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 Garden-Time.com

Copyright © 2003 Brian Ballard

Getting Creative with Compost Ingredients

There's no special trick to successful composting other than ensuring a good mix of greens and browns. But there is an endless list of things that might be good -- or might be wrong -- to add to your compost.

The Good

Though potentially off-the-wall, these items can actually increase the overall benefits of your compost:

• Clay - Soil should be a good mix of organics, sand and, yes, even clay. The clay in dirt can even help capture nitrogen (as ammonia) before it escapes from your pile. (Don't add modeling clay.)


• Egg shells - These add calcium to the soil. (Don't compost rotten eggs.)


• Granite dust or greensand - This adds several trace elements. You can get granite dust at tombstone manufacturers. (Greensand is just a term for rock dust.)


• Hair - This sounds icky to me, but human and pet hair contains lots of nitrogen and composts nicely I hear.


• Junk mail - This is the most satisfying thing to tear up and throw in a pile! (But see glossy paper below.)


• Oyster shells - Either whole or preferably crushed, these add calcium.


• Seaweed - Seaweed contains lots of nitrogen. Wash it down first to remove salt.


• Soil - Some gardeners like to jump-start their compost by introducing extra organisms from soil. Though all organisms needed for decomposition already exist on plant material, adding a few shovels-full of soil can't hurt.


• Wallboard or drywall - Crumble it up as much as possible, and don't over-do it! Drywall is made of gypsum, which includes lots of calcium.


• Wood ashes - Wood ashes from non-pressure treated wood are high in potassium and can be used sparingly in compost. Don't use more than one or two shovels-full per batch since ashes are very alkaline and will disrupt the pH of your finished compost. (But see below for coal or BBQ ashes.)

The Bad

These things can go into compost if you're really, really good at composting, but the average gardener should avoid them:

• Dairy products - Same issues as meat (below).


• Diseased plants - Your pile would have to get awfully hot to kill all viral pathogens. Bag diseased plants at your plot and take them home for disposal.


• Fat or lard - Same issues as meat (below).


• Human urine - Though urine from a healthy person is sterile and contains lots of nitrogen, don't include it at our community garden. You can give it a try in your home compost if you must.


• Meat scraps - It takes a very specific set of conditions to safely decompose meat or dairy products. It's very hard for the average gardener to reproduce these conditions.

The Ugly

Strictly verboten! Never ever, ever put these things in your compost. At best they won't decompose, at worst they may cause disease in humans!

• Animal urine, feces, or litter - Cat feces in particular can contain parasites that cause brain damage in toddlers, infants, or unborn children. (Pre-composted horse, cow, rabbit, chicken manure or manure from other vegetarian animals is fine to add to your compost.)


• Coal ashes - These ashes contain harmful chemicals, even after burning. Don't use BBQ grille charcoal ashes either.


• Glossy paper - Don't compost glossy paper, catalogs or magazines. They include special clays and inks that aren't the best for your soil. Don't compost paper with colored ink either.


• Human feces (including diapers) - Leave it to the city to try to figure out how to deal with your poop! It can contain all kinds of viruses, bacteria and other diseases like dysentery, e. coli, and cholera. Even composted human waste from municipalities should never be used on food crops.


• Metal, Plastic or glass - These items will simply never decompose. Broken glass in the soil is obviously dangerous to gardeners.


• Pesticides, herbicides or fungicides - These will kill the very organisms you're trying to encourage in your compost!


• Petroleum products - Liquid petroleum products will poison the soil and groundwater. One measly pint of gasoline can contaminate 750,000 gallons of water.


• Pressure treated wood or sawdust - Pressure treated wood contains cyanide and/or other chemicals that kill beneficial soil organisms.

More Reading

Lists of other things you can or should not put in your compost:

Compost Ingredients at MasterComposter

All About Materials by Steve Solomon

What can I compost? at the HDRA

Materials for Better Composting at the Chicago DOE


Copyright © 2003 Brian Ballard. All rights reserved.