for good nutrient adsorption and water retention, but also provide adequate drainage.  It’s really unlikely to have a soil that is 100% sand, or 100% clay, or 100% loam, so really, most soils fall into the “loam” category: sandy loam, silty loam, clay loam, etc.

With practice, you can determine your soil texture by feel, but the most reliable way to find out where your soil falls on the soil texture triangle would be to get it tested.  Which just so happens to be the topic for next time.  Until then, go outside, get into your garden, and see if you can feel any grittiness (sand), or stickiness (clay), or smoothness (silt).

1 Comment

  1. I am sorry I have not spent much time on here before! I’m beginning to feel smarter.

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