SWAT stands for soil, water and topography. It is a soil mapping system for characterizing your fields. 

Maps are made using a SWAT BOX mounted on a side by side or truck. This is driven over the field on 40-80 feet swaths. The maps are made using electrical conductivity measurements from the SWAT BOX and high-quality elevation data (LiDAR or RTK). 

It is critical to ground truth the maps as things like old manure piles or buried main drains can make a difference. Ground truthing is a very important step in using SWAT MAPS and is discussed later. 

The maps categorize variation due to soil moisture content, soil texture, salinity and topography into 10 predefined zones. 

For instance, Zones 1 and 2 are eroded knolls, hills, sands and low organic areas – these are typically the driest areas. Zones 3 and 4 are shoulder slopes and upper slopes where water runs off. Zones 5 and 6 are mid slopes and flat areas. Zones 7 and 8 are toe slopes and lower flats. Zones 9 and 10 are depressions, saline areas, clay water collection areas and high organic matter areas which are typically wet. 

Once the zones are established, the agronomy work starts. There is usually one central agronomist with field experience, who pulls everything together. This person relies on other professionals from seed and crop protection companies for insights. The teams are led by the agronomist who made the zones, and may include a custom operator who is capable of applying variable rate fertilizer, the seed rep who can help with seeding rates, and the crop protection person who can help with crop protection strategies. 

There is typically less reliance on retailers to apply fertilizer in western than in eastern Canada. Therefore, it is important that you have the equipment available in order to properly apply the prescriptions needed for maximum yield potential. The function of the retailers within this system is to have access to the correct tonnage at the right time with the right application volumes. 

This SWAT system gives better guidance for fertilizer application than does general soil testing or traditional zone sampling. The issues with zone sampling are numerous. When you have a low testing soil should you put on more or less than average? If it is low testing because it is eroded and will not hold moisture it doesn’t make sense to add extra fertilizer. However, if it is low testing but has great yield potential it makes sense to increase fertilizer application. 

Zone maps that also give an estimate of available moisture are valuable when determining N (nitrogen) rates. This is why SWAT MAPS are developed with pre-defined zones that are ground truthed. 

It is important when delineating specific zones for variable rate application to understand what the responsiveness to specific nutrients or herbicides will be for the different soil zones. A satellite image will not accomplish this. These images are a measure of plant biomass for soil potential, but plants of similar biomass in different areas within the soil profile will have different response curves. 

In order to build a proper map, a “soil potential” map is needed, which brings in the SWAT mapping process. This will combine the soil’s potential and the yield potential in that particular zone, making it possible to give nutrient recommendations that are based on nutrient responsiveness. 

Using SWAT mapping in this manner results in more accurate crop potential predictions than using the history of application rates and products. 

SWAT MAPS can be used in conjunction with yield maps. Together they can be a basis for fertilizer rates. This is where variable rate equipment enters. You have to be abbe to variably apply fertilizer to the zones created. You need to be able to work with the seed representative who aids in deciding seed drop in the various zones. What population do you need on the slopes and what population in the better areas? Since not all hybrids and varieties respond in the same manner to varying populations you need expertise to help decide seeding rates. 

The performance of many soil applied herbicides is determined by soil type. With some soil applied herbicides, a high rate on some areas of the field can lead to crop injury or carryover that can affect the next crop. In high weed pressure areas and areas with higher organic matter, higher label rates are often needed to get satisfactory weed control with certain herbicides. Again, you need to be able to apply herbicides at different rates in the same field. 

An integral part of the SWAT system is scouting by an experienced person. Satellite imagery or drone maps alone are not good enough. Once maps are made and seed drop decided you need to scout to see if the desired population was achieved. 

There can be various reasons for not having the desired population. These include equipment, seeding depth and mortality after emergence. You need to know this to verify your seed drop and make corrections for the next planting. 

Scouting is also critical for diseases. Many diseases only affect part of a field for example, sclerotinia will typically be worse in areas of a field with high amounts of plant growth, which affects the moisture retention within the canopy. 

SWAT zone maps can also be used for fungicide application. Typically, fungicides have the biggest payback in high yield situations. You can use SWAT MAPS to define the areas of highest probable fungicide payback. Typically, these areas may only represent 50-60% of any field. You can use yield maps but sometimes they will show a lower yield and it may have been because of diseases such as white mould. 

I talked to a number of consulting agronomists on their use of SWAT MAPS. Jason Voogt, CCA, P. Ag of Field 2 Field Agronomy Inc. states, “in south central Manitoba, we deal with a lot of transitional soils from heavy clay to sandy loam to sand or coarse textured soils, and a variety of cool and warm season crops. 

“We know that a large portion of the region we service has huge variability within each field and with increasing production costs and land values, we can’t keep applying inputs across the field equally. We know there are areas that respond differently. 

“In our region growers using variable rate (VR) based prescriptions have only had normalized difference vegetation index (NDVI) imagery as their guide. SWAT mapping provides a more comprehensive and complete way to measure true variability within the field. 

“Productivity management can be improved by using VR fertility and plant populations (for example, soybean seeding rates could be increased in Zones 1 and 2 to draw out the salts in high salinity areas). Potato growers can use SWAT MAPS to better define that area in a field that is better suited for potato production. The remainder of the field can then be planted to a different crop, like barley.” 

Tyler Kessler of Kessler Ag Ventures Ltd. is a SWAT MAPS partner in south Saskatchewan. He believes their team of agrologists can confidently create a zone map that will accurately represent field characteristics for growers, to precisely position seed and fertilizer at the appropriate rate and place within a field. 

Kessler states, “we have successfully used SWAT MAPS in all soil types and all topographies. 

“These range from highly variable fields having sands, loams, clays and salinity, with hundreds of feet of topography change, to very flat heavy clay soils with only 10 feet of elevation change within a field. 

“The link to making SWAT MAPS work on every farm and every field is between the agrologist, the soil, and the grower. 

“The agrologist represents the ‘boots on the ground’ to ground truth SWAT MAPS and scout fields, understanding the variables from soil, water and topography. 

“Communication between the grower, along with expectations and goal setting with our agrologists, is key to our success with SWAT MAPS. 

“There is no better map on the market to properly identify and treat specific soil zones. By grouping specific zones based on their soil potential, we can pinpoint specific nutrient responsiveness curves to that particular soil, and identify trends within the soil texture and landscape position to fully maximize the soil’s potential going forward. 

“As a base layer within our fertility program, SWAT MAPS then enable us to to better understand where our customer’s dollars are best spent on any individual field, maximizing their yield potential as well as their return on investment,” he adds. 

Michael Palmier of Max Ag Consulting Ltd. from Plenty, Sask., says “SWAT MAPS have taken our business to the next level because they enable us to make the best recommendation for each individual acre. We group soil zones to optimize dollars spent on each acre, and zoom in with regard to fertilizer, fungicide and herbicide recommendations. We were never able to do this just using NDVI imagery, as many of these functions are much more related to the soil’s response to the particular input, rather than the plant response.” 

Are SWAT MAPS for everyone? Absolutely not. You must have access to certain technology to make use of them. Optimal usage of SWAT MAPS means having the capability to do variable rate seeding, fertilizing and spraying. 

In the near future you could be switching genetics in different zones. One SWAT user is already doing this with some of his growers. 

Eventually, we could selectively apply an insecticide to certain parts of a field where wireworm or rootworm is always bad. 

You really need co-operation between the various people who advise you on seed, fertilizer and crop protectants, and someone to pull it all together. The people that can do this are the agronomists offering SWAT MAPS. 

This article is longer than intended. Folks using SWAT are passionate about how they do business. Thanks to Wes Anderson, VP of agronomy for SWAT MAPS, for helping. 

The final comment is one I once heard expressed by another CCA: “I find that using SWAT MAPS provides a repeatable process for defining soil fertility and seed response zones. 

“It removes a lot of the noise and focuses on fundamentals for a streamlined crop input decision making process."

Written by Patrick Lynch

Consulting agronomist Patrick Lynch, CCA-ON, formerly worked with OMAFRA and Cargill