MONDAY, JUNE 6, 2022. BY MATT COMREY, TECHNICAL NUTRITION AGRONOMIST, WILBUR-ELLIS AGRIBUSINESS.
This year will likely prove as challenging a year as any in the recent past due to supply chain disruptions, commodity pricing, and drought here in the Central Valley. Growers who can weather this year’s uncertainty will be poised for success in the future due to an increased focus on efficiency and proper input management. From proper sprayer calibration to increased awareness of nutrient use efficiency, the incentive for improving input efficiency is particularly strong as the supply of inputs remains questionable at best.
Product placement when completing a fertilizer application is vital, particularly when deploying dry fertilizers in a band and/or broadcast. The low solubility of many of the common dry fertilizers used up and down the state means the nutrient release is slow and nutrient movement through the soil complex is low. This is particularly problematic for potassium– and phosphate-based fertilizers whose primary method of uptake is via diffusion. The 4 Rs of Nutrient Stewardship are helpful in guiding growers through the considerations associated with properly applying nutrients: Right Source, Right Place, Right Form, and Right Rate. Turbulent times and uncertainty bring opportunities for growers to leverage new technologies in their ongoing pursuit to maximize efficiency.
Using GPS and soil mapping technology isn’t a new strategy for soil nutrient management but has gotten more and more attention in the past few years due to rapidly increasing fertilizer costs. Soil mapping coupled with a variable rate (VR) fertilizer application aims to maximize one of the 4 Rs: Right Rate by directly matching the fertilizer application rate with measured soil nutrient levels. This technology can effectively break a field or block into smaller management zones so different areas of the field can be treated differently. While this article highlights the use of GPS soil mapping as a tool to direct fertilizer applications, there are other uses, such as identifying soil variability, interpreting yield variability, and rootstock/variety selection.
GPS Guided Soil Mapping: How Does it Work?
The process of generating a soil map consists of pulling a specialized piece of equipment across the target field or orchard and generating specific measurements that are later used to direct composite soil sampling. Figures 1 and 2, show two different pieces of equipment commonly used to generate soil maps. Soil mapping equipment varies in cost and can be quite sensitive to soil conditions, including but not limited to soil moisture, soil texture, and/or salinity. Some sensors can be very sensitive to metal located throughout the field, making equipment selection very important in crops grown in a trellis system like grapes. Once the field is mapped, the raw measurements must then be processed into ‘management zones’ for further evaluation.
Raw or initial readings/measurements generated from the equipment are typically expressed as color-coated individual measurements compressed together (Figure 3). The result is a soil map that can seem pixelated and difficult to interpret and/or evaluate. Computer software is used to generate “zones” based on parameters such as measurement ranges or soil texture (Figure 4). Once the zones are created, composite soil samples can be taken to understand mineral composition throughout the management zone as well as the entire field. Once soil sample reports are back from the lab, results are uploaded into the computer software. When results are uploaded, nutrient maps that effectively display nutrient levels across the field or block can then be generated (Figure 5).
Fully processed nutrient maps can be useful when determining whether different areas of a given orchard or field should be managed differently from the rest of the field. The example image in Figure 5, illustrates potassium levels (ppm) across a fully established walnut orchard. We see that areas in the “green” zone of the field consist of approximately 306 ppm while other areas of the nutrient map reflect much lower values, Now that potassium levels are mapped across the entire field, the grower can make some actionable observations and decisions concerning fertilizer applications. Acting on the Information After nutrient needs are identified and well understood across the field, a grower can now move to determine the appropriate product needed to treat the field. To take advantage of GPS variable rate technology, a zone recommendation that includes both product and application rates must be generated.
The zone recommendation will simply outline how much of the product should be applied to the different management zones within the field while calculating the total product needed per zone as well as for the entire field. Growers should discuss products and application rates with their local PCA or CCA. In order to fully leverage VR technology to optimize fertilizer application rates, a VR controller file is needed. The VR controller file is generated from the zone recommendation and is formatted for upload into a VR controller (Figure 6) mounted to a tractor that operates the application equipment, whether it’s a spreader or a sprayer (Figure 7).
Now, the VR applicator will automatically adjust the fertilizer’s application rate based on the tractor’s location within the field. With this technology, we can now better match our fertilizer inputs to nutrient needs throughout the field or orchard with little alteration made to the fertilizer application itself. Over the past decade, I have worked with and helped many growers explore this idea of VR fertilizer applications and feedback is overwhelmingly positive. Most growers that I’ve worked with were initially apprehensive but were soon able to see the value in better reconciling fertilizer inputs with crop needs. The process of creating the controller file seems daunting, but with the help of a skilled technician, can be done in a few days’ time and requires very little involvement from the grower. While the true value of VR technology is the ability to properly place fertilizer products where they are needed, many times growers learn that by utilizing soil mapping and VR technology, they can see significant cost savings.
I’m reminded of when I worked with a particular grower that was seeing significant yield variability across a particular field of almonds. He attributed this yield variability due to both the variable soil texture as well as topography. He told me that he took an annual soil sample from the same general area of the field that he felt accurately reflected the average yield of the block. After I had a chance to visit this field, the soil texture and topographic differences were difficult to ignore. There were several distinct soil types within this field and, while it wasn’t large, it contained distinct flat areas as well as rolling hills. I asked this grower for his most recent soil sample from that block which reflected very poor potassium levels. After explaining the various ways I believed soil mapping coupled with a VR application could benefit his operation, he begrudgingly gave me permission to map this field using EC measurements, pull GPS-guided soil samples, and generate a fertilizer
Once I completed the entire soil mapping process and a fertilizer recommendation was generated, I met back up with the grower to review the results. We found that potassium levels in the area he was sampling were quite low and did not reflect the average of the rest of the field at all. The grower used this same sample to generate an application rate of 700 lbs. of sulfate of potash (SOP) banded on the orchard floor across the entire field. The fertilizer recommendation I was able to generate and share with this grower suggested that many areas of the field need much more than 700 lbs. and many areas of the field need little to no SOP applied. We found that the average application rate across the entire field was significantly less than previously calculated, resulting in a large reduction in product needed for the field. While these cost savings from implementing soil mapping are atypical, the real value is from improving the efficiency of our fertilizers by properly reconciling crop nutrient demand with our fertilizer applications.
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