Friday, July 18, 2008

Soil Basics

Good soil is more than just dirt; it's a complex mixture of sand, silt, clay, and living and dead organic matter. Soil is about 50% mineral matter, 25% water, 25% air and 5% organic matter. (Yes, I know that adds up to more than 100%, but these are approximate figures.) Plants absorb nutrients from the minerals in soil.

Loam, the most desirable mineral matter for growing vegetables, is made up of about 40% sand, 40% silt and 20% clay. Sand is made up of larger particles about 0.5mm to 2mm across. Silt particles are about 10 times smaller than sand and are usually coated in clay. Clay particles are 100 times smaller than sand.

Sandy soil has large spaces (pores) between particles that lets water run through too fast, leaving it too dry and washing away nutrients. The pores in clay soils are so small that they trap too much water, pushing out all the air and drowning roots. A good mix of all three components holds enough water and air for healthy root growth. Clay even carries a slight negative charge that helps it capture nutrients, which are typically positively charged (the main exception being nitrogen).

Simple Soil Composition Test

To test the mineral makeup of your soil, mix several handfuls of soil from around your garden plot together (taken from the root zone about 4-8 inches deep). Fill a medium or large jar about half full with some of the mixed soil. Fill the rest of the jar with water. Cover the jar and shake it well. Set the jar aside for a few hours or days until all the soil settles. Sand will settle first, then silt, then clay. Organic material may form a thin layer on top of the clay, or might still be floating. The layers of sand, silt, and clay should be fairly distinguishable. In optimal soil, each layer should be about equal (with maybe a slightly thinner clay layer).

Correcting large deficiencies of sand, silt, or clay is usually not feasible since you'd need much more of each ingredient than you would care to lug to your beds. If there's not enough sand, you might consider adding a few bags of play sand or greensand. If there's not enough silt or clay (I know that sounds like garden heresy, but clay really is important), you might bring a few buckets of local clay-laden dirt from your yard (if you own a house.)

The best solution for any type of imbalanced soil is simply to add more organic matter such as compost or manure. Organic matter helps aerate clay soils, helps sandy soil retain water, helps buffer pH levels, and feeds the living organisms necessary for good soil. Yet another reason to make your own compost!

More Reading

The Real Dirt on Dirt by John Harmon on HGTV

Soil Common Sense by Charlie Nardozzi at

Copyright © 2003 Brian Ballard

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