SWAT team attacks salt or cuts it out

I recently had an email conversation with Scott Day. He told me about his new approach to designing fertilizer prescription maps for his family’s Treelane Farm at Deloraine Man., an area notorious for spots of extreme salinity.

“This was the first time we variable rate applied all our nitrogen in accordance with the soil’s salt levels, which worked well,” said Day, adding that there are varying degrees of salinity affecting about one third of his farm.

“The very worst salinity is probably hurting 10 to 15 percent of the acres. And it’s getting a little worse every year, on this farm and for everyone in my area. This wet fall is probably going to accelerate it once again.

“It’s very hard to compare this new system to previous years because now we put more nitrogen on the good areas and next to nothing on the bad areas. In the past, I just variable rate applied my (nitrogen) manually. We only farm 1,600 acres so it was sort of easy to remember the worst spots in every field.”

Day said comparing the new saline-dictated system to his previous seat-of-the-pants system is difficult because most of the farm experienced a bad hail storm, followed by another hail storm when harvest was half finished, followed by snow. He still has unharvested crop in the field.

“So we don’t really have any valid yield comparisons, but I do know that the barley harvested before the hail was our best crop of barley ever on those fields,” said Day.

He said the logic behind using salinity mapping is the same as any nutrient management plan. Determine the limiting factor. Then attack it.

On Treelane Farm, he knew that salinity was the limiting factor on many fields, so that was the factor he attacked, with the assistance of the CropPro Soil Water and Timing mapping team.

“I think saline mapping is definitely going to gain popularity going forward. I’m using CropPro Consulting, which has been around a few years now. I imagine there are other farmers with more experience than I using the EC maps created from a precise EM-38. Wes Anderson in their Alberta office is working with me on this project,” said Day.

Anderson said the first step is to delineate the different soil types and soil textures throughout the fields, then compile their SWAT (soil, water and topography) map so they can deal with each area individually.

“That whole area of southwestern Manitoba is spotted with salinity pockets. You have areas ranging from five percent up to 20 percent of the field limited by salt,” said Anderson.

“On those fields, salinity trumps everything else. It doesn’t matter how much phosphate you have, or zinc, or nitrogen or anything else. The reality is that those saline areas have heaps of these nutrients they can’t use, deposited by more than a half century of over-application. We typically see big accumulations of phosphates and sulfates and usually nitrates too. These areas don’t need more fertilizer.

“Depending on how bad the salinity, we manage seeding rates for better establishment to try (to) use up some of those nutrients. We know plant mortality rates will be really high, but with cheap seed like wheat or barley, we can afford to increase the seeding rate. We want plant growth in those areas so we have transpiration instead of evaporation.”

Transpiration occurs when plant roots suck saline water up into the plant. Some salt is actually removed at harvest. If these saline areas are left without vegetation, saline water moves to the surface and evaporates into the atmosphere. But salts don’t evaporate into the atmosphere. They remain on the surface, and the saline spot gets worse year by year.

Saline spots are created by water moving through the soil, bringing natural soil salts along for the ride.

Rain and snowmelt catchment potholes higher up on the topography maps are known as re-charge potholes because they collect water. But they leak. And when they leak, the water flows down through the soil, bringing along the salts, bringing saline water into discharge potholes located lower on the topography maps. These discharge spots are the saline potholes.

“We have one field in northwestern Saskatchewan that’s an amazing example of re-charge. It’s on the higher elevation of the field. There’s a bunch of potholes that are like great big toilets. They’re very deep, sharp depressions that are very well drained. The water pours in and then just disappears. It doesn’t even flow down to a discharge slough. It seeps out the sides of the hills.”

He said anything a farmer can do to help is good. More vegetation grown in those areas will help buffer the impact of the salts. Even a cover of crop residue reduces evaporation. The crop residue limits the effect of water wicking up to the surface and pulling salts along to the surface.

“When we do the EC maps and ground truth with soil tests, it’s not unusual to find 150 pounds of nitrates in just the top eight inches of these saline zones. Plus what’s down below. We’ll see 50 or 60 parts per million Olsen phosphate.

“Sulfur is through the roof because a lot of salts are sulfate salts. Chlorides, too. There’s heaps and heaps of nutrition to be used, but that’s if you can get something to grow in the first place.

“Once we have the maps, we look at where the saline spots are located in the field. If it’s bad enough in a corner of the field or bathtub rings around a slough, I tell the client don’t even farm those areas. You’re losing money every time you turn a wheel over that area. So if you just cut out those spots, you’ll be losing less money.”

Anderson said the size of equipment sometimes makes it impossible to cut out just a couple of small potholes scattered around the field. As more farmers buy smarter seeding equipment with section control, they’ll be able to cut out specific sections when they get to the saline spots. That technology is advancing every year. However, it depends on a good salinity map that clearly identifies those spots.

“If there’s livestock on the farm, I always recommend seeding saline spots down to something (like) AC Saltlander intermediate green wheatgrass that can be harvested and fed to cattle. And there are some saline tolerant corn species. They’re not the highest quality forage for feed, but they are palatable and corn does suck up a lot of water. So if you cut and bale them, you remove a lot of salt. Your EC map will let you know what you can plant in which areas.”

When CropPro began EC mapping 10 years ago, they used Veris machines but have switched to a Geonics EM38 on a sled.

“Now we solely use what we call the SWAT box. That’s an EM38 or a topsoil mapper inside a plastic housing that rides a couple inches above the ground. It takes two readings at two different depths.”

Anderson said that when the CropPro agronomists map the field, they mount the SWAT box to the hitch at the back of a pickup. In areas with LIDAR data (light detection and ranging remote sensing) like in most of Manitoba and all of Alberta, they simply can install the SWAT box on the farmer’s drill or cultivator and the box gathers EC data during any regular field operation. It’s autonomous. It has its own GPS. All it needs is 12-volt power. As soon as it starts moving, it starts sending data to the server. When he’s finished mapping his target fields, the farmer simply removes the box.

“The data goes directly from the SWAT box to the mappers in Naicam. As soon as SWAT pulls off the field, the mappers can get to work. There’s no need for the producer to do anything other than secure the box to his implement. The very next day a CropPro agronomist can be at the field, taking soil samples to ground proof the data.”

Anderson added that an EC map remains relevant for years. Unless there is some sort of dramatic intervention such as tile drainage, salinity spots do not shift position or disappear, although new spots may develop. It’s not a map that needs to be re-done. The degree of severity will change, but that change will show up in the annual soil test.

He added that producers can’t use EC only and they can’t just use topography. They need a complete soil test with all 16 elements to make sense of it all.

“We developed the whole SWAT box technology at CropPro. It’s a patented methodology that combines multiple data layers into a single map. The SWAT box itself has a patent pending.

“Historically, most variable rate service providers relied on yield maps and NDVI satellite imagery to build prescription maps. The satellite imagery is free now, and with fairly high resolution. NDVI often corelates well with yield maps. A few providers use EC as well, typically with Veris machines.”

Anderson said using the EC rating is challenging. You can get high readings in the depressions because they’re saline. But you can also get high EC on the knolls because they’re eroded. It’s not easy to interpret.

“We get our RTK elevation data from LIDAR. We run that into a topography model that gives us water shedding areas (recharge) and water collecting areas (discharge). The model also creates water flow lines and accumulation areas. Then we add in the EC layer and the soil colour map, because that tells us where we have areas of high organic matter.

“All those layers are merged into what we call the SWAT map. But it’s not always EC driven like it is on Scott Day’s farm. Here in central Alberta where I’m located, we have topography and water flow, but EC isn’t a big factor. There’s no standard cookie-cutter approach to how the SWAT map is made. Our approach is different for each farm, depending on what the limiting factor is. We always ground proof. We have an agronomist on every client’s farm twice every summer.”

The basic EM38, made by Canadian company Geonics, now sells for about $20,000. The whole SWAT box unit with GPS, transmitters, data loggers and all related pieces sells for about $37,000.

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