Grit Blogs > The Creative Vegetable Gardener

Do You Know Your Garden Soil?

Karen NewcombMost garden soils are far from perfect, yet many gardeners plant without making any improvements and then wonder why their vegetables are far from ideal.  If you look in your own garden, you’re likely to find clay, sand, and silt.

Clay soil is composed of fine, flat, waferlike particles that fit together tightly and take in water slowly.  Chemically, clay is chiefly silicon and aluminum, with small amounts of sodium, magnesium, iron, calcium, and potassium.  When you sprinkle a clay bed, the water runs off instead of sinking in.  If the clay particles do absorb moisture, they hold it too tightly for the plants to use much of it.  When it’s rubbed between the fingers, wet clay soil feels smooth, soft, and slippery.

When clay dries, it often has the consistency of brick.  The particles are so compressed that there isn’t any space for air to penetrate and plant roots have great difficulty forcing their way down.  Plants grown in untreated clay soil are often stunted and have pale green or yellow leaves.

Sandy soil is the opposite end of the scale.  It is lighter than clay but has particles 25 times larger.  While sand is easy to dig in, it has almost no capacity to store water, which moves freely through the soil and quickly leaches out the nutrients.  Sandy soils warm up faster than clay soils and reflect a considerable amount of heat.  Most sandy soils, however, contain enough clay and silt to retain water and nutrients.

Sandy soil rubbed between the fingers feels grainy and gritty.  Plants grown in sandy or gravelly soils frequently have yellow or pale green leaves.

Silt falls somewhere between clay and sand.  It consists of medium-size gritty particles that pack down hard almost like clay and is seldom very fertile.  If silt topsoil covers a layer of heavy clay, the plants may be stunted because the clay layer traps and holds water.  Silty soil rubbed between the fingers feels slippery but has a grainy texture.  Plants grown in silty soils often have pale green or yellow leaves.

Loam is the kind of soil every gardener wants, and although it isn’t the theoretical ideal described earlier, it’s close enough to grow great vegetables.  Loam is crumbly granular soil that has close to even quantities of different-size particles and a good supply of humus (decomposed organic material).   A combination of root growth, worms, and bacteria gives this soil’s grains a good structure, enabling both adequate retention of water and proper drainage.  Similarly, air moves freely through this soil, so roots can find their proper depth.  Loam consists of 7 to 27 percent clay, 28 to 50 percent silt, and 20-45 percent sand.  Plants grown in loam are usually vigorous, healthy, and green.

Improving Soil Structure

You can improve clay, silty, and sandy soils by adding massive amounts of organic material.  Shovel 6-8 inches of compost, ground bark, sawdust, leaf mold, manure, or peat moss over the top of the garden area and then spade or rototill this layer into the soil. 

Organic particles in clay soil hold the fine clay particles apart while acting as a king of glue to hold the fine clay particles together in crumbs.  This opens up the soil and allows air and water to circulate freely, which gives vegetables a fighting chance.

Heavy clay soils can also be improved by adding 40 to 50 pounds of gypsum (available at nurseries) per 1,000 square feet.  The positive calcium ions in the gypsum neutralize the negative sodium ions in the clay and allow the clay particles to group together, into bigger soil crumbs that create larger air space and permit good water and air penetration.

Adding fine-textured organic material to sandy soil fills the spaces among the grains and helps retain water both by stopping the flow and by absorbing some of it.  As you get to know your soil, you may discover other structural impediments, such as rocky or shallow soil.  If rocks are your problem, you’ll have to bring in a foot or two of topsoil and do your gardening in raised beds or pick out the rocks a few at a time with a spading fork.  Neither solution is easy or fun.

To get the best possible vegetable growth, the roots must be provided with plenty of space straight down.  Some of these plants have roots that try to reach China.  Tomato roots, for instance, often reach a depth of 10 feet or more; cabbage, 7-8 feet; and carrots, 5 feet.  Since most vegetables can survive in a depth of 1-2 feet, and you have limited depth, don’t plant tomatoes or other long rooted vegetables.

Alkaline or Acid 

Tell a gardener that their soil is too acid or too alkaline and they will nod as if they already know all about it.  Tell them that their soil has a pH of 5.7 and as often as not you’ll get a blank stare.  Soil is either acid (sour soil) or alkaline (sweet soil), using a scale of 1-14 that represents hydrogen-ion concentration, or pH.  It’s actually simple:  7 is neutral; below 7 is acid; above 7 is alkaline.

Vegetables, however, are finicky.  Each type has its own particular pH requirements.  Since it isn’t practical to make one planting section of your garden one pH and another section a different pH, most gardeners compromise on a slightly acid to neutral soil (pH 6.5-7).

Soils turn acid when calcium and magnesium ions are leached out and replaced by hydrogen ions. This occurs frequently in areas of heavy rainfall.  Soils become more alkaline as calcium, manganese, and sodium ions accumulate and replace hydrogen ions.  This often occurs in areas of low rainfall and poor drainage, as well as in regions where there are natural limestone deposits.  The soil in the Southwest tends to be alkaline; in the Northeast is tends to be slightly acid.

If your soil is too alkaline, your plants may show yellow leaves, stunted growth, and leaf margins that appear burnt.  Alkaline soils are sometimes too salty; and, in extreme cases, heavy brown or white salt deposits are left on the soil surface.  Acid soil is not easy to detect visually and will generally require some sort of pH test.

Testing pH is a simple task.  Buy a pH test kit.  If you need to make a number of pH tests on a continuing basis, a pH meter is the most convenient device and found at most nurseries and seed catalogs.  If you’ll be making only a few tests in a small plot, a pH test kit will do the job.

To counteract acidity, add ground or dolomitic lime at the rate of about 4 pounds per 100 square feet for each unit of pH below 6.5.  To correct alkaline soil, add sulfur at the rate of about 4 pounds per 100 square feet for each unit of pH above 7.  Gypsum and soil aluminum sulfate can also be used.  Follow the directions on the package.

I recommend visiting Peaceful Valley Farm & Garden Supply.  They have soil mixture recipes that you might want to check out.  They also carry planting mediums and amendments that they ship.

© copyright by Karen Newcomb