John Breker

Soil sampling on drown-out, unplanted, and Prevented Planting acres

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Across the northern Great Plains and Canadian Prairies, weather patterns have ranged from too dry to too wet. For the too wet parts, excessive spring and summer rainfall has resulted in extensive stretches of unplanted (Prevented Planting) acres or drown-out acres. As people think about the fall soil sampling season ahead, we are starting to get questions about these unplanted or drown-out fields: When can I start soil sampling? What kind of residual soil nitrate-nitrogen amounts can I expect in the fall?

Extremely wet soil conditions can cause soil nitrogen losses to leaching or denitrification. Warmer soil temperatures and good soil moisture can promote more nitrogen mineralization from soil organic matter. Fallow fields without growing crops (or weeds) can accumulate nitrogen in the soil profile. There are a lot of variables in the equation, and soil testing is the only way to know how much nitrate-N is actually present in the soil profile. Sorry, no points for guessing! The soil nitrate-N level will depend on numerous management and environmental factors, which vary from field to field and zone to zone.

Management Factors

  • Did you apply nitrogen with intent to plant the field? What was the nitrogen fertilizer rate and application timing? Was it applied last fall?
  • Did you do any summer tillage? More tillage promotes nitrogen mineralization.
  • How was your weed control? Did the weeds get large and acquire a lot of nitrogen from the soil profile?
  • Did you plant a cover crop to take up excess water (and nitrogen)?

Environmental Factors

  • Did excessive rainfall cause nitrate leaching on well drained soils?
  • Did excessive rainfall cause denitrification on poorly drained soils?
  • Were summer temperatures warm? Warm temperatures promote nitrogen mineralization.

For immobile soil nutrients (e.g., P, K, Zn), you could start soil sampling anytime, as soon as you can collect good quality soil cores (not too muddy). If these nutrients were applied the previous fall or spring, a soil test will reflect their current availability in soil, following any fixation reactions and nutrient uptake from cover crop or weed growth. For soil nitrate-N, however, the timing will depend on tillage, nitrogen mineralization, and nitrogen uptake from cover crops and weeds.

For “clean” fallow fields (no cover crop or weeds), soil testing may begin in mid-August. It is important to prioritize soil sampling on fallow fields while you can still drive across them. Since these fallow fields have no plant growth to use excess water through fall, the field trafficability might become challenging if excess precipitation continues into fall. To help ensure you can collect good quality soil samples on fallow fields, start soil sampling in August and early September.

For fields with cover crops, soil testing should be delayed until the cover crop is terminated or growth has slowed and nitrogen uptake has stopped. A healthy cover crop can take up a lot of nitrogen through the fall, so you do not want to collect soil samples for nitrate-N too early. In NDSU cover crop projects, fall-planted cover crop mixes can contain 100 to 150 lb/acre N in the plant biomass, which is a sizeable amount of nitrogen that would not be measured as soil nitrate-N.

AGVISE has also performed fallow and cover crop comparison projects; we have seen 35 to 90 lb/acre nitrate-N differences in the 0-24 inch soil profile between fallow and cover crop areas of the same field (Figure 1). To best reflect the amount of residual soil nitrate-N available for next year, it is suggested to wait until cover crop nitrogen uptake has slowed or stopped in October. If more precipitation arrives in fall, the cover crop will continue to use excess soil water and also provide a nice plant residue surface to drive on.

Figure 1. Soil nitrate-N following fallow or cover crop. Cover crop planted in August; soil samples collected in October. AGVISE Laboratories, Northwood, ND. 2020.

We also recommend splitting fields into management zones for soil testing. The unplanted or drown-out parts of the field can very considerably from the rest of the field, which will skew the field-average soil test result and resulting nitrogen fertilizer rate for next year. Often, the unplanted or drown-out parts will have higher soil nitrate-N (no  nitrogen uptake), but sometimes the situation is oddly reversed for no good reason (Figure 2). This data highlights the importance of collecting separate soil samples for the planted and unplanted/drown-out parts of the field.

Figure 2. Soil nitrate-N variability in fields with unplanted or drown-out areas. Paired soil samples in close proximity from the cropped and unplanted/drown-out area in the same field. AGVISE Laboratories, Northwood, ND. 2014.

John Breker

Soil Testing Behind the Combine

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As harvest gets underway, savvy soil samplers are following right behind the combine and starting to collect soil samples. These soil samplers understand the many reasons why taking soil samples right behind the combine gives them the best quality soil samples and data.

In the past, the reasoning to wait until later in the fall to start soil sampling was that there may be additional nitrogen that would be converted to nitrate through the fall as small grain straw and crop residue start to decompose. However, we now know that small grain straw has a high carbon content, and it takes a long time for wheat straw nitrogen to convert to nitrate-N in soil for future crops. Research has shown that soil nitrate-N levels after small grain harvest are quite stable with small changes (up or down) through the fall. Soil sampling right after harvest provides actionable soil nitrate-N data for making fertilizer decisions for next year.

Soil testing behind the combine has several other advantages. If you sample right behind the combine, you beat chisel plows and disk rippers to the field. Taking soil samples before fall tillage allows you to obtain clean and consistent soil cores with your soil probe; this is important for high-quality soil samples. If you sample after tillage, you will be dealing with soil clods that do not feed smoothly into the soil probe. Soil sampling after tillage can also lead to inconsistent sample depths, which will affect soil test levels for P, K, Zn, etc.

Here are some comments by Dr. Dave Franzen, NDSU Extension Soils Specialist (retired) about soil testing right after harvest:

“It is more the rule than the exception that soil sampling begins in mid-September, rather than starting immediately following small grain harvest. However, many producers miss an excellent window for soil testing by waiting too long. The reason for waiting is the hope that additional nitrogen will be made available through mineralization (decomposition of crop residue and organic matter). A review of research has shown that soil nitrate levels change very little, up or down, following small grain harvest.”

Soil sampling right after harvest is recommended and has numerous advantages

  1. Producers are more likely to use the actual soil test results for deciding fall nitrogen fertilizer rates if the soil test results are in their hands before fall fieldwork begins.
  2. Soil sampling before fall tillage provides more consistent 0-6 inch soil cores, which gives the best soil sample quality for phosphorus, potassium, zinc, organic matter, and other non-mobile soil nutrients tested on topsoil.
  3. Soil sampling right after harvest guarantees that fields will be soil sampled on time and not missed due to weather problems that could happen later in the fall.
Brent Jaenisch

Early Summer Grid Soil Sampling

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The interest in early summer topsoil grid sampling (1.0- to 2.5-acres per grid) continues to increase, especially in traditional corn-soybean growing areas. In Minnesota alone, 30-40% of all grid soil samples are now collected in the summer months. The early summer period (late May to late June) is an excellent period of time to collect grid soil samples, instead of waiting until after soybean harvest when workload and time constraints are heavier.

These early summer soil samples are collected from unfertilized soybean fields, and the soil samples are collected when the soybean plants are in early vegetative growth stages while you can travel across soybean fields with ATVs or UTVs without causing unnecessary damage. These are fields that would have been fertilized two years prior ahead of corn planting, and the fertilizer rates were high enough to cover the following soybean crop as well.

The early summer timeframe works well for 0-6 inch soil sampling and analyzing non-mobile nutrients and soil properties. The commonly tested nutrients and soil properties are P, K, Ca, Mg, Na, B, Cu, Fe, Mn, Zn, pH, buffer pH, salts, organic matter, carbonate (CCE), CEC, and base saturation. It is not applicable for 2-ft residual nitrate-N testing, which must wait until after the crop has been harvested. The mobile soil nutrients like nitrate-N, sulfate-S, and chloride should wait for fall soil sampling.

Advantages to early summer grid soil sampling

  • High-quality soil cores with consistent depth (moist and firm soil profile)
  • No more chasing around in the fall trying to soil sample fields that have been harvested and before any fall tillage occurs
  • More time in summer to develop fertilizer management plans with growers
  • Fields can be fertilized immediately after harvest
  • Avoid post-harvest soil sampling rush in the fall
  • More available labor (interns) in the summer timeframe compared to the fall season
  • On-ground assessment of soybean stands, especially if iron deficiency chlorosis (IDC) is observed

You will want to avoid soybean fields that have been fertilized or manured in the fall or spring prior, as the recent fertilizer or manure application can skew soil test results. In these situations, it is best to wait until after the soybean crop has been harvested to collect soil samples in the fall. In small grain production areas, if soybean or pulses will be planted next year (both crops not requiring nitrogen fertilizer), the early summer timeframe can also offer another opportunity to accomplish grid/zone sampling in the early vegetative growth stages of the small grain crop (barley, oat, wheat), just make sure to avoid any fertilizer bands (seed-row or mid-row fertilizer bands).

John Breker

Soil Sensors: Helpful Gadgets or Hapless Gimmicks?

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This article originally appeared in the AGVISE Laboratories Winter 2024 Newsletter.

A number of new handheld sensors have hit the market, claiming to accurately and precisely measure soil nutrient content in the field, similar to traditional wet chemistry analysis at a soil testing laboratory. The draw for any person soil sampling is the ability to receive soil analysis results right in the field in real time. We know that our clients have a lot of questions about these types of sensors because we are getting these questions too. For almost 50 years, AGVISE has been an early adopter and innovator of new technologies in soil and plant analysis, and these new soil sensors are among the newest to gain popular attention in agriculture.

First, handheld sensors in general are nothing new for soil analysis. There are a number of handheld pH and electrical conductivity (EC) sensors available on the market that are often used for assessing and mapping environmental sites for reclamation and remediation projects. The environmental consultants still need to collect field soil samples and send them to the laboratory for calibration and validation in their official reports. The handheld sensors are used to help them assess the site size and variability.

Second, the type of sensor for the intended soil nutrient or property for measurement is important. After all, you should not try to measure something that the sensor cannot detect, right? The new handheld soil nutrient sensors often rely on near-infrared (NIR), mid-infrared (MIR), or X-ray fluorescence (XRF) spectroscopy methods. These technologies have long existed as benchtop instruments in analytical laboratories for various applications, and each method has its strengths and limitations.

For example, NIR spectroscopy is widely used in feed and forage analysis, food processing, and even meat science. The American Society of Agronomy compiled an 800-page book on NIR applications in agriculture (https://doi.org/10.2134/agronmonogr44.c10). There is one chapter on soil analysis at the end of the book. The strengths of NIR for soil analysis include soil organic matter, total carbon, organic carbon, organic nitrogen, and even pH. However, it does not perform well for nitrate-N, P, K, sulfate-S, Ca, Mg, Na, Cu, Fe, Mn, Zn, or soluble salts (EC). Simply put, NIR fails at measuring the actual soil nutrients we are trying to manage! This is why we do not use benchtop NIR for any soil analyses at AGVISE, let alone a handheld unit with less accuracy or precision. You might see handheld NIR sensors being used for some things, but you will not see them replace soil sampling or soil nutrient analysis soon.

Third, the handheld sensor outputs are often correlated and converted, in the end, to traditional wet chemistry analysis methods, like Bray P, Olsen P, or ammonium acetate K. These are the plant-available soil test methods that we are all familiar with and have decades of soil test calibration research behind them, which allow us to make fertilizer guideline calculations from the soil test result. Whenever a correlation and conversion step takes place, this introduces error for any subsequent calculations, like fertilizer rates. It is important to know what is actually being measured versus what is being reported.

As new sensors hit the market, a person thinking about trying them should be asking a lot of questions. AGVISE is always evaluating new analysis technologies, which can help us do a better or faster job while providing high-quality data to our clients. The questions outlined above are those that we use when we evaluate new analysis technologies for our own operation, and we hope the same questions can help guide you through the gamut of new soil sensors too.

John Breker

Sticky Wet Soils? Try Adding a WD-40 Holster

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This article originally appeared in the AGVISE Laboratories Fall 2024 Newsletter.

Do you have challenges collecting good quality soil cores in sticky wet soils? You are not the only one! WD-40 has been the soil probe lubricant of choice for over 30 years to help obtain better quality soil samples. University researchers have also tested WD-40 and found it does not contaminate soil samples.

Spraying WD-40 on your soil probes with the spray cans can get messy inside the pickup cab. A smart idea to make the WD-40 application process simpler and cleaner is making a WD-40 holster with some PVC pipe. The PVC pipe holster lubricates the soil probe with WD-40 between each soil core and also keeps the soil probe within easy reach. The clever idea came from a client who had spent too much time fiddling with WD-40 spray cans and losing them underneath the pickup seat.

The WD-40 holster is made from 2-inch diameter PVC pipe with a cap glued on the bottom and a threaded fitting on the top with a screw-in plug for storage when not in use. The PVC pipe should be fastened so that the open end faces the soil sampler and the soil probe can be easily placed into the pipe. Fill the PVC pipe with about 3-4 inches of WD-40 in the bottom. With the PVC pipe opening near the hole in the pickup floor, any excess WD-40 drops coming off the soil probe will go down the hole and reduce the mess of spraying WD-40 in the pickup cab.

John Breker

Zone Soil Sampling: How Many Zones?

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Zone soil sampling has become a standard practice in precision nutrient management, but the grand question remains – How many zones should you be soil sampling?

Well, it depends! It just makes sense that a field with more variability requires more zones than a field with little variability. Zone soil sampling separates parts of fields that behave differently into similar zones that can be managed together. Common data layers used to build zone soil sampling maps include satellite imagery, plant vegetation indices, crop yield, salinity, topography, and even bare soil color.

As a soil testing laboratory, AGVISE does not know what data layers are used to create the zone maps, but we do know the soil nutrient levels in each zone. Clients often ask how many zone soil samples should be collected in each field to get the best soil nutrient information. Common sense tells us that splitting fields into more zones should provide more detailed soil nutrient data.

With soil test data from thousands of zone soil sampled fields, we mined the AGVISE database to see what the average range in soil test levels per field (high testing zone minus low testing zone) could tell us about field variability and the number of zones that should be sampled. The table summarizes the average range in soil test levels for over 24,000 zone soil sampled fields in 2023. The number of zones ranges from 3 to 8 zones per field. You can see, as the number of zones increases, the difference between the high zone and low zone gets larger and larger.

This data reminds us that more zones per field can tell us more about the soil nutrient status in each field, providing more powerful information to develop variable-rate fertilizer applications. If you have variable landscapes with rolling topography, diverse soil types, or salinity problems, you may have to take more zone soil samples per field (5-7 zones) to see the greatest differences in soil fertility and to take full advantage of variable-rate fertilizer applications. If your landscapes have less variability with fewer soil types, relatively flat topography, and no salinity problems, then you can probably take fewer zone samples per field (3-4 zones).

John Breker

Winter Soil Sampling: You Need the Right Tools

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Snowfall in late October and November slowed harvest and soil sampling across the region. This means some fields will be soil sampled in December and maybe January as harvest for late-season crops continues in the snow.

The right equipment is the key to any project, and winter soil sampling is no different. AGVISE heavy-duty (HD) chromoly soil probes were designed for hard, frozen soil conditions. Chromoly steel is much tougher than stainless steel, and it handles the stress of sampling frozen soil. To punch through several inches of frost, you will also require additional weight. Most soil sampling trucks have the hydraulic cylinder mounted inside the truck cab, where you can take advantage of the entire truck weight to push through the frost. This enables you to take soil samples through 4 to 6 inches of frost on most medium- and fine-textured soils in winter. For receiver hitch-mounted hydraulic cylinders, you will need to add extra weight in the truck box, and it may limit you to pushing through only 1 to 3 inches of frost.

AGVISE offers wet and dry soil probe tips for the HD chromoly soil probe. The wet soil probe tip is best suited for frozen soils. The HD chromoly soil probe is available with or without a slot.

You can view examples of in-cab and receiver hitch-mounted hydraulic soil sampling systems on our website (https://www.agvise.com/installed-soil-sampling-kit-examples/). You can also find videos of soil sampling in frozen soils with the HD chromoly soil probe and wet soil probe tip.

Jodi Boe

Sampling Depth: Be consistent!

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This article originally appeared in the AGVISE Laboratories Fall 2022 Newsletter

Soil test results are only as reliable as the soil samples collected in the field. A crucial part of soil sample quality is consistent sampling depth. This is important because all the soil test calibration research and fertilizer guidelines for non-mobile nutrients (e.g., phosphorus, potassium, zinc) are based on a soil core depth of 0-6 inches, thanks to the historical tillage depth. If soil cores are taken too shallow or too deep, you can skew soil test values and the resulting fertilizer guidelines. Getting the most accurate and useful fertilizer guidelines starts with a good quality soil sample. To help illustrate this point, we did a simple demonstration project, showing how soil sampling depth consistency affects soil test results in a long-term no-till and conventional-till field.

Soil nutrient concentrations can vary greatly throughout a soil profile, even more so in long-term no-till where soil nutrients are not regularly mixed. This leads to stratification of nutrients near the soil surface, meaning a soil core that is too shallow or too deep can greatly affect soil test results. You can clearly see the effect of no-till stratification in soil test potassium (STK) levels in Table 1. Between the 0-2 and 0-4 inch soil cores, there is a 53 ppm difference in STK. Although nutrients in conventional tillage systems do not concentrate at the surface to the extent they do in no-till, a concentration gradient still exists. This is most obvious near the tillage depth, where soil mixing below that depth stops. In Table 2, the 0-2 and 0-4 inch soil test results are similar, but the differences become apparent at the 0-6 inch depth. Soil sample depth is just as critical in conventional tillage as it is in no-till. In addition, it is important to collect soil samples before any fall tillage occurs
to collect good quality soil cores with consistent depth. Tillage creates uneven clods and a “fluffy” soil surface, making it hard to determine what actually represents the 0-6 inch soil depth.

Tips to increase soil sample depth consistency

• Collect soil samples before any tillage occurs. If tillage does happen before you can take a soil sample, try to make a firm surface with your foot or sample in a tire track.

• If you are using a hand probe, mark the target soil core depth on the soil probe clearly. A metal file works great to cut a notch in the soil probe at 6 inches. The file mark does not wear away like a piece of tape or permanent marker can.

• If you are using a hydraulic probe and use your hand to measure the soil core length, calibrate often to ensure you are measuring a true 0-6 inch soil core.

• If you train new soil samplers, reiterate the importance of soil sampling depth consistency. Provide clear instructions on measuring the proper soil sampling depth in the field.

• Be sure the soil sample submission information sent to the laboratory (online or paper) matches the actual soil sample depth obtained in the field. The correct soil sample depth can be noted on the paper forms or edited on the AGVISOR online submission before it reaches the laboratory.

 

John Breker

Soil Sampling for Nitrogen in a Delayed Spring

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Spring planting is clipping along in some parts of the region, while other parts are still waiting to hit the field, as excessive rainfall and cold temperatures have delayed spring field work and planting. Who would have thought last fall that this is what spring 2022 would look like, after the worst region-wide drought in 30 years? Mother Nature always reminds us to stay prepared for anything.

A delayed spring start means that every day in the field is important. AGVISE delivers next-day turnaround on processing soil samples. The soil samples are analyzed and reported the next business day after arrival at the laboratory. Soil test results are posted to our online AGVISOR portal for quick and easy access. If you need any soil sampling supplies for spring, please let us know and we will send them to you right away.

So, what is the best strategy for spring soil testing and assessing soil nitrogen losses after the rain? The compressed fertilizer and planting window might not leave enough time to adjust preplant fertilizer rates, especially if the field is just barely dry enough to plant. If soil nitrogen losses have occurred following spring rains, a spring soil test collected now will be helpful to create a split-applied nitrogen plan or to direct a supplemental nitrogen application later. In the AGVISE Spring 2022 Newsletter, we answered some questions on split-applied nitrogen application strategies, so please take a look at those options for applying nitrogen during the growing season.

Short-season crops develop quickly, so additional nitrogen should be applied in the upcoming weeks. A soil sample collected before or shortly after planting will provide the best assessment of preplant soil nitrogen supply and losses. Do not wait too long to collect the soil sample because, as we move into June, plant nitrogen uptake and nitrogen mineralization from soil organic matter will make the soil nitrogen result more difficult to decipher. To maximize yield in small grains, apply all topdress nitrogen before jointing (5-leaf stage). Any nitrogen applied after jointing will mostly go to grain protein. In canola, apply nitrogen during the rosette stage, before the 6-leaf stage.

Long-season crops like corn offer more flexibility and time for in-season soil sampling and nitrogen application. Rapid nitrogen uptake in corn does not begin until after the V6 growth stage. The Pre-sidedress Soil Nitrate Test (PSNT) can help you decide the appropriate sidedress nitrogen rate. For more details, take a look at the PSNT article link for instructions on collecting and submitting PSNT soil samples. The PSNT requires a 0-12 inch depth soil sample taken when corn plants are 6 to 12 inches tall (at the whorl), usually in late May or early June. Late-planted corn may not reach that height before mid-June, but PSNT soil samples should still be collected during the first two weeks of June. If spring rainfall was above normal, Iowa State University guidelines provide additional PSNT interpretation criteria for excessive rainfall, manured soils, and corn after alfalfa.

If you have any questions on the best strategies for spring soil sampling and in-season nitrogen application options, please call our technical support team and we will be happy to answer any questions you may have.

John Lee

Probe stuck in the ground? Don’t let it wreck your day.

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If you have ever had a soil probe come off while in the ground, you have experienced a rare but stressful event!

A customer recently called with this situation and a success story about how he recovered the soil probe “MacGyver” style.  I think his solution involved barb wire, duct tape and some chewing gum.  While a soil probe with quicktatch collar can come off the roll pin on the cylinder shaft, it is very rare.  If you have had that happen to you, you may have used a shovel to dig the probe out or pulled it out with some other MacGyver device you created.  In the winter with frozen soil, your options are more limited.  We wanted to give others who may experience this rare event an idea to create your own “probe puller” if this happens to you.  The materials you need are an extra quicktatch collar, snap pin or small bolt, short chunk of light chain, and another bolt with two nuts.   Here are some pictures to show you the probe puller we “MacGyvered” with stuff laying around the shop (sorry no duct tape or chewing gum involved).  I am sure several of you may have come up with even simpler/better designs for this life-saving device.  We would love to see other designs to share with customers who need to create a device that will save the day for poor samplers who find themselves in this situation!