So, after week two of my class, an overarching theme has appeared: Surface area per unit soil volume is important.

**Soil Particles**

Let’s start with the basics. There are three general types of soil particles: sand, silt, and clay. The ratios of their relative diameters and specific surface areas (surface area divided by volume) are listed below.

Particle | Relative Diameter | Specific Surface Area |

Sand | 10,000x | 1x |

Silt | 100x | 100x |

Clay | 1x | 100,000x |

While sand is the particle with the largest diameter, it has the smallest surface area per unit volume. This means that for a fixed volume, a soil composed of all sand particles will have a smaller total surface area than a soil composed entirely of clay particles.

So, why is this important? Well, in the post last week we learned that plant roots need three things: water, nutrients, and air. Both water and nutrients are held on the surface of soil particles, and the air is found in the spaces between the particles. A soil’s composition and particle size directly affects the amount of water, nutrients, and air in the soil. The surface area also affects the internal drainage, runoff potential, and how herbicide is applied.

**Aggregates**

If the smaller soil particles have better nutrient and water holding capacities but not a lot of space between them for air, and the larger particles have more space between them, but not as good nutrient and water holding capacities, how can we have the best of both worlds? If your first though was like mine, you answered compromise and choose a medium size particle and would have been wrong. The answer is soil aggregates.

A soil aggregate is a group soil particles that are associated with each other in more or less stable packets (clumps) of soil. If each grain of sugar is a soil particle, then a sugar cube is an aggregate. Aggregates allow for the benefit of a high surface area (from all their smaller components) while allowing the space for air that a larger particle would have. A well aggregated clay soil, with aggregates the size of sand particles will have more water holding capacity, more nutrient holding capacity, and the same amount of aeration as a sand soil of the same volume. Organic matter acts as the “glue” that makes the aggregates possible, which is the real reason why it is so important to add to soils (Until this class I thought it was for the nutrients).

Okay, well that concludes this week’s soils lesson/summary of what I’ve learned. Stay tuned next week for hopefully more (potentially on aeration and drainage of soils).

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I gotta say, I’m confused. If a grain of sand is the largest of the soils, how can it have the least surface area?

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Hi!

That’s a great question. Just looking at surface area, a grain of sand does have the largest total surface area, but since we’re looking at the specific surface area (surface area divided by volume), sand has the smallest of the three.

Think of it this way. We have 2 cubes, one with an edge length of 1cm and the other with an edge length of 2cm.

For each cube, the surface area is the area of each side times the number of sides, so for cube one the total surface area would be 6cm2 and for cube two the total surface area would be 24cm2. Cube two has the larger surface area. The volume of each cube is the length multiplied by the width multiplied by the height. For cube one the volume is 1cm3 and for cube two 8cm3, so cube two also has the larger volume. Now, if we divide the surface area by the volume for cube one we get 6 cm2/cm3 and for cube two we get 3cm2/cm3. This means that cube one has the larger surface area to volume (or specific surface area) even though it had the smaller total surface area and the smaller volume.

Cube 1:

Length = 1 cm

Width = 1 cm

Height = 1 cm

Surface Area of one side = 1 cm^2

Total Surface Area = 6 cm^2

Volume = 1 cm^3

Surface Area/Volume = 6 cm^2/cm^3

Cube 2:

Length = 2 cm

Width = 2 cm

Height = 2 cm

Surface Area of one side = 4 cm^2

Total Surface Area = 24 cm^2

Volume = 8 cm^3

Surface Area/Volume = 3 cm^2/cm^3

Does that make sense? I hope I didn’t go too overboard with the explanation. Thank you for the excellent question!

Jen

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No, definitely not overboard. Math is one of my worst subjects, so the more explanation the better, and I think I actually understood the way you walked me through it. You’d make a great teacher!

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Yay! I’m glad it made sense. And thank you, that’s quite a compliment!

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Good job on the instant question of diameter vs surface area.

Only better teachers merit time & effort explaining omissions, please accept it as praise.

You wrote, “…plant roots need three things: water, nutrients, and air.”

That omits the 4th critical component, w/o which the others are ornamental, at best.

The only known source of energy for life forms is solar energy. In the ground, they’re stored as Schumann waves, and like human cavities, organs, etc., hum along at the same wavelength.

That stored solar energy is the spark for the fuel you mention in water, nutrients, and air”

Of course, if you’re including Schumann waves under “nutrients,” these comments become delete-worthy 🙂

Agreed, you’re a good teacher. You post succinctly, in comfortable language, in each post. Very effective. Thank you on behalf of the 97% who appreciate what you do and omit the thanks. You’re a definite standout on the internet.

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