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Study looks at potato response to microenvironment and opportunities for precision ag management

This is a condensed version of a more thorough and detailed analysis, involving over a dozen sites between 2019 and 2020. Sites included irrigated low organic matter sands and sandy loam soils in central Minnesota. The study was done by Anez Consulting, based in Little Falls and Paynesville, MN. The article below was authored by Precision Agronomist, Michael Dunn, CCA; SSp. The original article was published on LinkedIn here. It is republished on Potato News Today below with permission and thanks.

Soils in central Minnesota can change quite rapidly, sometimes in less than 30 linear feet. High resolution soil mapping using remote sensing techniques, coupled with surface flow modeling and other data sets can accurately, efficiently, economically, and consistently delineate and classify features that affect crop performance.

The result of the modeling is a multivariate analysis, or zone map, that has a strong relative relationship to soil moisture, organic matter, exchange capacity, and fertility attributes. The specific relationship to certain fertility attributes like potassium and manganese can vary with region and soil composition, but the overall relationship to moisture, organic matter and exchange capacity is consistently strong across all regions and soils that have been analyzed.

Zone Map Overlaid on 3D Relief Model:

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Generally, red zones are low organic matter, shed water, low exchange capacity, and low base fertility. Purple zones represent the cooler, wetter, heavier and more fertile soil zones.

In order to turn this zone map into something functional, we need to ground-truth the zones. Most often this takes the form of site-specific soil sampling using hand-held GPS devices. Soil sampling several features of the same color and submitting samples to a soil testing laboratory is a common practice. The results of the analysis are then extrapolated based off the original zone map to create individual maps for each attribute tested.

Gravimetric Soil Moisture, Base Zone Map (water by weight of field moist soil) and Other Layers with 3D Relief Overlay:

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High-density grid sampling has confirmed what these extrapolated maps imply, a very consistent relationship between the zone map and actual soil moisturego here for more details:

While mapping soil moisture is important, we often cannot manage water as effectively as we can manage fertility. Therefore, it is important to explore the ways fertility is varying with the landscape, soils and various microenvironments.

Zone Map with 3D Relief Overlay and Soil Test Results:

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Above: This example field is split between irrigated and dryland zones because often times the dryland corners will be planted into rotational crops like soybeans. Note how much potassium levels vary from the driest irrigated (yellow) zone to the wetter zones (blue, purple). Due to the low holding capacity of the driest soils, it is difficult to build potassium (and other nutrients), meanwhile the wetter zones have significantly more holding capacity and potassium starts to build in these zones; especially where water movement is impeded.  

Below: this field had some of the most extreme concentrations of potassium that this agronomist has scene. These soils were zones sampled the fall before potatoes were planted, which means that this field had not seen significant quantities of applied potash for 3 or more year before the soil samples were collected.

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Bulking Curves and Microenvironment Digs for Assessing Crop Response by Microenvironment

Each point in time represents three digs within the same row, but different microenvironments (zones); Red=dry soils, Green=field average soils, Blue=wet soils (see soil attribute tables for more details on variability by microenvironment). Each dig represents 3.33 ft for a total of 10 ft of row. Weight is expressed in grams on the y axis. %H2O represents gravimetric moisture at 12” depth, sampled from the top of the hill. TEC is total exchange capacity, a function of the soils holding capacity.

All of these fields had flat-rate fertilizers applied to them, nothing was variable rate applied before sampling in these examples.

Keep in mind that there is often a lot of variability in potato size and sets. Variables such as seed size, whether seed was cut, seed spacing, planting depth, and variability in starter fertilizer applications can have a significant impact in crop development, especially early on.

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Above: weight (g) of tubers in 3.33 ft of row by zone over time, same row per zone (color). Red zone was very slow to bulk and stayed behind wetter environments much of the season. The blue zone eventually suffered more soft rot and aerial stem rot, which contributed to lower yields there. Green zone is often the ‘goldilochs’ zone, because it is often the right balance of moisture, heat, nutrient use efficiency, and mineralization potential. While the spring started off dry, by the end of the season, moisture was much less limiting.

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Above: Zone map with 3D relief model overlay. This is a moderate variability field, but with considerable variability in soil test potassium in just a couple hundred linear feet. 

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Above: Three consecutive plants pulled from each microenvironment in spring. 25% difference in canopy (biomass) between red and blue zones. Note the signs of senescence on the lower leaves in the blue zone. Fungicide could be required in the blue zone up to a week before the red zone requires it, based on canopy coverage, in this particular growing season. Once again, early season drought stress delayed maturity in the red zone, resulting in daughter tubers that were physiologically younger than the tubers in green or blue zones; also smaller sets.

Below: Tuber shape showing some bowling-pin characteristics in the red zone, likely from mid-season drought stress. More conspicuous scab present in the red zones, slight scab pressure in the green zone. Soft rot was present in both the red and blue zones, but more consistent in the blue zones. Soft rot is often a secondary disorder and requires some initial agent to open the tuber up to infection. In the red zone, soft rot was often associated with an initial rhizoctonia infection. Rhizoctonia was present in the other zones, but at significantly lower pressure. Due to the more consistently wet soil conditions in the blue zones, soil compaction is more prevalent than in other zones, this can lead to tuber shape being contorted and less uniform.

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Conclusion

Site-specific (zone) fertility management of potatoes in central MN is justified agronomically on several levels. The first level is base soil fertility, which can vary significantly even in short distances. 

The second level is yield potential and risk mitigation. Wetter microenvironments tend to produce lower yields (under flat-rate irrigation at least), by nearly 10% on average in this analysis. Lower yield potential, agronomically, implies lower demand for added fertilizer. There is also a higher risk of flood stress and storage issues in the wetter zones. 

The third level is specific gravity, which is consistently lower in the wetter microenvironments (albeit, not by a very wide margin, but the trend is consistent nonetheless); it is well known that excessive K, N and salts can contribute to lower gravities and these tend to be higher in wet zones. Conversely, the opposite tends to be the case with the dry zones. However, in this analysis, the driest zones were only slightly higher yielding than the field average (green) zones. Worth noting, however, that the dry zones tend to produce a higher percentage of larger tubers than the field average and wet zones. Specific gravities in the dry zones tend to be highest, but not by a large margin.

Another level that may justify site-specific fertility management is marketability. Increasingly, customers are putting pressure on industry to produce food more sustainably, even to the extent that growers who adopt sustainability practices may enjoy financial incentives, such as premiums paid on their crop. Even without premiums, a sustainable producer may have a competitive edge in marketing their product compared to conventional producers. 

4R nutrient management (right place, right product, right rate, and right time) is a pillar of sustainable farming practices and is optimized with site-specific management in at least 3 of the four R’s.

Finally, there is considerable evidence that site-specific management of irrigation, seeding rates, and crop protectant products may be justified in this crop.

Author: Michael Dunn, CCA; SSp.
Contact: [email protected] On Twitter: https://twitter.com/michaeldunnCCA
Original article: Potato Response to Microenvironment & Opportunities for Precision Ag Management
Related: More articles by Michael Dunn

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