GARDEN SOIL

SOIL OR DIRT?

The old gardener's joke goes that "soil" is what you grow your plants in, and "dirt" is what's under your fingernails.

Garden soil, where it all begins...
During several Pennsylvania winters, Bob taught adult-education Landscaping classes in Peters Township.

Bob's students always felt that way too much time was spent on soils.
B-O-R-I-N-G stuff they said!
But his curriculum never changed.
Why?

Because everything begins with the soil. Once you understand the basics of soil science and begin to apply what you've learned, plants will start to grow much better for you.

Hopefully, this page and its links will teach you the basics of soil science. Even though these pages specifically address soil conditions in Southwestern Pennsylvania, much of the material here also applies to soil everywhere.

SOIL. . . by the book!

Soil is like a layer cake:

HORIZON	NAME	DEPTH	FORMED BY
A	Topsoil	6 - 12 inches	Intermixing of organic matter with mineral matter
B	Subsoil	10 - 20 inches	Losses from the "A" horizon
C	Parent material	-	The material from which topsoil evolves

So what is soil?

Average soil profile (by weight):
95% to 99% - mineral matter
5% to 1% - organic matter

Average soil profile (by volume):
50% solid -- 25% water -- 25% air

Therefore, 50% of the average soil profile is "pore space" (25% air + 25% water = 50% pore space). It's this pore space where plant roots actually grow. Think about it: Plant roots actually grow "between" the soil, not "in" the soil.

50%
SOLID

25% WATER

25% AIR

Also, it's this pore space that gets reduced when soil is "compacted." Everyone knows heavy equipment can compact soil, but most people don't think about how much compaction can occur from ordinary foot traffic, such as on a golf green. Compaction is an ongoing problem on athletic fields, especially if fields are used when the soil is wet.

Sand, Silt or Clay?

Soil is classified by particle size, with sand being the coarsest and clay being the finest. Pittsburgh soils tend to be predominantly clay.
Clay soils have the "bad rap" since they can exhibit poor drainage characteristics. However, clay soil has good nutrient-holding capability (a high C.E.C.) and will hold moisture much longer than sandy soil. Most eastern U.S. subsoils are 70% clay.

*Soil separates*	*Diameter (mm)*
Clay	< 0.002
Silt	0.002 - 0.02
Sand	0.02 - 2.0

Soil Structure

DEFINITION: The combination or arrangement of the primary soil particles (sand, silt, clay) into secondary particles ("units" or "peds"). Rototilling soil that is too wet breaks down soil structure and causes a crusted surface.

Soil pH

The activity of Hydrogen ions in the soil determines its pH as being either acidic, neutral or alkaline.

Rainfall (H2O) in S.W. Pennsylvania continually introduces Hydrogen (H) ions into the soil solution, causing soil to revert back to an acidic condition. (This is why soil should be tested every 3 to 5 years to determine its changing needs for lime)

The logarithmic scale measuring pH extends from zero to 14, with 7 being neutral. Soil with a pH of 6 is 10-times more acid than soil with a pH of 7, while soil with a pH of 5 is 100-times more acid than pH 7. Generally speaking, most plants grow best in a soil pH of 6.5 to 7.2

Most soils we have tested in Washington County and Allegheny County range from pH 5.5 to 7. Unless "acid loving" plants are growing in these areas, agricultural lime is used to raise the soil pH into a better plant growing range. (See Soil pH for more information)

Some other "soil basics"

Never rototill or "work" soil when it is too wet! Tilling wet soil will ruin soil structure and result in the formation of hard clumps. Always allow time for your soil to dry enough for proper cultivation.

When tilling the soil, don't overdo it. It is best not to rototill the soil into a powdery consistency.
Heavy clay soils can be improved by working in organic matter. (See amendments for more information) The addition of lime (if needed) or gypsum, will also help improve "heavy" soils.
Correcting your soil pH will "unlock" nutrients which are already present in the soil, and help maximize dollars spent on fertilizer.
Some chemicals move very slowly down through the soil profile, and should be mixed-in with the soil at planting time. These include phosphorus and calcium.

Other soil pages