Wednesday, 25 January 2012

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.

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