Soils Primer

I like to tell people when on the topic of soil that “soil isn’t dirt.” Dirt is what you end up with in the dustpan after sweeping out your garage.”

Not all soils are the same. They range from very good to very bad. In this article I will discuss soils that are essential to our landscape trees, shrubs, and gardens plants. Their health and longevity depend on it!  Soil science is a very complicated topic especially when discussing soil chemistry. To understand soils, we must have some understanding of what a good soil is made up of and how it relates to plant health. 

Good soils consist of a blend of sand, silt, clay, and organic matter. A good blend gives us the more desirable loam soils. Too much clay gives us clay soil. Likewise, too much sand gives us sandy soil. A moderate blend might give us a clay/loam soil. Tip: Fill an empty mayonnaise jar one third full of soil and fill the rest of the way with water. Shake vigorously and then let it settle for several hours. As it settles, the particles will settle with the heaviest particles on the bottom and the lightest on top. You will see four layers. Organic matter, clay, silt, and sand in that order from top to bottom.

Just as important as the component blend is their particle shapes and sizes. Visualize sand (for comparison’s sake) as a granule the size of a basketball, silt as a granule the size of a marble and clay the shape of a pancake and the size of a pin tip. Why is this important? Too much sand and the soil will drain quickly and dry out. Sand also doesn’t retain important mineral elements found in fertilizer and a deficiency could develop. Too much clay and the pancake particles stack and compact. Clay doesn’t drain well and contains little pore space for both root and oxygen penetration. This explains why, in heavy clay, we find roots growing on the soil surface. Clay does contribute to a soil blends mineral holding capacity, so it’s not all bad.

Soil structure describes how the blended soil particles come together creating the all-important macro- and micropores. All of this influences every aspect of soil management and root penetration.

When wet, for example, clay soil becomes sticky and mucky. It excludes oxygen because the macropores have been compacted or filled with water and should not be worked until it dries. A loam soil has large macropores. It drains well and is friable, a term that describes the soil’s ability to easily break up into smaller particle sizes.

A good soil is alive with soil microorganisms, earthworms, and other arthropods. Their job is to break down the organic matter in the soil, releasing mineral elements and maintaining good soil structure. There are beneficial fungi and bacteria, which inhabit the soil and assist the root system with water and mineral absorption.

Topsoil has most, if not all, of the beneficial attributes we are looking for. Subsoil is the next layer you come to when the topsoil is removed during the construction process and is not as desirable.

Soil chemistry is essential to nutrient cycling and availability. All plants require 16 elements for their life-giving processes, most of them coming from the soil. A good soil blend ensures that these elements are available in a form available to the plant.

Soil pH can have a great influence on their availability. The most desirable soil pH range is from 6.5 – 7.

Many of our urban soils are what we refer to as disturbed soils. They are poor in text structure and organic matter, often very compacted. Very few beneficial organisms find it hospitable. Urban soil is often contaminated with toxins and trapped gases. The pH has probably been altered leading to poor nutrient availability. It drains poorly and is lacking in oxygen.

In closing, if you go to dig in the soil and find it dark, moist, deep, and crumbly, chances are your plants will do well.