There are many types of maps that can be used to make on-farm management decisions.
While they all have a useful place, it is important to understand what they are telling you, and what they are not. This allows for better management of expensive and logistically challenging inputs.

NDVI MAPS

A Normalized Difference Vegetation Index (NDVI) map measures the greenness of vegetation from a satellite image. It quantifies vegetation by measuring the difference between near-infrared (which vegetation reflects) and red light (vegetation absorbs). NDVI maps are great in-season maps to determine where the vegetation is, in the current year. The maps will show concentrations in different areas depending on the year – a dry year will tend to show more biomass in lower areas where water is available, and wet years will tend to show higher biomass on hill tops or slopes, where plants are not waterlogged.

It is also important to note the time of the year the map image is taken – if it is from too early in the season, it will be more of a weed map as the crop has not had a chance to emerge.
NDVI maps that are taken after herbicide application can be useful in fungicide and top-dressing applications, especially if the crop is variable. This map is more reliable as a map of the crop itself, rather than weeds.

Satellite NDVI maps are also at the mercy of atmospheric conditions such as cloud cover. If there is too much cloud cover, imagery will be spotty, and an accurate map cannot be made. Figure 1 shows drastically different imagery where the 2020 conditions were extremely wet and 2021 was a drought year. Cloud cover prevented the imagery from being useful earlier in the growing seasons.

Yield Maps

A yield map utilizes data from GPS and sensors to track crop yields across a field. It will tell you where the yield was good or poor, relative within a field, in a single year. Areas of higher yield vs lower yield will change, dependent on temporal conditions of the growing season. This is helpful in determining where crop was more productive and for calculating nutrient removal, but it does not tell you why.

Yield data needs to be cleaned and processed to create an accurate map – factors such as calibration, multiple combines, or changing settings mid-field can affect data.

Why was the yield better in a depression this year than in the past? If you have a few years of yield maps it may help to identify continually problematic areas.

The picture below shows the yield maps from the same field under vastly different conditions, resulting in quite different maps.

SWAT MAP

A SWAT MAP is a soil-based map that takes soil characteristics, water flow, and topography into consideration. EC (Electrical Conductivity) is mapped, elevation data is collected and a SWAT CERTIFIED agronomist ground truths the field to ensure that a map is selected that correctly identifies the characteristics into management zones. Soil samples are taken within these zones, analyzed at the lab, and interpreted by the agronomist to understand similarities and differences within a field that allow us and the grower to manage a field with confidence. Fertilizer, seed, lime, herbicides, or amendments can all be managed well when we understand each zone’s soil characteristics such as nutrient availability, organic matter, or salinity.

A SWAT MAP is a stable map that will stay the same year over year. Zones 1 and 2 are typically the hill-tops, coarse textured soils, or are possibly eroded. Zones 5 and 6 move into the slopes of a field, often a consistently productive area of the field. Zones 9 and 10 are reserved for low-lying or saline depressions. Each zone will be different between fields but are always relative to each other with a field.

In this field, Zone 1s (red) are limited by coarse textures with less water holding capacity and low pH. Zone 10s (dark green) are limited by salinity. These are some of the characteristics, along with nutrient reserves, that help us determine the yield potential across a field, regardless of the conditions that the current year may bring.

Understanding spatial and temporal differences in a field is key to planning and making in season crop management decisions. Having multiple layers of data to work with is helpful, if we know the background and what that data is telling us. Every year will have varying temporal constraints, but if we can map the stable spatial properties, we can manage a crop to its highest potential.

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Rachelle Farrell
Senior Precision Agronomist, AB

Everyone in agriculture knows the importance of soil organic matter (OM). It provides many soil functions, including nutrient cycling, improved water infiltration, and reduces soil bulk density that helps reduce compaction. For these reasons, OM is an important measurement to guide nutrient management decisions.

The challenge, like all things we can measure with a soil test, is variability. Organic matter varies no different than nutrients, pH, and texture across the landscape due to several reasons:

1. Climate – more plant growth driven by water availability typically results in more vegetation and carbon input. Warmer climates have higher microbial activity, decomposition and cycling of organic matter, making it more difficult to accumulate soil carbon.
2. Type of vegetation – grasses typically accumulate more carbon in the soil than trees, which is why grey wooded soils (luvisols) tend to have less OM than soils developed under grasses and mixed vegetation (chernozems). 
3. Soil texture – higher clay content allows more mineral associated organic carbon to accumulate, which tends to be more stable and resistant to loss.

These factors have impacted how much OM has accumulated over thousands of years during soil formation. More recently, farming activities such as tillage, practices that incur erosion, and the addition or removal of organic materials have resulted in further variability across the landscape, and in most cases a net loss of soil carbon reserves.

Confused about the difference between carbon and organic matter? Organic carbon, often referred to as SOC or soil organic carbon, is just that – carbon. Organic matter, however, includes all material originating from living organisms and the nutrients within it, at various stages of decomposition. Typically, the amount of SOC is about 58% of organic matter %. So, if a soil has 6% OM, the SOC would be about 3.5%. Both OM and SOC can be measured in a soil lab. SOC can be used along with a bulk density measurement to calculate the total tonnes/ha of organic carbon in the soil to a given depth.

Typically, in the process of soil mapping with SWAT MAPS, agronomists will observe higher OM values in lower landscape positions and in soils with higher clay content. Lower landscape positions have more water available for plant growth that has accumulated OM for many centuries. In many fields, erosion has also deposited topsoil in depressions, resulting in both high percent OM and increased depth of the A horizon. I’ve soil sampled many fields in northern Alberta with topsoil depth ranging from 10 cm on the top of a knoll to more than 60 cm in wet depressions.

Not all hills and depressions are made equal though! A sandy knoll will not have the same capacity to store organic carbon as a loamy, productive knoll. Historically anaerobic depressions have developed peat or muck soils that are extremely high in OM (I’ve seen as high as 88% OM in a soil test). It is the responsibility of precision agronomists to delineate all these soils and landscape positions for proper variable rate management.

Another interesting SWAT related observation is the long-term effect of soil constraints such as salinity. It is not uncommon to see lower OM in saline-sodic areas that have not produced much plant biomass for a very long time, and perhaps lost existing OM at a faster rate since cultivation. There are native plant species that can thrive in severe salinity, but very few annual crops will grow well at all. These areas are better off to be mapped and planted back to perennial, salt tolerant species whenever feasible.

Most importantly, OM is a huge reserve of nutrients that are mineralized throughout the growing season, enabling farms to reduce fertilizer inputs in certain areas, or address nutrient limitations where OM is too low to supply the crop. As a rule, for every 1000 kg of carbon in soil OM, there is 100 kg of N, 15 kg of P, and 15 kg of S that can become available to the crop as it is mineralized by microbes.

SWAT zones allow us to define this organic nutrient pool more accurately. An example from a field in Alberta, Canada where Total Nitrogen (TN) was tested by zone, showed just how much nitrogen there is in the organic nutrient pool. Note that the last column is using an assumed bulk density of the soil that is unlikely accurate due to texture differences by zone.

SWAT Zone	OM % (0-8”)	TN % (0-8”)	Approximate Total lbs N in 0-8”
1	4.0 %	0.23 %	6160
3-4	4.6 %	0.28 %	7333
5-6	6.9 %	0.33 %	8667
7-8	9.9 %	0.44 %	11,813
10	27.1 %	0.96 %	25, 573

Following a similar trend, sulfur deficiency is often observed on eroded, low OM sandy knolls (SWAT zone 1), while only meters away a depression with deep, black soil may not need any sulfur applied at all! Crop lodging in part due to excess nitrates is often observed in lower landscape positions with high OM and mineralization rates. Reducing applied nitrogen in these areas can not only reduce lodging, but improve crop yield, quality, and harvestability! Organic matter mineralization is also an important source of nutrients like phosphate, boron, and zinc. To accurately understand how different soils respond to these nutrients, we need to understand how much OM there is, how deep it is, and relative carbon cycling rates across the landscape.

These are just a few examples of the economic advantages that precision ag technology and high-quality soil mapping with SWAT MAPS can contribute to agriculture. The carbon economy and opportunity to increase stored soil organic carbon will add additional farm revenue in the future. But to understand where we are going and how we get there, it is best to know where we are coming from. A good soil map is that starting point.

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Wes Anderson
VP of Agronomy
wes@swatmaps.com

By the Canola Council of Canada

The canola industry has a goal to see 4R Nutrient Stewardship practices used on 90 per cent of canola acres by 2025. Farmers following 4R use the right source of fertilizer at the right rate, right time and right place to get more from each tonne of fertilizer. This nutrient use efficiency improves economics and reduces nutrient loss to the air and water.

Many farmers follow 4R practices – perhaps unknowingly – because of the inherent benefits associated with proper nutrient management. This case study describes the 4R practices for one Saskatchewan farmer who knowingly follows 4R. At the end, CCC agronomy specialists comment on this farmer’s practices and how all farmers can take 4R to the next level.

By applying fertilizer at the time of seeding, Chantal Bauche doesn’t see an economic reason to use enhanced efficiency fertilizer products. For more on EEFs, read “When do enhanced efficiency fertilizers make sense?” at canoladigest.ca.

NAME: CHANTAL BAUCHE
LOCATION: REDVERS AND RADVILLE, SASKATCHEWAN

Chantal Bauche is an active part of two farms in Saskatchewan – she helps on her family farm at Redvers and with her partner and his family at Radville. Bauche is also a senior precision agronomist with Croptimistic. For 2023, the Redvers farm will take a major step forward in its 4R practices, moving to zone-based soil sampling and variable rate (VR) fertilizer application. Bauche soil tests some fields every year, but she ramped up soil testing this fall to prepare for their first VR fertilizer application next spring.

Bauche will use Croptimistic SWAT maps for each field, which are based on soil properties, water modelling and topography. It divides fields into 10 zones, from eroded knolls with low organic matter (zone 1) to depressions with high organic matter, high nutrient content and high salinity (zone 10). Both of these areas can result in lower yields but for completely different reasons, which is why fertilizer rates for each zone should not be the same. Cropimistic combines zones 1 and 2, 3 and 4, and so on, to create five soil sampling management zones for each field.

This fall, Bauche will capture 0-8” samples from five zones in each field. Each sample will be a composite based on numerous sample sites from each zone that get GPS located and returned to each year afterwards. Cost for analysis of the five samples will be $150 to $250 per field. This doesn’t include the sampling service for farms that don’t collect their own samples.

“The benefit is huge,” Bauche says.

Saline areas often have higher nutrient carryover and moisture, but salinity keeps a lid on yield. These areas don’t need much, if any, fertilizer. Hilltops will need some fertilizer to support yield, and possibly higher rates of sulphur, because these areas have low organic matter and lower nutrient reserves, generally.

Annual soil tests also indicate year-to-year fluctuations in soil nutrient reserves. “After the drought of 2021, soil nutrient reserves were high for many of my Croptimistic clients, and we had a lot of fields where we didn’t recommend any fertilizer in 2022,” Bauche says. The same was not true for her family’s Redvers farm. “We had good yields in 2021,” she says, “and soil samples in the fall of 2021 showed that fertilizer was needed.”

To put VR prescription maps to use, the farm upgraded to a new Väderstad drill with four tanks and sectional VR control. They will use it for the first time in 2023. “Our old drill was our Achilles heel keeping us from adopting variable rate fertilizer application,” Bauche says. With four tanks, the new drill does not require any pre-blending of fertilizer. It has separate tanks for urea, phosphate, potash and seed. “Without pre-blending, it is much easier to tailor variable rates,” Bauche says. With sectional control, the drill can also adjust rates section by section across its width. “When crossing over two zones, the drill will compensate,” she says.

The drill, which applies nitrogen, phosphorus and potash (when required), into the ground at the time of seeding achieves three principles of 4R in one pass – right rate, right place and right time. For these reasons, Bauche doesn’t use enhanced efficiency products. The only fertilizer they apply in the fall is ammonium sulphate.

November can be a great time to soil test if soils are not frozen. Cool soils reduce the microbial activity that can mobilize nutrients, and soil samples collected after this activity slows down will more closely reflect spring nitrate (NO3–) contents. Read “The right time for soil sampling” at canolawatch.org
/fundamentals.

CCC ANALYSIS OF THIS PRACTICE

WHAT DO WE LIKE ABOUT BAUCHE’S 4R NUTRIENT MANAGEMENT PLAN?

Jason Casselman, CCC agronomy specialist, Cleardale, Alberta: I like that Chantel Bauche is building a database of information for each of her fields. She can use that database not only for variable-rate fertility but also to map out and highlight areas like the saline patches for tile drainage.

Keith Gabert, CCC agronomy specialist, Innisfail, Alberta: By using zone mapping and management on her own family farm operations, Chantal Bauche builds on her ability to offer these services to her customers.

HOW CAN BAUCHE AND OTHER FARMERS USE THIS CASE STUDY TO TAKE 4R TO THE NEXT LEVEL?

Casselman: Tile drainage on saline areas may fix a problem that continually affects yield. Those zones could become more productive than ever before, and make it possible for Bauche to farm fields with fewer management zones.

I encourage other farmers to dig a little deeper into some of the causes of variability on their land and evaluate long-term solutions to a problem that will improve profitability. For other growers who aren’t ready to trade in the drill to be able to do variable rate, look for other options. For example, farmers can use a sprayer with rate control to top-dress liquid fertilizer in season at variable rates.

Gabert: Growers tend to appreciate simplicity. A new drill with separate tanks for each nutrient sounds like a logistical challenge. To improve 4R, a farm might simply split some nitrogen out of the primary fertilizer blend to allow an additional top up of nitrogen on fields or zones were soil tests indicate it is required. While 10 zones managed as five is a really valuable level of precision, for growers that aren’t using VR yet, they can achieve significant improvement in nitrogen management across the farm by choosing which acres receive additional nitrogen, rather than a single blend.

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This article originally appeared in the November 2022 issue of Canola Digest. You can download that issue here or read the article online, here.

Grid soil sampling has historically been the gold standard to measure nutrient variability in soils. In many regions, 1 hectare or 2.5 acre grids have been a very common strategy, with not a lot of technology needed to do it.  But it is time consuming and expensive. It’s A LOT of sampling.  But perhaps the biggest issue is that it assumes, to some extent, that these made-up squares that are completely unrelated to topography or soil differences will be enough to capture all the true variability that exists in a field. So, is grid sampling good enough? Or can we do better with newer technology?

This is the question we at Croptimistic wanted to answer, so we picked a few fields in different regions and soils in Canada. It’s simple really – you lay out 2.5 acre grids, and then take several soil samples at different points within the grid.  In our case we were curious if SWAT soil based management zones captured significant variability within the 2.5 acre grid.

The results were very much what we expected – there can be a lot of soil and nutrient variability within a 2.5 acre square! There can also be a big difference between soil water status due to landscape position, and therefore crop yield potential.  Below are field and soil test results from the University of Alberta Breton Farm, which is currently established perennial hay.  The whole field is only 6 acres, so in this case the soil test points are within only 1 acre.

There is significant variability in several attributes including organic matter, pH, P, S, and most micronutrients. The trends are ones we commonly see – lower landscape positions (in this case, the zone 9 point) that collect water and eroded soil from knolls are often high in organic matter with deep top-soil (depicted in subsoil OM%) and have higher nutrient levels. The difference that doesn’t show on the soil test was soil moisture. In zone 9, the soil probe easily went in the soil to 12”, with soil moisture likely close to field capacity. In contrast, the zone 1 point took some effort to get the probe in, and the soil was already quite dry.

The site at Breton Farm was unique in that it was grass hay (brome) and was unfertilized. So, any variability in the brome grass was due to soil and water variability, not from historical site-specific management. For this reason, we took a tissue test at the exact same points where soil tests were taken, shortly before the brome entered the reproductive stage – typically a time of high nutrient demand for all crops. The tissue test results are shown below.

As expected based on visual observation, the grass appears to be quite nutrient deficient and would benefit from applied nitrogen and phosphorus. Several trends in the tissue results followed the soil test well – particularly N, P, Ca, S, and Cl.  Tissue N and S reflect the soil’s ability to mineralize these nutrients from organic matter. Phosphorus availability is likely reflective of the extractable soil P as well as potential fixation from Al in acid soil in zones 1 and 5. Mn levels are also quite high in zones with acidic soil – something we’ve observed in canola as well in this area, to the point where Mn toxicity is causing visual symptoms on canola leaves and likely root pruning. Stunted roots then lead to poorer uptake of all nutrients, particularly relatively immobile P, Cu, and Zn. It’s a vicious cycle, and one that can only be solved with pH amendments such as lime, cement kiln dust (CKD), or wood ash. 

The combination of soil and tissue data clearly demonstrate the nutrient supplying power of different soils within a small area of the field, and that these differences can be efficiently mapped and measured using modern precision ag tools. The benefits are multi-faceted – economic advantages for the farm resulting from applying the right rates in the right areas, and environmental benefits or reduced nutrient applications where they are not needed.  It’s simply 4R Nutrient Stewardship in high resolution.