UGA precision ag specialist Simer Virk says start with precision soil sampling and variable-rate technology.
⋅ BY AMANDA HUBER ⋅
As harvest winds down and planning for 2024 begins, it is time to consider what fields will be planted so soil samples can be taken. But is it also time to examine how that sampling is done?
At various opportunities throughout 2023, University of Georgia precision ag specialist Simer Virk has presented data from a study on grid size efficacy versus cost considerations.
“If I had to recommend one thing to a farmer who is not doing any precision agriculture, it would be to start with precision sampling and variable rate technology because that’s the first thing that will pay off on your farm,” Virk says.
Variable-rate application of lime and fertilizer is a common practice to address soil nutrient variability within the agricultural fields. When it comes to precision soil sampling to determine site-specific nutrient requirements, grid sampling remains one of the most widely used methods because of its ease of implementation and it does not require additional data layers.
Start With Precision Soil Sampling
In a grid sampling strategy, the field is divided into grids of a pre-defined size, and soil samples are collected from each grid to determine the spatial soil pH and nutrient variability within the field. But what is the most optimal grid size?
“Five acres is the most common grid size sample taken in the Southeast,” Virk says. But there’s also a lot of eight-acre and 10-acre grids being pulled, he adds.
There is a cost factor and also an accuracy of nutrient application in the optimal grid size per field. Sampling grid size can range from one to five acres, or larger, but a smaller grid size is generally recommended to accurately capture the difference in nutrient levels. However, smaller grid size also means more samples and greater sampling costs.
“When making a grid-size sampling decision,” Virk says, “Often, the farmer is looking at the cost of sending samples to a lab for analysis. He is not looking at how much of the field is being over or under applied or at that cost.”
When sampling on a grid size greater than five acres, it may help cut down on sampling costs, but it also presents an argument about the effectiveness of larger grids in accurately depicting the spatial variability within the field, he says. The question is important for growers who want to make better data-driven nutrient-management decisions while also being cost effective with their soil sampling strategy.
What Is The Optimal Grid Sampling Size?
To answer this question, Virk says they conducted soil sampling using grid sizes of 1, 2.5, 5, 7.5 and 10 acres in nine different fields ranging from 20 to 93 acres in South Georgia. The actual spatial nutrient variability within each field was also determined using high-intensity sampling at two to three samples per acre. Spatial nutrient maps and corresponding variable-rate prescription maps were created using each grid size. From that, the application accuracy, total fertilizer applied and the application costs were calculated for each strategy.
In Figure 1, a spatial map for potassium was made for soil sampling grid sizes 2.5-acre (top), 5-acre (middle) and 10-acre (bottom) grid size for one of the fields.
Virk says the difference among the maps depicting soil K variability is pretty obvious and was expected. A similar trend was noticed for soil pH and phosphorus in all fields.
The table shows application accuracy and total application cost averaged across lime, P and K for each grid size for three of the fields. Virk says the application cost includes the cost of the soil sampling plus analysis and the cost of total amount of fertilizer recommended by each strategy.
“As expected, the application accuracy is highest, greater than 85%, for the smallest grid size for all three fields. The accuracy decreases, significantly in some cases, with an increase in size of the grid,” he says.
Under- And Over-Applying Is A Cost
While the application cost also decreases with the increase in grid size, Virk says, it is worth noting that not all of that is due to the reduced sampling costs but more so from the under-application in certain parts of the field.
“In some fields, such as Field 3, the lower accuracy at the 2.5-acre grid size can also result in similar or greater application costs than smaller grid sizes due to high fertilizer over-application. Thus, the larger grid sizes do help lower the sampling/application costs but not without the expense of reduced accuracy,” he says. “Additionally, the under-applied areas within the field can further affect the crop yield potential, especially if the nutrient levels are critically low and therefore would impact overall profitability.”
Virk recommends to not pull soil samples in greater than a 2.5-acre grid because it defeats the purpose of precision soil sampling. “In the larger grids, you are doing more over and under application than trying to address the variability in the field.”
Use A Smaller Grid Size On New Fields
He says that the data emphasizes that the choice of grid size largely depends on the amount of variability and how much historical data is available for a given field.
“On newer fields or fields that have not been in production for a long time, I think you should sample on a one-acre grid initially so that you can learn more about the variability in the field,” Virk says. “The reason is you may have fields that are very highly variable, or you may have a field with little variability and future sampling could be on a five-acre grid.”
Once that baseline information on nutrient variability is established, in subsequent years, other data such as soil type, electrical conductivity or yield could be used to either sample on larger 2.5-acre grids or eventually transition to a zone-based management, which Virk says tends to provide the better combination of accuracy and cost effectiveness.
“In most cases, for soil sampling to remain valuable and provide any realistic depiction of soil nutrient levels in the field, grid size should not exceed 2.5 acres,” he says. PG