Many people may wonder why SWAT MAPS variable rate is named the way it is. While it may sound like just a “cool name,” it stands for three critical factors for unlocking a strong and stable variable rate system: Soil, Water, and Topography. These factors are essential for applying inputs such as seed, fertilizer, soil amendments and even soil applied herbicides within soils to provide the best ROI. Soil, water, and topography are all factors that have influence over and interact with one another, such as seen in Figure 1. These interactions impact both yield and fertilizer response of a crop and should be looked at in depth when making variable rate recommendations. Other tools such as yield data and satellite imagery show you variability without explaining the why. Unlike SWAT MAPS. They do not differentiate the most important factors in determining fertilizer behaviour and responses: soil potential.

Figure 1. Soil water and topography interactions that influence soil fertility.

Soil is the most important factor to look at when it comes to a strong, stable and successful variable rate program. Many soil components influence the yield of a crop from year to year and determines how a crop will respond to added fertility.

Two key factors are topsoil depth and organic matter levels, which both influence mineralization rates and must be considered when making fertilizer recommendations. Some fields can have pockets of extreme organic matter levels (peat) that have excess nutrient mineralization rates, causing severe lodging and uneven crops. These areas need to be treated separately from other areas within a field and often need little to no nitrogen fertilizer applied.

Soil salinity and solonetzic areas can also greatly impact how a crop will yield and respond to added fertility. Identifying where these occur within a field and treating them separately Is a good way to save on fertilizer costs, as they typically have excess nutrients.

Soil texture is another important soil feature that should be considered when making fertilizer recommendations. Separating soil texture differences by zone can greatly improve fertilizer ROI. Areas that are sandy will be more moisture limited and have less mineralization compared to areas that have higher clay content. Sandy soils have lower CEC (ability to hold cations) than clays, and also have greater potential for leaching mobile nutrients like nitrate and sulfate. These differences should all be incorporated into a variable rate fertilizer recommendation.

Figure 2. Different landscape positions within a field along with their soil properties.

Water is the most important factor for crop growth, yield, fertilizer response, and nutrient-use efficiency. When using variable rate, you need to be able to separate out areas in a field where there will be different degrees of available moisture (Figure 3). Zones with limited, adequate, or excess moisture should be managed separately when it comes to applied fertilizer and seed rates. Knolls or sand/gravel seams are commonly moisture limited and should be treated differently than mid-slope positions and depressional areas which typically have more moisture. However, areas with adequate or excess moisture shouldn’t necessarily receive excessively high fertilizer rates, either, as these areas are more prone to lodging and can cause uneven staging.

Figure 3 Example of water dynamics in a field.

Topography is the third crucial factor that SWAT MAPS variable rate uses. Topography is the landscape position within a field (knolls, mid slopes, or depressions). Topography influences how the water sheds or collects and how it moves through a field (Figure 4). Topography has also influenced erosion history in fields. When summer fallow was a common farm practice, we ended up with a lot of eroded knolls due to the movement of topsoil from knolls to lower slope positions.

Figure 4. Example of water flow accumulation through a SWAT MAP field.

Organic matter levels within a field typically follow topographical position, with levels increasing as you go downslope. In some cases, this is due to historical farming practices such as such as tillage, while other cases are natural occurrences such as peat depressions and gravel/sand ridges. Soil pH also varies with landscape position. In regions with calcareous soils, eroded knolls usually have higher pH due to exposed subsoil with containing calcium carbonates, or natural lime. In other regions we find pH is influenced more by texture, with sandier soil types being most acidic. Topography influences soil fertility through the movement of water containing mobile nutrients such as nitrogen and sulphur. The movement of topsoil and organic matter will impact the overall nutrient availability and mineralization in different landscape positions. Finally, the overall differences in soil pH from land scape positions will impact the availability of all nutrients.

Many tools in the agriculture industry – such as yield mapping and satellite imagery – can be used for making variable rate decisions, but they are not a stable or standalone approach that should be taken when it comes to fertilizer and seed rates. These methods show us historical field performance and variability, but they lack the “why.” Soil, Water and Topography are the three main factors that influence fertilizer response and yield potential. Understanding these factors through a SWAT MAPS variable rate approach helps explain the “why” and helps to unlock your soil potential.

For decades, the primary concepts of modern soil fertility management have focused on utilizing semi-dense soil sampling (1 to 5 acres per sample) to obtain spatial soil test results, utilize some form of interpolation to estimate soil test levels between soil samples, and invoke any number of fertility recommendation calculations and algorithms to generate a fertilizer application map. Much software has been built and potentially millions of acres of fertilizer have been applied in this manner with the goal of applying the correct fertilizer to the right parts of fields. This has been the cornerstone of “Precision Agriculture.”

While this approach is relatively simple (determining sample placement on a grid using a defined extent), it doesn’t incorporate useful characteristics of the field, including soil water holding capacity, drainage and parent material.

SWAT MAPS provides the process to create SWAT zones, a much-improved way to represent the soil characteristics of a field. SWAT stands for “Soil, Water and Topography”. Zones created using the SWAT process incorporate the natural terrain of the field and help break areas down into common elements of similar soil characteristics that have similar topographical and water influence. These zones can be used to place samples or sub-samples, allowing for better potential correlation between soil test values and the underlying soil properties.

In recently published research entitled, “Soil test phosphorus predicts field-level but not subfield-level corn yield response,” scientists set out to answer if, at a sub-field level, a recommended amount of phosphorus fertilizer showed a yield response in corn when the soil test phosphorus level was below the determined Critical Value. For soil fertility recommendations, the Critical Value is a soil test level of a nutrient above which provides little to no economic benefit from additional fertilization, and below which has been shown to have economic response to fertilization.

More than 100 plots were placed across the landscape each at two field-sites in Kentucky, with each plot containing subplots with either a control (no phosphorus) or a treatment (with 29.5 kg/ha or 26.3 lbs/ac of P2O5 nutrient). Each plot was either 9 x 9 m or 12.2 x 12.2 m (29.5 x 29.5 feet or 40 x 40 feet). Applications of liquid ammonium polyphosphate with the planter in a subsurface band were applied in the no-till planting program. This placement was utilized as it offered the highest probability of yield response. These plots were harvested using a plot combine, harvesting the middle two of the four rows of each plot and yield captured by a yield monitor. Soil samples were taken in each plot prior to each of the corn cropping seasons and analyzed using the Mehlich 3-ICP methods. Seven site-years were captured using this process between 2016 and 2021.

Across all site-years, Soil Test P ranged from 1 to 63 ppm M3P and averaged 14 and 12 ppm for each site, respectively. 94 to 98% of the plots had M3P values below the 30 ppm critical value set by University of Kentucky guidance.

Interestingly, soil pH was also low at both sites; one site averaged 4.6 pH and ranged from 4.0 to 7.3 in the first two cropping seasons before being limed and raised to 5.2 (range of 4.6 to 7.2). The other site started with an average pH of 5.9 and ended at 5.3 at the conclusion of the study.

Comparing the yield between the untreated and treated subplots, it was found that 31 to 70% of the plots per site-year responded to phosphorus; on average, 51% of the plots responded across all site-years, and 96% of the plots had soil test P levels of less than 30 ppm, the critical value.

At the field scale, 5 of the 7 site-years did show that phosphorus application was effective enough to raise yield significantly. However, at the plot level, this was not always observed. In fact, some negative responses to phosphorus application were observed at the plot level.

These results indicate that while current soil test recommendations may be useful at the field level, they may not be so useful at the sub-field level. Unfortunately, the sub-field level of management is what is considered as “Precision Agriculture,” the state of the art.
Does this suggest that soil sampling should be engaged at even higher resolutions to capture even greater sub-field variability? Work by Lauzon et al., (2005) included the sampling of 23 fields in Ontario at 30 meter (96 feet or 0.21 acres/sample) spacing. Using autocorrelation analysis, they found that 13 of the 23 fields would require sampling of 30-meter or less to adequately address their spatial variability. At only one field was it found that the typical 100-meter grid (2.5 acres/sample) pattern was adequate for phosphorus and potassium assessment. This is not a ringing endorsement of the current generally accepted approaches used in Precision Agriculture.

On the other hand, would field-level composite samples be the way to overcome the sub-field variability question? This would inevitably mask portions of the field that may respond differently. Ignorance is bliss.

Instead, the authors of the paper suggest future research should consider “incorporating mechanistic factors such as soil texture, climate zones, and crop production history;” in other words, incorporating sub-field “context” into corn yield response algorithms. This would point towards utilizing tools that can parse out the variability of the field’s soil composition, water movement characteristics, and topography, like SWAT MAPS.

This research work was very comprehensive, but it would be worth considering at other locations with varying soil and weather conditions to see if these results hold true. If so, this work may lead us towards a better “Precision Agriculture,” one that doesn’t hold a blind eye to the conditions of the soil but instead welcomes the natural (and human-influenced) variability to help explain yield response to nutrients.

Vaughn Reed, Jenni Fridgen, Bronc Finch, John Spargo, Josh McGrath, James M. Bowen, Gene Hahn, Douglas Smith, Edwin Ritchey, 2025. Soil test phosphorus predicts field-level but not subfield-level corn yield response, Agronomy Journal, Volume 117, Issue 1

John D. Lauzon, Ivan P. O’Halloran, David J. Fallow, A. Peter von Bertoldi, Doug Aspinall. 2005. Spatial Variability of Soil Test Phosphorus, Potassium and pH on Ontario Soils. Agronomy Journal, Volume 97, Issue 2.

Nitrogen (N) plays a critical but complex role in carbon (C) sequestration in soils, influencing both the accumulation and decomposition of soil organic matter (SOM). Depending on how it is managed it can either enhance or reduce the storage of carbon in soil. Proper management of nitrogen fertilization is essential in maximizing soil carbon sequestration while minimizing the loss of carbon as carbon dioxide (CO2) through microbial respiration.

One of the primary ways nitrogen influences carbon sequestration is by enhancing plant productivity. Nitrogen fertilization increases plant biomass production, which in turn leads to higher inputs of organic material into the soil through root exudates, fine root turnover, and crop residues. These carbon inputs provide the raw material for SOM formation, an essential process for long-term carbon storage in soil. When plants have adequate nutrition, their increased photosynthesis results in greater carbon fixation from the atmosphere, subsequently enriching the soil organic carbon pool. (Tiefenbacher et al., 2021)

Sequestration of carbon in SOM is inherently tied to nitrogen because organic matter contains both elements in varying proportions. As SOM builds up, it sequesters not only carbon but also nitrogen, effectively tying up nitrogen in forms that are less available to plants and microbes. While this is beneficial for long-term soil health and fertility, it also means that some nitrogen inputs are locked in stable organic forms and are not immediately available for plant uptake. The process of microbial immobilization is governed by the carbon-to-nitrogen (C:N) ratio, meaning that soils high in carbon but low in nitrogen will experience increased nitrogen immobilization by microbes. As a result, additional nitrogen fertilizers may be required to offset the extra nitrogen that is immobilized while SOM is increasing in soils. (Karimi et al., 2020)

However, it is crucial to avoid over-application of nitrogen, as excess fertilization can decrease carbon sequestration rather than enhance it further by accelerating microbial decomposition of SOM. A key challenge in carbon sequestration is the dynamic balance between carbon inputs and the decomposition of SOM. Microbial activity, which is influenced by nitrogen availability, plays a central role in determining whether soil gains or loses carbon over time. Nitrogen fertilization enhances microbial activity, leading to the accelerated decomposition of SOM and the release of CO2. While this process releases plant-available nutrients that support growth and further carbon input, excessive microbial breakdown of SOM can counteract sequestration efforts by increasing carbon losses from the soil system. (Khan et al., 2007)

The balance between carbon inputs and decomposition is crucial in determining changes in soil carbon content. When nitrogen fertilization is applied in appropriate amounts that match plant demand, it fosters plant growth and organic matter accumulation without significantly accelerating SOM decomposition. However, when nitrogen is supplied in excess, it stimulates microbial decomposition beyond the rate at which carbon is replenished, leading to net losses of soil carbon. Excess nitrogen can tip the balance toward decomposition rather than accumulation, thereby reducing the effectiveness of soil as a carbon sink.

To optimize carbon sequestration, nitrogen application should be carefully managed to match spatial and temporal plant requirements. Precision nitrogen management with SWAT MAPS—applying the right amount of nitrogen to each SWAT zone within a field—ensures that plant growth is not limited by nitrogen deficiencies while also preventing excessive nitrogen levels that would accelerate SOM decomposition. Areas within fields that are nitrogen deficient will experience reduced plant growth and, consequently, lower carbon inputs as well as increased immobilization as the C:N ratio of the soil rises. Conversely, areas within fields with consistently excessive nitrogen levels will exhibit heightened microbial activity, leading to SOM depletion and increased CO2 emissions. By optimizing nitrogen applications to match plant demand and soil conditions, it is possible to sustain agricultural productivity while maximizing carbon sequestration potential in soils.

The SWAT ECOSYSTEM allows for fine-tuning nitrogen application rates and distribution. It can therefore enhance soil carbon sequestration while maintaining or even improving crop yields. Technologies such as soil testing, moisture monitoring, remote sensing, yield monitoring, and variable-rate fertilization enable more precise nitrogen management, reducing the risks associated with both nitrogen shortages and surpluses. Ultimately, managing nitrogen effectively in agricultural systems requires a strategic approach that optimizes fertilization withing zones of varying capabilities for both productivity and long-term soil carbon storage. And long-term carbon storage benefits everyone.

Karimi, R., Pogue, S.J., Kröbel, R, Beauchemin, K.A., Schwinghamer, T. & Janzen, H.H. (2020) An updated nitrogen budget for Canadian agroecosystems. Agriculture, Ecosystems & Environment. 304: 107046. https://doi.org/10.1016/j.agee.2020.107046

Khan, S.A., Mulvaney, R.L., Ellsworth, T.R., & Boast, C.W. (2007) The myth of nitrogen fertilization for soil carbon sequestration. J. Environ. Qual. 36:1821–1832 (2007). doi:10.2134/jeq2007.0099

Tiefenbacher, A., Sandén, T., Haslmayr, H.-P., Miloczki, J., Wenzel, W., & Spiegel, H. (2021). Optimizing Carbon Sequestration in Croplands: A Synthesis. Agronomy, 11(5), 882. https://doi.org/10.3390/agronomy11050882

Eroded knolls – those high spots within a field where crop production struggles year after year – is developed from years of tillage, wind and water erosion. This erosion caused knolls with low organic matter, poor fertility, water infiltration and holding capacity. The topsoil is often very thin or in some cases has even disappeared.

A tool that can be used to determine the severity of eroded knolls is electrical conductivity (EC) mapping. EC is a measure of soil electrical conductivity, which is largely determined by salinity (soluble salts) and soil texture (clay content). High EC areas are indicated by higher clay content while low EC areas are indicated by higher sand content. In a typical field where there will be low EC hilltops and high EC depressions. This is often reversed when dealing with eroded knolls because erosion has removed much of the topsoil from the hilltops exposing the subsoil with higher clay content and depositing the low EC topsoil further down the slope into the depressions. Including a topography layer in map development allows us to define where water sheds and where it collects throughout the landscape. When combining EC with topography (SWAT MAP) and site-specific soil tests it is an accurate way of delineating areas of the field that may have eroded knolls.

Figure 1. Field mappings route, paying attention to knolls and depressions

How to identify an eroded knoll?
Eroded knolls lose soil organic matter, the loss of which results in a surface soil lacking good granular structure. When erosion occurs, there are many different physical, chemical and biological characteristics to look for. Some physical characteristics of an eroded soil can include:
• Loss of granular structure
• Reduced water-holding ability
• Reduced stored water due to increased runoff
• Crusting on the soil surface
• Compaction of exposed subsoil
• Stony soil surface

Figure 2: Stony soil surface

Physically mapping a field will only get you so far in determining the severity of eroded knolls. A tool like SWAT MAPS, which properly delineates a field into management zones ranging from eroded knolls that are the driest areas of the field (Zone 1) to a water collecting depression (Zone 10), will allow you to visually see what the field looks like.

Figure 3: SWAT zone Delineation

Pairing soil sampling alongside the SWAT MAP will help to determine the chemical and biological characteristics of an eroded knoll.
These characteristics can include:
• Low organic matter
• Loss of biological activity
• Reduction of nutrient levels
• High pH (8.0 or more)
• Reduced nitrogen efficiency

Figure 4: Soil test result of a SWAT MAPPED field with Eroded Knolls

When interpreting a soil test result to help determine if you are working with a field that has eroded knolls there are a few characteristics to look at. First, look to see what the organic matter levels are at, often seeing the lowest values in zone 1 of the field and the highest values in zones 8 to 10. Second, higher pH values are a key indicator of an eroded knoll. Usually, pH increases from surface to subsoil however, when the surface soil has eroded away this exposes the higher pH subsoil on the hilltops. Third, cation exchange capacity (CEC) can also be an indicator, a CEC greater than 25 typically means clay soil while a CEC less than 10 means sandy soil. When the CEC is highest in zone 1 of the field and lowest in zone 10 this typically means that soil erosion has occurred. There are nutrient indicators to looks for as well - low phosphorus steadily increasing further downslope as well as high calcium steadily decreasing further downslope. Calcium levels can often indicate the presence of carbonates at the soil surface which are most often present in subsoil horizons which have now been exposed due to erosion. These carbonates are the primary reason the pH is so high as well.

How to restore productivity of eroded knolls?
With eroded knolls usually being deficient in phosphorus and micronutrients, repairing these nutrients is an approach that can be taken to improve the productivity. Research has been done showing that moving the topsoil from the water collecting depressions of a field to the eroded knolls is a successful approach. While it offers quick results, it can be a very expensive strategy. The combination of applying high rates of phosphorus above normal application rates as well as applying manure can help improve the soil. Beginning a phosphorus building program with a SWAT CERTIFIED Agronomist allows targeting of the areas of the field that will be of most benefit while reducing application in areas that don’t require as high of application rates.

Figure 5: SWAT Based Phosphorus Build

When phosphorus is brought up to a sufficient level, any further erosion needs to be prevented. This can include improving root biomass below ground which should result in more stubble above ground. To do this variable rate seeding can be a very successful approach which will allow for increased plant populations in targeted areas of the field. Over several years the improved crop establishment and biological activity that comes with it helps improve these soils and redevelop an A horizon.

For example, zone 1 of a SWAT MAP usually has higher mortality because they are dry, sandy/rocky with a poor seedbed (Eroded Knoll) so increasing seeding rates there should allow for better establishment. Variable rate seeding will consider crop specific population response as well as mortality rates. Once stubble has been better established further attention at keeping tillage to a minimum in those areas of the field is essential.

With SWAT MAPS, there are tools in place to manage eroded knolls separately from the rest of the field, helping reduce the added costs of high input methods of restoring eroded knolls. Interested in hearing more about ways to better improve productivity? Reach out to your SWAT MAPS agronomist today or get in touch on our website.

When fertilizer is applied, we want all those nutrients to be taken up by the crop, right? And the more fertilizer that gets into the crop the better? Yes… and no. That’s the paradox of nutrient use efficiency (NUE).

This starts with the complexity of how NUE is measured. There are many ways to measure NUE, explained in “Defining nutrient use efficiency in responsible plant nutrition” (The Scientific Panel on Responsible Plant Nutrition, 2023). The method used depends on what you want to learn and measurements that are available. But the end goal is largely the same: To understand how agriculture can provide nutrients to crops while minimizing their environmental impact.

While it would be outstanding to measure 90+% uptake of an applied nutrient in the year of application, it’s not very realistic for most nutrients and crops grown in soil. Crops acquire most of their nutrients from the soil. A recent example of this with phosphate uptake in corn was published, showing 78-92% of early – mid season corn phosphorus came from the soil, not fertilizer. (Chatterjee et al., 2024). Griesheim et al. (2022) measured a range of only 7% to 46% fertilizer nitrogen uptake efficiency in aboveground biomass in corn. The relative amount from soil versus fertilizer depends on the nutrient and the soils’ ability to supply the nutrient, either as “available” when we soil test or through mineralization during crop growth. The more deficient the soil is, the higher the percentage taken up must come from applied fertilizer. So, in a very deficient soil, NUE by many measures will look very good! But is this what we want?

Assuming all other 4Rs are adhered to, very little response to the applied nutrient means the soil is functioning the way we want it to and supplying enough nutrition. All we need to do is continue to monitor soil levels and ensure supply remains sufficient but not excessive. An example is the P response graphs in figure 1. Is the steeper response curve more exciting, or the flat one? Many would say the steep curve - it shows an impressive return on applied phosphate. But ideally, we want the curve to be relatively flat, since that tells us that we’ve done a good job of managing soil nutrient levels and our soil is functioning the way we want it to. This is the intersect of psychology and agronomy – the steep response curve gives us the excitement of knowing there’s an opportunity to fix something, and while that information is valuable, we’d be better off on the flat curve since yield isn’t limited by that particular nutrient.

Figure 1. Average corn response to fertilizer P on high P soil vs. low P soil (Leikam et al., 2010).

Given what we do with SWAT MAPS, we are particularly interested in responses by SWAT zone to demonstrate opportunities to use variable rate technology to improve returns on applied nutrients. What if the high yield, flat response curve above was from a well-drained depression with deep topsoil and high soil P, and the lower yielding, steep response curve from an upper landscape position with historical erosion and little topsoil? That’s incredibly valuable information and needed if the industry wants to drive precision ag adoption.

To recap, we want soils that can supply sufficient nutrients to the crop, because that means we don’t have to apply more from fertilizer. But if we achieve that it also means that a very small percentage of fertilizer that we do apply to maintain fertility levels will be taken up in that year, and that’s the paradox. The annual “efficiency” that is often chased is not actually what we want. Whether that H2PO4– molecule taken into a root hair came from a fertilizer granule applied 4 weeks ago or 4 years ago doesn’t really matter. The important thing is it’s there where the root can access it, it’s in high enough concentration to supply what the plant needs, but not so high it risks excessive environmental loss. That SO4-2 molecule that will contribute to high quality protein in your wheat? It may have cycled through many, many microbial proteins before finally entering a crop root years after it was applied as a sulfur fertilizer. The important part is it didn’t leach out of the root zone a week after it was applied. Soil is the ultimate nutrient laundering scheme. It supplies nutrients but we don’t usually know the original source, and frankly does it matter?

Before the inundation of “yeah buts...” I will say that nitrogen in some ways is an exception. Soil can’t reliably “store” nitrate-N. Because nitrate is susceptible to many forms of loss and it has numerous environmental effects in both surface water, groundwater, and the air, we need to be cautious about how much nitrate is available for loss at any given time. The way to mitigate those losses is by following 4R nutrient stewardship, nurturing a healthy, well-structured soil that has a suitable pH, and is nutritionally balanced. All these characteristics tend to reduce the need for applied N, maintain crop yields in a broad variety of climate conditions, and reduce losses.

It is obvious from recent research that healthy agricultural soils provide the majority of nutrient uptake rather than the fertilizer we apply. Which is why we need to understand the soil’s ability to supply these nutrients for a successful precision ag strategy. And no, it is not just a single nutrient value on a soil test. It requires an understanding of carbon cycling across the landscape that is influenced by many things – both chemical and biological.

At Croptimistic we are currently scoping out ways to measure yearly and long-term nutrient use efficiency using a partial balance method. While it's a data intensive process, it will be relatively easy to calculate for farms participating in the Yield Potential Program. It allows us to analyze whether we are building or depleting nutrient levels at the field and zone level or risking environmental losses. It’s a valuable measure of sustainability and helps us make better fertility recommendations.

So, remember, the next time you do a fertilizer trial on your farm and see no results, don’t be disappointed! It might mean you can pat yourself on the back and thank your soil for doing its job.

References:

Chatterjee, N., Li, C., & Margenot, A. J. (2024). 33P-isotope labelling ammonium phosphate fertilizers reveals majority of early growth maize phosphorus is soil-derived. European Journal of Soil Science, 75(5), e13578. https://doi.org/10.1111/ejss.13578

Griesheim, K. L., Mulvaney, R. L., Smith, T. J., & Hertzberger, A. J. (2023). Nitrogen-15 evaluation of fertilizer placement at planting for corn production. Soil Science Society of America Journal, 87, 309–323. https://doi.org/10.1002/saj2.20503

Leikam, D., G. Randall and A. Mallarino. (2010). Are current soil test-based phosphorus and potassium recommendations adequate? Crops & Soils. Vol. 43-6. Pp 27-32.

Scientific Panel on Responsible Plant Nutrition. (2023). Defining nutrient use efficiency in responsible plant nutrition. Issue Brief 04. Available at www.sprpn.org