Soil pH Basics

A pH value indicates how acidic or alkaline something is on a scale from 1 to 14. Pure rainwater or distilled water is neutral with a pH of 7.0. Acids have pH less than 7 (lemon juice and vinegar are around pH 2). A pH above 7.0 is more alkaline (baking soda is around pH 8, ammonia is about pH 11).

Most vegetables like a neutral or slightly acidic soil (pH 6.0-7.5). In areas with wet, rainy climates like the Pacific Northwest, soils tend to be acidic. Soils with lots of leaf matter also tend to be acidic, as do clay soils. So the soil in your Seattle garden is more likely to be slightly acidic than alkaline - but don’t worry, it might not be too acidic.

Simple Soil pH Tests

Here are a few basic pH tests. These test are not exact by any means, but should give you a general idea of whether your soil pH is too far from neutral. If either test indicates your soil is too far from basic, you should get a more accurate test from an expert, or try the mild corrective actions mentioned.

To test if your soil pH is too low (acidic, also called sour): Add a pinch of baking soda to a tablespoon of wet soil from your garden. If you don’t hear a lot of fizzing, then your soil is not too acidic (try testing for alkalinity). To correct acid soil, add a few scoops of lime, dolomite lime, or even wood ashes to your plot and dig it into the top few inches of soil.

To test if your soil pH is too high (alkaline, also called basic or sweet): Add a tablespoon of white vinegar to a tablespoon of dry soil from your garden. If you hear fizzing, then your soil is probably too alkaline (try testing for acidity). If you don’t hear a lot of fizzing, then your soil is not too basic. To correct slightly alkaline soil, dig peat moss into your planting beds.

Copyright © 2003 Brian Ballard. All rights reserved.

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.

When Your Garden Sleeps, It Dreams

This winter it may seem nothing is happening in your garden. Biological activity slows down in the colder months, organisms go dormant or migrate deeper into the soil, cool season weeds like chickweed get their first footholds, and rains wash nutrients and important minerals away. You don’t have to be a helpless observer of this hibernation. Use the winter months to rebuild your soil.

Once your garden is done for the season, clear out the last of your annual plants saving them for compost. Reserve a small part of the garden for your fall and winter greens and for planting garlic cloves in November. Cover the rest of your soil with a nice layer of compost materials about 8-12 inches deep. Be sure to use both “greens” (grass clippings, coffee grounds, fresh chopped plants, composted manure) and “browns” (fall leaves, non-pressure treated sawdust, dry plant material) in the mix.

A variety of amendments help replenish the supply of important minerals. Agricultural lime is a good source of calcium, and dolomite lime also includes magnesium. Calcium helps plants build cell walls. Magnesium is the working ingredient in chlorophyll. Both types of lime help neutralize acidic soil, which is also a side effect of too much rain.

A compound called greensand includes a different mixture of beneficial minerals including potassium, iron, and phosphorus. It’s usually available at better garden centers.

You can even use granite dust to add a wide array of minerals to your soil. If you look closely at granite, you’ll notice it’s made up of thousands of different crystals fused together. Each crystal is essentially a different mineral, including many that plants require in tiny amounts: potassium, calcium, iron, magnesium, phosphorus, manganese, and others. These minerals become more available when ground into gravel, or even into dust, and are broken down further by organisms and naturally occurring weak acids in the soil. Ready sources of granite dust include stone yards, or gravestone monument carvers.

Spread a few handfuls of lime, greensand, or granite dust over your compost mix and work it in a bit. Water the mulch well, and cover it with two layers of burlap. The burlap works as insulation, keeping the soil and sheet mulch slightly warmer than the air, and provides the dark, moist environment soil organisms prefer. Most coffee roasters will gladly give you discarded burlap bags their raw coffee beans are shipped in.

Once or twice during the winter, pull back the burlap and turn your compost mix, covering it again with the burlap when you’re done. In the spring, you should be able to remove the burlap and either plant directly into the finished compost, or work it into your soil a bit if it’s not completely done.

Copyright © 2006 Brian Ballard. All rights reserved.