Currently a wide range of technology exists to locate the producer in a given field, to vary the rates of one or more inputs on-the-go, and to measure the yield on a point-to-point basis during normal harvest operations. Unfortunately the agronomic information that producers need to make best use of this new technology is currently lacking. Research will emerge over the next several years for producers to use in planning precision farming operations; however, producers can use the information currently available on soil fertility variations in typical Saskatchewan landscapes to decide if precision farming may be suited to their landbase.
Soil scientists have spent many years studying the pattern of soil properties in fields and the processes responsible for these patterns. In this section I'll concentrate on the types of soil variation which occur in the most typical of Saskatchewan landscapes - a gently rolling surface. Formed on glacial till deposits, these are landscapes which have a series of knolls and sloughs in a given field; the knolls typically have a grayish colour, and the surface of the soil has scattered stones on it.
Different soils in a field occur because of the action of soil-forming processes through time. In Saskatchewan soil formation in most of our landscapes began about 14,000 to 10,000 years ago, after the great ice sheets retreated to the north. Most of the soil forming processes which have influenced the landscapes since that time are controlled by the amount of water present at any point in the landscape. The pattern of water movement in a landscape is in turn controlled by slope and soil properties, and the pattern of water movement is well understood (Figure 1).
Figure 1: Pattern of water redistribution in a typical glacial till landscape
Because the soil-forming processes are so closely linked to water movement, a clear pattern of soils occurs in many Saskatchewan landscapes (Figure 2). Water is shed from knolls or upper slope positions and drier conditions occur. These dry conditions limit the amount of organic production that occurs in both natural conditions and in agricultural fields. The result is a soil that typically has lime close to the surface and a thin layer of topsoil. Erosion of soil by wind, water, and tillage since the breaking of the land has removed topsoil from the knolls, and in many cases has mixed the grayish lime layer in with the remaining topsoil. This gives knolls the grayish appearance that we see in many Saskatchewan landscapes. Soils with very thin topsoil layers, or where the lime has been mixed in, are called Regosolic soils.
Figure 2: Distribution of major soil groups in typical Saskatchewan glacial till landscapes.
Soils in the midslope position typically show a consistent change in their properties as we move downslope. These soils typically have lime-free topsoil, which increases in thickness as we move from beside the knolls to the edge of the lower slope positions. In some cases tillage has mixed some of the lime-free layer underneath the topsoil into the plough layer, and these soils can have a reddish-brown appearance in the field. These midslope soils are called Chernozemic soils by soil scientists.
The lower slope soils show the greatest variations between fields and even within fields for it is in these positions where the interaction between the soil and the groundwater becomes a factor. For most fields in the agricultural portion of the province the groundwater table is far below the surface. The lower slope positions receive the snowmelt water and runoff water from upper slope positions, and this water ponds in the lower slope position for some period of time. The water displaces oxygen from the pore space of the soil, and a set of soil processes typical of oxygen-depleted conditions occurs. For example, in oxygen-poor conditions, the plant available forms of nitrogen (called nitrate) undergo a series of chemical changes and can be converted back to nitrogen gas and is lost to the atmosphere. As the ponded water percolates through the soil it carries with it dissolved ions and other larger particles such as clay. These processes associated with water-saturated conditions give rise to the Gleysolic soils.
In other fields (or even within the same field) the groundwater table is closer to the surface, and water moves from the groundwater table to the surface. This means that the ponded water from runoff cannot drain as rapidly from these areas. As well, when the groundwater reaches the soil surface and is evaporated, material dissolved in the groundwater are deposited in the topsoil. If the groundwater is high in dissolved salts, then the evaporation of the groundwater over time causes the build-up of salinity in these lower slope soils. Hence in a given field the lower slope soils may have a deep, well-developed soil such a thick Chernozems or Gleysols in the lower position while another lower slope position a 100 meters away may have plant limiting levels of salinity.
The soil differences discussed above translate into differences in both the nutrients and moisture available to your crop in a given year and these two factors are two of the main determinants of crop yield. The relationship to moisture is clear - soil conditions are driest on the knolls, increase through the midslope areas and are wettest in the lower slope positions. This pattern will be the same in all years; however the actual difference between the three positions will depend on the amount of precipitation received in a given year.
As well, the differences in moisture between the three positions are, for the most part, unmanageable. Certainly a good residue cover acts to even out moisture variations between different parts of the field by trapping snow in winter and by allowing the soil to absorb more moisture during rain storms. Generally, however, it is the fertility variations that we are trying to manage by using precision farming technology.
The primary focus of many of the current precision farming research projects is on nitrogen fertilization. Nitrogen levels are known to differ in fields and, at least at the broadest scale, these differences are well understood. The largest pool of nitrogen (N) in the soil occurs in the organic matter in the topsoil layer. Nitrogen tied-up in the organic matter is unavailable to plants, and must be converted to a mineral form before it can be taken up and used by growing plants (the fertilizer you add is already in a mineral form and does not need to undergo this conversion). This process is called mineralization and depends on a number of factors, but a recent summary suggests that between 5 and 15% of the organic N can be mineralized in a given year. Hence, at the simplest level, the amount of organic matter in the soil will control the maximum amount of N available to be mineralized, and the moisture and temperature conditions in a given year will determine the fraction of that maximum available amount that will be mineralized.
This basic understanding of N fertilization can be linked with the soil differences discussed above. Soil organic matter will increase as we move from the knolls through the midslopes and will usually be at its highest in the lower slope positions. Hence the maximum amount of organic N available for mineralization will also increase as we move from knolls to depressions. As well, the temperature and moisture conditions for mineralization are usually optimum in the lower slope positions, and we could expect that the rate of mineralization would be greatest here. Overall, then, we would expect that the ability of the soil to supply N to the growing crop will be strongly related to slope position.
As discussed above, the pattern of soil moisture in a field also shows a clear pattern. Knolls or upper slope positions will have the driest conditions and will often have the lowest ability to deliver N to the crop; midslope positions will have increasing moisture levels and N availability as we move from next to the knolls to next to the lower slope areas; and moisture and N availability may be highest in the lower slope positions. The challenge for researchers and producers alike is to determine how we can use this understanding to achieve optimum use of the soil resource in crop production.
The question most producers have at this point is - do these slope differences actually translate into differences in yield? Recently the Agri-Food Innovation Fund provided funding for a group of researchers from the Department of Soil Science, Agriculture and Agri-Food Canada, the Saskatchewan Irrigation Development Center, and the Saskatchewan Wheat Pool to develop some of the agronomic information required for the successful adoption of precision farming. We began our research in the summer of 1997 at four sites (shown in Table 1). At each of the sites we are imposing different N, P, and seeding rate treatments on the slope positions discussed above and we are also measuring a number of soil and crop properties at each of the sites.
The 1997 results show us that there are real differences in the productivity of these different slope positions at our research sites (Table 1). The Watrous and Swift Current sites are on the type of glacial till landscapes discussed above. The St. Louis site is located on a rolling silty-very fine sand field. All three sites are very typical of the region they are located in. As you can see from Table 1 the lower slope positions on average yielded about 10 bushels per acre more than the upper slope positions under the same fertilization regime. The midslopes differed in their productivity - at Swift Current they produced crops similar to the lower slope positions and at Watrous the yields were closer to the upper slope positions.
At the irrigated site at Outlook the degree of soil variation was much lower than at the three sites discussed above. As well, the application of irrigation water removes the moisture limitation that typically limits yields on knolls. The average yields of the three slope positions at Outlook are the same - irrigation and the limited soil variability removes the differences between the slope positions.
Table 1: Average grain yield for the slope positions at the four main AFIF sites in 1997. The crop at all sites was spring wheat. The average is for the five nitrogen treatments used at each site.
|
Site |
Slope Position |
||
|
Upper |
Mid |
Lower |
|
|
Grain Yield (bu/ac) |
|||
|
Swift Current |
20 |
27 |
29 |
|
Watrous |
28 |
30 |
39 |
|
St. Louis (Conservation Learning Center) |
29 |
---- |
38 |
|
Outlook (Irrigated) |
63 |
63 |
61 |
Clearly many factors come into play to determine overall crop productivity in a given year. The variability of other nutrients other than N such as phosphorus or sulphur can be critical for certain crops; competition from weeds may be greatest in the lower slope positions; or in a wet year, problems with water-logging or root rot in lower slope positions may lower the yields. Hopefully, through a combination of research trials on non-level fields and of producer trials of different fertility-weed control scenarios a more complete understanding of the management of variable productivity conditions will emerge.
Producers thinking about adopting variable rate fertilization can draw a few points to help them in their decision from the information presented above. If your fields have a range of soil conditions in them which are associated with the type of topographical variations discussed above then variable rate fertilization may be suited to your operation; if not, then the adoption of variable rate may be more complicated (in other words, more costly!). How can you assess this? - here are a few ideas.
The soils of the agricultural areas of the province have all been mapped. These maps are not at a scale which is suitable for quarter-section scale field maps but they can tell you the range of soils which you are likely to find in your field. In other words, they may tell you that a given quarter-section is likely to have a certain percentage of Regosols, Chernozems, and Gleysols but they won't tell you where in the field you will find them.
This information is available from the Land Resource Unit of Agriculture and Agri-food Canada located at the University of Saskatchewan. Contact them at 1-306-966-4060 (fax: 966-4226) with the legal location of your land and ask them about the type of soil association which this land was mapped as.
Nothing can replace walking through your fields with a shovel or auger and looking at the range of soils associated with the different slope positions. The three basic positions discussed above are knolls or upper slopes, midslopes, and lower slopes. These three positions might all occur within a small distance from each other - typically on the order of 30 to 50 yards apart. Try digging a small pit on each of the three, and looking at the thickness of the upper, organically-enriched layer of the soil. The thickness may vary from as little as 4-6" on the knolls to 18-30" in the lower slope positions. If this amount of variation occurs on several knoll and lower slope sequences within your field, then the basic framework outlined above probably applies to your field.
The thickness of topsoil is a good (and cheap!) measure of variation. To get more quantitative data on the three positions, try splitting up an area of your field into the three positions. For example, a 10-acre area may have 3 or 4 of the knoll to lower slope sequences in it. Take 2 or 3 samples from each of the three positions from each of the sequence and then composite (mix) them by slope position (in other words, mix 10 or so samples from the lower slopes to get one lower slope sample, 10 to get a midslope sample, etc.). Send this off to a soil testing lab and ask from them to do organic matter as well as the normal N-P-K-S test. The organic matter levels are an indication of the capacity of the soil to mineralize, or supply, N. Higher levels of organic matter are typically associated with a higher potential to supply N. This can also be a good time to do micro-nutrient testing if you have concerns about this. When the recommendations from the three samples come back you'll be able to see if significant differences exit between the positions - significant in the sense that applying different rates of inputs would make economic sense.
The type of information outlined above will give you a good starting point for making a decision regarding precision framing. The information on your land base can then be compared with the agronomic information currently being developed to allow you to make a sound decision about the suitability of variable rate fertilization for your farming operation.