As soon as the ground is thawed enough to sink in a trowel, you can perform these hands-on experiments.  Knowing more about your particular corner of earth only takes a few minutes, but gives you insights which set the stage for your most bountiful harvest.

What’s in your soil?

In fact, half of the ideal loam consists of elements we don’t consider to be soil at all: water (25%) and air (25%).  But the solid components determine how much air and water the soil can hold.  Organic matter — humus (decaying plants) and soil organisms — may only account for about 5% of your garden soil.  The balance is mineral particles of varying size, including sand (largest), silt (finer), and clay (finest).  The more sand, the more air the soil will hold, but water will drain away too quickly if sand content is too high.  Silt and clay hold water more effectively, but too much and there may be no room for the air which is essential for root respiration and nutrient exchange.

And so, when we asses soil for gardening, we’re looking for an ideal balance of elements.

When sand, silt, clay, and humus are each present in roughly equivalent quantities, you have a good loam to bring a smile to any gardener’s face.  Once you discover what’s in a spade-full of your own dirt, you can choose to add various amendments to make conditions more hospitable for your garden’s intended occupants.  Or to take another approach, you can choose your plantings based on your soil.  Perennials and fruit trees like sandier (though still moist) soil, while many vegetables such as melons, squash, and brassicas including broccoli, cabbage, and brussels sprouts will do well in denser, wetter soil.  Talk to other local gardeners who may have similar soil.

How to get a good soil sample

Choose a representative spot in a garden bed or planned planting site.  You may wish to test a few different areas, as results can vary even within a small area.  Remove any plants and debris from the surface.  Use a shovel to remove a chunk of soil about 6-8 inches deep and set it aside.  Now you are ready to scoop your testing material into a container.  Insert your trowel vertically along the edges of the hole to obtain a cross-section of topsoil.  This is the crucial layer for most garden plants, especially annuals.  Mix up the resulting strips of soil until you have a fairly uniform substance.

Use your senses
Rub some dirt between your fingers and take a close look.  Is it gritty, crumbly, sticky, fluffy, silky?  You will begin to understand the texture and composition of your garden just by looking and touching.  Next, bring a handful near your face and take a deep breath.  How does it smell?  If your soil is fertile with an abundance of healthy microorganisms, it will smell pleasant and “earthy”.  Any offensive odor indicates your soil is putrefying with anaerobic bacteria and needs aerating — just like tender roots, the “good” bacteria need oxygen to thrive.

Test 1: Soil Composition

Trowel four inches of soil into a quart-sized glass mason jar.  Fill the jar with water up to the neck, tightly screw on the lid, and shake vigorously.  Now set the jar aside for at least 24 hours.  When you return the next day, the sample will have settled into visible layers showing the proportions of your soil components.  Sand goes quickly to the bottom, with silt just above, then clay, then organic matter.  Bits of undecomposed plant matter will float on top.  If the water is still opaque with dissolved clay, try leaving the jar in a dark place (to prevent algae growth) for a few more days.  Measure each layer.

A perfect loam would show around 45% sand, 25% silt, 25% clay, and 5% organic matter.  Notice that sand takes up more room because it will not pack as tightly, owing to large particle size.

What do I learn?  A glance at your sand layer tells you a lot.  If sand is much more than half the mineral content (over 60%), you may struggle to keep your garden adequately watered and nourished.  Add more organic matter such as compost or manure.  However if the sand is much less than half (under 40%), sensitive roots may suffocate or struggle to penetrate the density of your terrain.  Again, adding lots of organic matter is key — but take care to mix it evenly with existing soil rather than just scooping amendments into planting furrows or holes.  If the plant’s immediate soil density differs too much from the general conditions, the roots may fail to penetrate outside of that little pocket of nutrients, leading to an unstable plant.  Talk to your local Cooperative Extension office or experienced local gardeners to learn more about your soil type.

Test 2: Soil Compaction

Hold a wire landscaping flag (or other 2-foot length of straight metal wire) vertically over an untilled garden bed.  Plunge it slowly but firmly into the ground.  When the wire bends, buckles, or stops, mark it at ground level.  Pull it out and measure the portion which sank easily into the ground.

What do I learn? If your garden has a foot or more of penetrable soil, new roots will explore and expand with ease.  Overly dense, compacted soil may have a high clay content and will prevent roots from spreading optimally.  Other causes of compaction include walking or driving machinery over the area.  Tilling can help soften hard ground, but aerating amendments such as compost, sphagnum peat, or coconut coir offer a longer-term solution.

Test 3: Drainage

Dig a roughly cubic hole: 12 inches wide, long, and deep.  Fill this hole completely with water and allow it to empty completely.  Then fill with water a second time.  How much time elapses before the hole is again empty?  Check the hole every thirty minutes until draining is complete.

What do I learn?  If the second emptying took less than two hours, your soil drains too quickly.  Thirsty plants will suffer without very frequent watering, which is particularly impractical if you live in a drought-affected region.  Look for crops that need less water; adding organic matter should help retain water longer.  If any water remained in your hole after six hours, your soil drains too slowly.  Again, adding organic material improves the situation by giving loft to the soil.  Switching to raised garden beds is a good option for those with chronically poor drainage.

Test 4: Living Organisms

Who lives in your garden?  One of a child’s greatest pleasures in the garden is the marvelous abundance of creatures found below the surface.  In various areas of your garden, turn over one spadeful of earth, then gently sift the material and count the visible inhabitants.  Note the number of earthworms, as well as the total insect population.

What do I learn?  A healthy loam has 5 or more earthworms per shovel.  Any fewer, and your soil needs nourishing.  (One exception to this rule: in some parts of the Southwest, earthworms cannot survive in the hot ground, and will be absent from even fertile gardens.)  At least half a dozen other bugs, such as millipedes, beetles, and ants, indicate a thriving ecosystem where beneficial microorganisms can flourish.

Test 5: Tilth

A soil’s “tilth” is a summary of its agricultural prospects.  Tilth combines the properties of composition, moisture, aeration, and drainage to impart a sense of a piece of land’s readiness for planting success.  For this final test, we return to where we started: pick up a handful of earth.  Hold a mound in your palm and compress it between two cupped hands.  Does it hold its shape when released?  Now poke it gently with a finger.  Does it crack easily apart?

What do I learn?  If your soil will not easily cohere into a smooth lump, it is probably too dry, sandy, or barren of healthy bacteria.  If it holds together too persistently when poked, it is likely to be too wet, dense, or clay-heavy.

In Conclusion…

You can choose to go further with more technical testing, but these five tests give you a good “grounding” of your soil structure.  Short of trucking in massive amounts of soil, you may not be able to change your soil’s fundamental composition.  The French concept of terroir (loosely translated as “a sense of place”) celebrates the uniqueness of every regional soil and ecosystem, and recognizes that each brings its own blessings and challenges.  Work with what you’ve got.  Find out what plants love your garden and your dirt.  For those species which never seem to thrive, consider organizing a trade with other local gardeners who till a different terrain.  Every soil, however, can and should be enriched.

For an abundant source of organic material to enhance any soil, consider compost.  Faced with sandy, arid soil, using a drip-system and a soil moisture meter can help.  In heavy, sodden soil, various amendments such as Greensand or coconut coir can promote aeration and drainage.  To improve all soil types, try planting a cover crop, or even simply mulching with straw or fallen leaves during the off-season to provide abundant organic matter to be turned over into the soil in early spring.

Once familiar with these five ways of getting to know your soil, you may find it comes naturally to re-evaluate regularly.  Test at different times of year, and notice how your soil responds to certain crops, amendments, and gardening methods.  Much as a mother can tell at a glance if her child is fatigued or feverish, an experienced gardener develops a close relationship with his or her soil.  Through consultation, trial, and error, you become the expert on maximizing your garden’s potential.

Sources:

Where to Buy a Soil Test Kit

Know Your Garden Soil: How to Make the Most of Your Soil Type

Soil — Site Assessment — Introduction to Perennial Garden Design — University of Illinois Cooperative Extension