Thoughts about soil texture

 Soil texture is one of the most important and fundamental properties of soil. It not only governs the behaviour of  soil with respect to water storage and movement, but additionally influences a range of other properties such as natural soil fertility, soil structure, soil erodibility etc. In Western Canada finer textured soils, which possess excellent water storage capability and superior fertility status, are considered the most desirable for semi-arid crop production. Figure 1 provides a very generalized look at the distribution of various soil textures in the province. The areas labelled fine or very fine would generally be clay or heavy clay textured. The medium and moderately fine soils would be some form of loam texture and the course and moderately course would be classified as sand to loamy sand. We will learn shortly that soil texture and parent material deposition are directly related. You may notice that the medium textured materials (orange color) dominate the Saskatchewan landscape, they represent about 60% of agricultural land of Saskatchewan. Soils of medium texture are normally formed from materials deposited during the advance of the last glacial period in Saskatchewan's prehistory. Soils of fine and very fine texture were generally deposited in the still waters of lakes formed during the melt and subsequent retreat of those great ice sheets.

Figure 1. A generalized map of soil texture in Saskatchewan, courtesy of D. Cerkowniak, SK Land Resource Center, AAFC, Saskatoon, Saskatchewan.

Saskatchewan is blessed with a surprisingly large acreage of soils formed from fine materials deposited in glacial lakes (areas mapped in dark green). These lakes were formed during the melt phase (deglaciation phase) and remained for long periods of time because the melt water was trapped between the retreating ice sheet to the North and the land of higher elevation to the south. In a subsequent section of the course that examines soil formation (genesis) in Saskatchewan, we will look in more detail at the various depositional processes that occurred during the last glacial period.

What is the best soil texture?

Based on my previous discussion you may have come to the conclusion that a clay to heavy clay soil is the "best" texture. A better view is;  the best soil texture depends on its intended use.  Dry land farmers tend to pay top dollar for fine textured soil for a reason. These soils posses excellent natural fertility and water holding capacity which supports good dry land crop production.  These soil usually have additional benefits such as level topography and little to no stones. These characteristics further add to their attractiveness for crop production. That same clay soil, however, if located in a high rainfall area, would be much less suitable unless you were considering rice production.  A gardener, in contrast, may prefer a medium textured soil (loam, silt loam to clay loam) because he /she often has a supplemental source of water so moisture holding capacity is not as important. An additional benefit of medium textured soils is the ability to absorb water more rapidly and therefore experience less runoff.  Medium textured soils are also easier to work (till) than there heavy textured counterparts, because they are less sticky when wet and less likely to get rock hard when very dry. This point of view would also hold in a field situation if it were being developed for irrigated production. A medium textured soil is generally preferable, because irrigation systems apply water at fairly high rates and these soils are able to absorb the water with little or no runoff. In contrast, irrigation on heavy soils is difficult and requires specialized application equipment.

In the world of engineering and construction, the view of ideal soil texture may be quite different. The construction of roads and building requires soils which are stable and allow water to drain away quickly. In this case coarse textured soils are preferable, they are less compressible and do not exhibit shrink/swell behaviour that clay soils do during wetting and drying cycles. Therefore cities like Regina or the east side of Saskatoon which are built on clay to heavy clay soil are notorious for cracked and heaving basements. Houses built in these area must employ specialized construction techniques to avoid damage to basements.

Wet clay soils can be a challenge

Behavior of heavy clay when dry is also challenging

A final thought about soil texture and crop production. 

The adoption of modern crop production techniques including direct seeding/minimum and zero tillage has narrowed the traditional productivity gap between the fine and medium textured soils. Clay soils were particularly superior when farmers employed the practice of summer fallow. The objective of summer fallowing was to store extra soil moisture for subsequent crops during the 18 month fallow period. The storage capacity of the fine textured soils is high which made them especially effective for that purpose. Modern minimum or zero-till production systems in combination with continuous cropping have a much shorter period (6 months) of moisture storage,  so storage efficiency is a less important soil characteristic.  The reduced tillage systems we now employ retain more crop residue on the soil surface. This decreases moisture loss via evaporation and therefore contributes to improved storage.  Improved fertility practices have also improved crop water use efficiency so in the end the yield differences between the heavy and medium textured soils has narrowed substantially.  In the end, to rationalize the extra $ to buy C to HvC soil, one must place a high value on level topography and few stones, because the yield differential between medium and heavy textured soils is often slim to none.