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The ABC’s of Soil Science

By   /   May 13, 2013  /   2 Comments

If you want to know how to create better soil, you’ve got to know what makes soil work for you, and what you’re starting with. This is the first in a series to get you on the dirt road.

Full confession: I am a soil geek. I love thinking about soil, working it, and smooshing it between my fingers. If you ask me a question about soil, I’ll bend your ear. Kathy and I were talking the other day, and she said she wasn’t completely clear about soil pH.  Who is? we wondered. And what about soil texture? Cation exchange capacity? Anions? Did you know that when you’re standing out on the pasture, half the soil’s volume is pore space, and only half is solid material? If any of these topics intrigue you, read on! The more you know about your soil, the better!

Click here for a little fun, and a flashmob rendition of Julie Andrew's singing Do A Deer at the Antwerp Train Station

Click here for a little fun, and a flashmob rendition of Julie Andrew’s singing Do A Deer at the Antwerp Train Station

Putting names to things is a way of understanding them better.  In this case, we’re going to put names to soil characteristics as a tool for helping us understand our soil’s potential so we know what we can realistically expect from it.

To really understand soil, let’s see it as something with physical properties first and foremost. Not to sound like Julie Andrews in The Sound of Music, but let’s start with the ABCs of soil. When you read, you begin with ABC, when you sing, you begin with do, re, mi. When you think of soil, you can start with color, texture, and structure. Each property tells us something important about the soil.

Soil color comes from where and how it developed, and what minerals and materials it contains. Soils in cooler, wetter climates are usually grayer and browner in color. The colors may come from

Checking soil color against Munsell color book. Smithsonian Institute - Dig It! project

Checking soil color against Munsell color book. Smithsonian Institute – Dig It!

organic matter that hasn’t decomposed. Soils in dryer and hotter climates are usually lighter in color and redder. There, organic matter doesn’t stick around as long, and iron and other minerals have oxidized, showing off their brighter hues.

Soil color can be measured using fancy lab equipment or using something called a Munsell color book. The Munsell book is like a book of paint chips, ranging from reds and yellows to grays, greenish and bluish colors too. Each page and chip has a name. Clump of soils of soil are compared to chips to pick the color name for recording and management.  When soil is moist, though, it will be darker than when it is dry.

Soil texture is related to a soil’s ability to hold and retain nutrients and.  It is described by a math equation. Do you remember trying to figure out what x was? If you knew that x + y + z = 100, you have to know what y and z are.  In this case, soil is made up of three types of soil particles, like the x, y, and z. There are the larger particles, sand, which are between 0.05 and 2.0 mm in diameter. Then there is silt, which is about 0.005-0.05 mm in diameter. And finally, there is clay, all the soil smaller than 0.005 mm.  All together, the 3 particles are in soils in proportions that total 100%.

Two soil aggregates that look the same. But are they? Let's drop them in water and see what happens.

Two soil aggregates that look the same. But are they? Let’s drop them in water and see what happens.

LOOK OUT! It's a 3-sided graph!!! It's also a cool tool to check out the texture of a soil with, say, 35% sand, 25% clay, and 40% silt. Follow the arrows, and you will find them intersecting ---at loam!

LOOK OUT! It’s a 3-sided graph!!! It’s also a cool tool to check out the texture of a soil with, say, 35% sand, 25% clay, and 40% silt. Follow the arrows, and you will find them intersecting —at loam!  Click on the picture to make it bigger and easier to see.

There are different names describing soil textures, textural classes, each name based on the proportion of sand, silt, and clay particles present. For example, a sandy loam has 60% sand, and 25% silt, meaning that the remaining 15% is clay. Similarly, a clay loam could contain 30% clay, and 30% silt, along with 40% silt. There is a nifty 3-sided graph you can use to check this out.

You may have noticed a difference between the proportions in the two textural classes. Clay loam gets called “clay loam” with a lot less clay in it; sandy loam has twice as much sand as clay loam has clay to earn its “sandy” name. That difference comes from the wallop packed by clay particles; they have a lot of personality, conveying physical and chemical properties to the soil.

How can you figure out the texture? There are lab procedures that separate the soil particles and determine how much of each is present. Your soil test may tell you. Or you can finesse the fun field option of “texture by feel”. It might feel like you are going back to your play-dough days, but it’s a truly scientific form of play.

The third physical property of soil is soil structure. The structure of the soil is how the particles are held together, or aggregated. What shapes are the aggregates? Are they well aggregated? Do they hold together well? A lot of the structure is due to management, but some is related to the soil’s texture.

For example, you’re not going to see well-aggregated sand dunes. The stickiness of clay will help hold soil together.  Organic matter, though, makes fabulous glue. Soils that have not been tilled and have accumulated organic matter will have strong and lovely aggregates. And that is all about management.

So what, you may be asking? So what if I have a sandy loam or a clay loam? So what if my soil aggregates are Superman strong or non-existent, and my soil seems to be one big clump in the field?

Soil texture and aggregation makes a world of difference in how your fields and pastures will collect and hold water and organic matter.

On opposite ends of the spectrum: Sandy soils will allow water to infiltrate quickly, but they won’t hold it long. Clay soils will hold water longer, but may take longer to dry out. Silty soils provide a happier medium, but they don’t provide some of the other funky chemical qualities clay particles offer.

Once you’ve bought your farm, you can’t do much about the soil texture. The color is what it is, too. But you can do something about the aggregation. Well-aggregated soils will give you the best of both worlds. The soil aggregates, or small clumps, will provide spaces between them, macropores, where water can infiltrate quickly. And they will provide micropores that will hold water for plants to take up in dry periods.

This pie is what makes up the ideal soil: half solid, half pore space. The pore space is evenly mixed between air and water.

This pie is what makes up the ideal soil: half solid, half pore space. The pore space is evenly mixed between air and water.

A well-aggregated soil will be made up of half solid particles and half pore spaces. Those pore spaces will be a mix of water and air.

The best way to build up aggregation is to allow organic matter to accumulate (this might darken the soil color a bit, too). Avoid tillage; avoid bare soil. Let your animals graze, but not overgraze. Feed the soil, and it will feed you.

You’ll get strong soil aggregates, glued together by amazing organic compounds. Soil aggregates give you lovely pore spaces, which means better homes for soil organisms. Better homes, along with more organic matter (food) will promote biological activity, which in turn will speed up nutrient cycling, which will help plants grow. It won’t happen overnight, but it will happen. And that is the goal.

The nitty and gritty behind this, the stories of organic matter, soil biology and soil chemistry, are other wonderfully yummy soil topics for other days. Till then, happy smooshing!

 

 

 

About the author

editor and contributor

Rachel's interest in sustainable agriculture and grazing has deep roots in the soil. She's been following that passion around the world, working on an ancient Nabatean farm in the Negev, and with farmers in West Africa's Niger. After returning to the US, Rachel received her M.S. and Ph.D. in agronomy and soil science from the University of Maryland. For her doctoral research, Rachel spent 3 years working with Maryland dairy farmers using management intensive grazing. She then began her work with grass farmers, a source of joy and a journey of discovery.

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