What constitutes desirable soil structure?

My previous blog discussed changes in Western Canadian cropping systems over the last 40 year period and the benefits that have accrued to soil organic matter, soil structure and water infiltration because of those changes. The question not explored thus far is, why have those cropping system changes had a beneficial impact on soil structure? The answer lies largely in the impact of these system changes to the biological processes that influence soil structure development. The other key issue is that tillage, which is so deleterious to soil structure, has been greatly reduced with the advent of  min-till /z-till systems.

Before discussing soil structure development further it is appropriate to describe what constitutes good soil structure. Fundamentally it involves the development of a relatively stable soil aggregate system that produces many crack/pores/fissures etc which support/enhance the normal vertical movement associated with water infiltration, gas exchange and root growth. In contrast, soil structures with a horizontal orientation produce barriers to the normal vertical processes and hence are considered undesirable.

Figure 1. A soil profile showing very good soil structure.

Figure 2. A soil profile showing undesirable structure in the second horizon.

Plant root systems, particularly dense fibrous root systems, produce beneficial soil structure.  Roots have the ability to enmesh or knit together soil particles into larger desirable soil aggregates. In addition the normal metabolic processes of roots involve the release of various slimes, gums and mucilages. These organic compounds  act as nature's cement, to further stabilize aggregate formation. Development of stable aggregate structure is desirable because well-aggregated soil can more rigorously resist the destructive powers of wind and water erosion. The forces of tillage however are likely to destroy even relatively stable aggregates.

Crops and cropping systems also enhance water infiltration because of the influence of old root channels associated with previous crop growth. These channels provide a rapid pathways for water infiltration
(see figure 3); however, tillage will destroy these channels or at least destroy its continuity. Pore continuity refers to a pore system connectedness. An optimum pore system will consist of cracks, pores, or root channels that support a continuous (uninterrupted) flow of water from the soil surface to subsurface. Intensive tillage tends to disrupt the entire pore system to the depth of tillage. The subsurface pore system is therefore no longer connected with the surface, reducing water infiltration to greater depth.

Figure 3. Microscopic view of an old root channel.                                                                                   

Another important component of water infiltration is preferential flow. Preferential flow describes water movement in soil via preferred pathways. In essence, soil water moves via the path of least resistance rather than moving  through the entire soil matrix. In this context I recognize two types of preferential flow, movement though macro pores and root channels.  This concept is illustrated in an experiment by Dr. Bing Si, College of Agriculture and Bioresources, University of Saskatchewan. Dr Si applied water, amended with a greenish dye, to the surface of a heavy textured soil to assess water infiltration. The irrigated area of the soil profile was later exposed with the use of a small backhoe to examine water movement. The path of the dye clearly shows the importance of the macro pore systems, particularly at depth (figure 5), in enhancing the naturally slow infiltration associated with clay textured soil.

Figure 4. Moisture movement into soil at surface.

Figure 5. Moisture movement in the 9 to 14 inch segment.                                 

In Figure 4, the dye pattern reveals a more general infiltration process through the bulk surface soil with some indication of preferential flow. Deeper in the profile (figure 5) the influence of preferential flow is more clear, water infiltration is more rapid and deeper than expected. Water and associated dye has taken the path of least resistance and has not infiltrated the entire soil profile. This concept is important when considering possible contaminant movement into a heavy textured soil. Extensive crack development supports preferential flow and thus has the potential to move a contaminate more swiftly and to a greater depth than expected based on its natural slow infiltration properties.

In summary, a reduction in number and intensity of tillage operations combined with a continuous cropping system, supports desirable soil structure development and superior water infiltration. Increased organic matter and reduced mechanical disturbance allows the development of stable soil aggregates which produce a vertical pore system that makes possible more rapid entry of water, even in fine textured soils. This improved infiltration is the result of stable soil structure, production and maintenance of root channels, enhanced pore continuity and the potential for preferential flow.

Soil structure, water infiltration, cropping systems

The rate and nature of water movement into soil (infiltration) is governed by a number of soil and climatic characteristics. They include soil texture, structure, organic matter and slope, crop surface residue, kind and density of crop growth, as well as rainfall intensity/duration and soil moisture content. Western Canada is located in a water limited, semi-arid environment, therefore from a crop production perspective, infiltration and storage of incidence precipitation is clearly a more desirable outcome than runoff. Fortunately changes to our crop production systems over the past 30 to 40 years have helped to improve water infiltration and storage.
During the first 80 to 90 years of crop production in Western Canada, tillage was a dominant practice. In the beginning ploughing of the native prairie removed the thickly rooted perennial grasses to make way for cereals such as wheat, oats and barley. Cereals root systems are much less extensive than the native grasses they replaced and therefore have considerably less beneficial impact on soil structure. In addition the practice of summer fallow (leaving the land idle for an entire growing season) necessitated multiple tillage operations in the fallow year to control weeds. While effective in controlling weeds, routine tillage had a serious negative impact on soil structure. To make matters worse, the lack of crop residue during the fallow season did not support production of new organic matter rather it accelerated the degradation of the existing soil organic matter reserves. In short, the system had severe negative consequences to soil organic matter levels (on average 40% reduction in organic matter content) and to soil structure (beneficial structure destroyed). Lack of crop cover and deteriorating soil structure reduced water infiltration. Summer fallow fields therefore experienced greater runoff and associated soil erosion during heavy rainfall episodes. In rolling terrain, see figure 1, it was not unusual to see the landscape dotted with potholes and sloughs, great for waterfowl, but not desirable from a crop production perspective.

Figure 1. Typical example of potholes in a gently rolling summer fallow field

By the 1980's advances in seeding equipment and fertilizer technology ushered in the era of minimum till/zero till seeding. This was combined with continuous cropping which revolutionized Western Canadian crop production systems. Min-till / Z-till conserved crop residue on the soil surface, and made possible more intensive annual cropping systems than had previously been practiced. Greater crop residue production and hence organic matter input, coupled with less tillage, produced significant improvements in soil structure. The sloughs and potholes which dotted the landscape during the 60's and 70's began to disappear as the combination of decreased tillage, increased surface residue, increased soil organic matter and improved soil moisture infiltration, significantly reduced water redistribution in the landscape, see figure 2. While good for farmers, this cropping system is definitely less desirable for water fowl.  I have personally observed fields that have been converted to Z-till and continuous cropping for more than 30 years. These fields have experienced substantial productivity gains, with the greatest improvement occurring on the upper slope positions. I attribute part of that productivity gain to improved infiltration and  moisture retention on the upper slopes.  An added benefit of narrowing the productivity potential between upper and lower slope is crop maturation.  Crops mature more uniformly on these fields. This is very beneficial at harvest because now upper slope and lower slope crop is ready to harvest at the same time.

Figure 2.  Rolling Z-till field with uniform stubble cover, no potholes visible

Before I am called to task over my remarks, I want to point out circumstances where sloughs and potholes do return. The likely scenario is when soils enter the winter period with better than average soil moisture content. This soil condition is coupled with heavy winter snow fall and rapid spring snow melt. The frozen soil prevents normal infiltration and a rapid melt of a heavy snow pack will generate allot of water with no place to infiltrate.  Theses conditions lead to greater than normal runoff, therefore sloughs and potholes  return to the landscape.

Soil Moisture - too much of a good thing?

Anyone growing up in Saskatchewan is well acquainted with soil moisture even if they don't realize it.  Everyone can remember a summer when farmers would say things like, "we need at least two inches of rain in the next week for the crops to be decent this year."  People like to talk about moisture a lot in Saskatchewan, or more accurately, the lack of moisture.  Saskatchewan is a mid-continental, semi-arid province which means that we rely on precipitation to replenish the majority of soil moisture.  It isn't just rainfall that is important either, snowfall accounts for approximately one third of the annual precipitation.  As well, there is a geographical distribution of precipitation in Saskatchewan with higher annual precipitation in the north declining to the south.  However, it seems more often than not, that precipitation is  widely variable and can strongly affect how crops will perform from one year to the next.

2009 and 2010 are recent examples of how dramatically moisture can change in two consecutive years.  In 2009, conditions were cool and dry.  Most of the stress experienced by crops was due to lack of moisture and lack of heat.  Soil moisture completely changed in 2010 when the rain wouldn't stop all summer.  It was difficult to seed crops in the spring because it was so wet and many fields remained flooded for much of the summer.

Map of Saskatchewan showing cumulative rainfall throughout growing season in 2009.  Map is from the Government of Saskatchewan 2009 Crop Report.  A full version of the report can be found at http://www.agriculture.gov.sk.ca/crprpt091222

Map of Saskatchewan showing cumulative rainfall throughout 2010 growing season.  Map is from the Government of Saskatchewan 2010 Crop Report.  A full version of the report and maps can be found at http://www.agriculture.gov.sk.ca/crprpt101104

Just at first glance, it is obvious that the two years were very different in regards to rainfall.  The different rainfall regimes influenced soil moisture and crop production dramatically.  Many of the crops grown in Saskatchewan are more drought-tolerant than moisture tolerant, so the large surplus of moisture did not benefit crops particularly well in 2010.  According to the Saskatchewan Crop Reports, in 2009 crops were average to above-average in quality and yield.  However, in 2010 the yields were average to above-average, but the quality was below-average.  There were several reasons for this including seeding late, crop loss due to flooding, plant stress from too much moisture, and plant disease.  All of these issues were related in part to excess soil moisture.

The soil moisture surplus in 2010 also carried through to 2011.  In the fall of 2010, much of the soil was fully saturated or had a higher moisture content than usual for that time of year.  This meant that when the ground froze, much of the pore space was filled with water.  In the spring of 2011, areas all over Saskatchewan experienced flooding because the winter snowpack started to melt and there was very limited infiltration into the soil.  My field site was not spared from the flooding.  All of my study plots were submerged for most of the spring in 2011.

Water as far as the eye can see.  Me (Morgan) at my field site near Scott, SK in April 2011.

Do you have any stories about the excess moisture we experienced in 2010? Or the flooding in 2011? Please feel free to share your stories in the comments section.

The role of soil moisture will also be discussed in more detail later in the soil formation and organic matter posts; however, it is clear that soil moisture is vital to crop production, but sometimes you can have to much of a good thing.

Information and maps from the Government of Saskatchewan Crop Reports was obtained from

http://www.agriculture.gov.sk.ca/Crop-Report.  For more detailed information, check out the Crop Reports from various years.

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.

Morgan's connection to soil

I have been asked numerous times "why soil?" when I tell people what I do.  Being a soil scientist is not something most people are familiar with.  Its not something recognizable like doctor, lawyer, or accountant, but I love studying it.

I suppose a love of soil is in my blood.  I come from a long line of farmers who have worked on the land for many generations.  I grew up on the original homestead that my great-great grandparents settled in 1903 and there was always a sense of how important the land around us was.  I spent my childhood helping out on the farm and being surrounded by farming activities constantly.

My childhood spent on the farm.

However, it took me a while to figure out that I wanted to study soil.  No one ever tells you that you can be a soil scientist.  I thought if I went into the College of Agriculture, I would become a farmer and that wasn't something I was interested in when I was 19.  I wanted to be an environmental scientist, so I started out in Physical Geography; however, eventually I started hearing about some amazing courses being taught in Soil Science.  It was then that I started taking a few of these classes and quickly realized there was a lot more to soil than just farming.  When you think about it, after air and water, soil is right up there with things necessary for sustaining life on Earth.  It provides us with food, stores and purifies water, provides and recycles nutrients for plants.  Soil is connected to so many aspects of our lives and yet we often don't take good care of it.  I was so enamored with soil by then, that I spent a summer working in the Department of Soil Science as a summer student.

I finally finished my undergraduate degree with a BSc in Environmental Earth Sciences and headed to Calgary to find a job.  I worked as an environmental scientist with a consulting company for a couple years and I soon discovered that most of the work I did involved soil.  Soil was usually what was being contaminated and impacted by pollutants. But I also learned that soil microorganisms have amazing capabilities to reduce those contaminants if we supply them with the substrate that they need.  Eventually I realized I wanted to learn more about soil and decided to pursue my MSc in Soil Science back in Saskatoon.

Photo courtesy of Rich Farrell

Studying soil science has allowed me to combine two areas that I never realized went together so well: environmental issues and agriculture.  Soil is key to agriculture, but environmental issues are becoming more and more important to look at in every aspect of our lives.  Most of my work now focuses on greenhouse gas emissions from different crops and crop rotations.  So 'why soil?' - well, studying soil allows me to combine my interest in the environment and my life long experience with farming.

Collecting greenhouse gas samples from agricultural plots

My connection to soil - Terry Tollefson

Welcome to SLSC 240 and to our course blog. In the next three months we will examine key physical, chemical and biological components of soil and relate those properties to soil productivity. Our course consists of 39, fifty minute lectures; not nearly enough time to explore these concepts in any detail. I hope  this blog will fill some of those gaps by offering you practical Saskatchewan examples of these soil properties. Before proceeding however, I have decided to give you a brief look at my personal background which may explain why I have made soil science a career.
    I believe that my farm background was responsible for awakening an early interest in soil. I  grew up on the family farm in southern Saskatchewan near the little elevator town of Ettington. That early interest in soil has evolved into a life long connection with soil, both practically and academically.  My claim is based on the fact that for the last 30 years I have taught soil science in the College of Agriculture and maintained an active role in the family farm at the same time.
   My family ties to the land go back to 1909.  My grandfather came to Canada as a young Norwegian Immigrant hoping to take advantage of the promise of farm land to homestead in Western Canada. There was no opportunity for him to farm in Norway, family holdings were very small with little chance to expand. Only 3% of the Norwegian land base is suitable for agricultural production.  The rocky and mountainous landscapes which dominate most of Norway may have influenced my grandfather's choice of land when he arrived in Saskatchewan.  He appeared to be unafraid to select land that was stoney or hilly or both. Figure 1 is an early view of the farm and grandfather seeding one of those rolling stoney fields.

Figure 1: Planting on the farm - precise date uncertain, approximately 1940

In 1948 my father took over the farm and by the 60's my brother and I were old enough to shoulder some responsibility. In those days farm kids did chores as soon as they were able and worked in the field  when old enough to pilot a tractor. I shudder to think how young that was.  One field operation in particular allowed me to be up close and personal with the land, that job was stone picking.  There was a constant need to pick stones to reduce wear and tear on seeding and harvest machinery. Back then, rocks were picked by hand and loaded into a truck or tractor and wagon. I was fortunate that horses were no longer in use by the time I was old enough to pick stones, so I never experienced picking rocks with a team of horses and a stone boat. I became very familiar with every farm field because I walked them many many times over the years. I realized early on that the ability of these fields to produce crops was quite variable and at times I wondered why.

Figure 2: Planting equipment on the farm in 1976

During the seventies I studied at the U. of S. College of Agriculture and received an M.Sc. in Soil Science.  After graduation the farm was still beckoning so in 1981 I  made a career choice that lasted 22 years. I farmed during the summer months and taught soil science during the winter at the U of S. The science of agriculture grew dramatically during that time and I was part of one of the largest teaching and research Colleges of Agriculture in Canada. I was continually exposed to the most up to date production information and used it to advantage on the farm.  In 2004, I decided to move to a full-time position within the College so I now limit  my "farmer habit" to 2 week at seeding and 2 weeks at harvest.
     I intend to bring my practical experience to the classroom to  ensure that you recognize the importance/application  of soil science principles in current production practices. I also hope that you will grow in your appreciation of just how precious our Saskatchewan soil resource is.

Figure 3: Direct seeding on the farm in 2008

Please consider joining the SLSC 240 blog. Tell us about your experiences with soil, good or bad? Where do you live, what is the soil like in your area?