Soil texture and soil structure

As a professional gardener, I have encountered some beautiful and well structured soils in my time. As with most aspects of nature, there are plenty of mediocre and poor soils to maintain the overall balance. I’m not exaggerating to say that I’ve also brushed up against a few of these from time to time. From personal experience, I’m convinced that poor soils can be drastically improved with a degree of planning and effort. Having said that, I always suggest that beginners attempt to find the most suitable soil types for the plants they wish to grow. The ability to understand various descriptions of soil types depends upon some very basic understanding of soil texture and soil structure.

When separated into its basic components, soil consists of rock or mineral particles, organic material, air and water. Included in the category of organic material are the millions of living organisms whose function is to break organic compounds into simpler molecules and nutrients which will sustain the growth of new vegetation.

In most soils, the solid matter will contain approximately ninety percent rock or mineral particles, the remaining ten percent being organic material. It is the relatively small percentage of organic material (also known as humus) that enables a soil to support the growth of plants. When the humus is depleted, soil fertility will decrease and most plants will struggle to survive.

Soil texture

Soil texture refers to the type and size of mineral particle that dominate the solid matter. The common types of particle are described as sand, clay, and silt.

Sand particles are large in size and tend to bind loosely with pockets of space around each particle. This will provide plants with the aeration needed for good root growth. Sandy soils also promote excellent drainage as water moves freely between the pockets. The negative side of this free movement is the tendency for such soils to dry out quickly and also to become leached or deprived of water soluble nutrients.

Clay particles, on the other hand, are comparatively small in size and bind together tightly without much space between them. A clay soil will, therefore, hold water and nutrients but may suffer from poor drainage if water fails to escape the impermeable barrier of tightly bound particles. For a similar reason, plant roots can be deprived of air in a heavy clay soil.

Silt particles rank between sand and clay particles in size. Silt is fortunate to possess the good drainage qualities of sand as well as the nutrient holding capacity of clay. Soils principally composed of silt are unusual while combination soils containing a mixture of the different particle types are relatively common. A balanced soil with equal parts of sand, clay, and silt is called loam.

If you’re unfamiliar with the physical properties of soil, the best approach is to venture outdoors in different types of weather and apply all of your senses (and common sense) to investigate. An experienced farmer or gardener will begin to evaluate the texture and structure of a soil by considering it’s behaviour across a range of conditions. I tend to place considerable emphasis on the surface appearance. When the surface appears hard and cohesive, there is likely to be a high proportion of silt and clay particles. Depending on the underlying structure, a heavy watering of clay soils can pool at the surface before absorption occurs. The weight of this surface water may have the effect of packing the underlying particles more tightly and increasing the overall resistance to moisture. Sandy soils tend to be unformed and relatively permeable near the surface. I have heard them described as dusty or loose. The sand particles themselves are usually quite dense and uniform in size. The dust appearing on the surface is probably the type of surface erosion which is common for both coastal and inland soils with a high sand content.

Soil structure

Soil structure describes how the various particles are organised. For example, vertical cross sections may reveal submerged layers of clay supporting a bed of loam. The structure of a soil will develop naturally through the effect of weather cycles. Moisture will cause a soil to swell and expand whereas drying encourages shrinkage and contraction. Some layers will be affected by the action of freezing then thawing. This is important in forming the aggregate crumbs contained within a loam. These consist of sand, clay, and silt particles grouped together with humus. The relative movement of the soil layers, in combination with the particles combining to form aggregates of various size and shape, will contribute to the soils ability to hold or release air, moisture, and nutrients.

The “no dig” approach

Have you come across any of the organic gardening publications recommending the no dig approach to gardening? They’ve been around for a while now. I must admit that when they first arrived, I was advising my friends and colleagues to toss them directly in the rubbish. I strongly believed in working a soil. Perhaps I’ve mellowed a bit since then. Advancing age has left me less enthusiastic about vigorous physical exertion. I’ve also had some persuasion from experienced gardeners who dug too deep and regretted it, so to speak.

One of my old colleagues from Britain had been treating a heavy clay soil with gypsum to improve its physical characteristics. The soil had originally been quite acceptable until his employer decided on an expensive collection of plants better suited to a well drained loam. In his efforts to please, my colleague borrowed a rotary hoe and used it several times to incorporate large quantities of gypsum and river sand. Unfortunately, his disruption of the soil’s fragile structure was extreme. Following such treatment, he discovered the previously cohesive soil lost most of it’s mechanical strength and stability. When exposed to heavy rain, it’s surface layers rapidly deformed to a spewy bog (his description). After struggling against this for several years, my embarrassed colleague succumbed to defeat and consulted a professional drainage contractor. This alleviated the immediate problem, although the soil structure was never fully restored.

In most cases, natural soil structure needs to be protected. This requires care during any activity which involves excavating below the surface or increases the risk of surface erosion. Intensive cropping, poor management, ploughing or harvesting with heavy machinery, even the tread of livestock can contribute to structural changes in a soil. Some farming and gardening practices can disrupt the structure so extensively that soil fertility will be affected, usually in the negative sense.