Clay, sand, silt & loam: What’s the ideal mix for garden soil?

By now, you probably know what kind of soil you have out there in your “back 40.” If planting that rose bush brought up wads of gummy goo, you know to call it clay. If instead you scooped up gritty particles that didn’t clump together, you have the other extreme, a sand.

Both extremes in soil have their advantages and shortcomings.

These soils act the way they do mostly because of the size of the particles that make them up. Sand particles are relatively large (by definition from 2- to 5-hundredths of a millimeter across). At the larger end of this range, you can easily see them with your naked eye and feel them between your fingers. Clay particles are very small (by definition less than 2-thousandths of a millimeter across).

It’s all about pores

Tiny clay particles have tiny spaces between them — small enough to draw in water and cling to it by capillary action. That can be a bad thing this time of year, when you’re likely waiting for the soil to be dry enough to become crumbly for planting, or anytime if there’s not enough pore space open for roots to get air. Don’t let clay soil get too dry before planting, though, or it becomes rock hard.

The way clay slurps up and holds water makes it a good thing as summer weather turns dry. Some of that water, though, is held so tightly that even roots cannot get at it.

In sandy soils, the larger spaces don’t hold water. This makes for pleasant digging in spring mud season, but has plants gasping for water in dry summer weather.

Another asset to many kinds of clay particles is that they are negatively charged on their surface. That means that positively charged plant nutrients like potassium and calcium can latch onto those clay particles instead of being washed below the roots by rainfall or watering.

Sand particles are uncharged, letting plant food just wash right through and leaving these soils naturally infertile.

The cure-all

You can cure the ills of either clay or sandy soils in basically the same way: with plenty of organic matter. That includes compost, manure, leaves, peat moss, straw, hay, grass clippings, wood chips, and anything else that is or once was living.

As these materials decompose and feed microbes, a witches’ brew of natural compounds is created that affects a soil’s physical, nutritional and biological properties — all for the better. Sandy soils become more spongy so they hold water better, and charged so they retain nutrients. Gums form that stick clay particles together into larger units with larger spaces between them, from which excess water can drain away and let in air.

Be careful about tilling sandy soils. They already have plenty of air and charging them with more causes organic matter to “burn away” too quickly. And be careful about treading or driving on clay soils; compaction, especially when such soils are wet, breaks down those aggregated particles, creating a mass of small particles and pores.

Most soils, of course, are not all sand or all clay. They are a mix of different-size particles that might also include silt, which is intermediate in size between sand and clay. Silt particles make a soil feel silky.

The ideal soil is not one that is predominantly sand, silt, or clay, but one that has all these particles in amounts that let each express itself. The result is a range of pore sizes, some to hold water and some to hold air. Such soils — the holy grail to gardeners — are called loams.

It is neither practical, wise nor necessary to improve any soil by hauling in material to change the particle sizes. All soils though, even loams, benefit greatly from regular additions of organic matter.