KICG Agronomy Services

Precision farming has allowed growers to become more efficient and be better stewards of the land. Farmers must be able to keep up with technology to remain competitive, as managing variability has become a crucial part of many farming operations’ success. The methods for determining soil and nutrient variability continue to evolve. 

Soil sampling methods have changed vastly over the last decade. During the 1990s, many growers were introduced to grid sampling. 2.5-acre grid sampling provided growers with the information needed to manage variability better than they could have done before. However, sampling by grids does not accurately represent fields with variable soils. Interpolation methods are used in mapping software to define soil test values between points. Therefore, the interpolated values predicted for unsampled areas are only valid if there is a spatial dependence between the sample sites (see Figure 1). Please note that the sample points are not placed in a way that would allow for spatial dependence. 

We overlayed the 2.5-acre grid sample points and the soil type map to show the possibility of errors. Keep in mind samples pulled using this method only take into account the 2.5-acre grid. As you can see, the 2.5-acre grid method leaves much room for errors. Therefore, newer methods have been adopted. 

A few years ago, smart sampling became popular. Smart sampling is a soil sampling method that does not rely on a fixed-size grid but instead tries to position sample points in a way that will capture the variability within the field. The number of samples taken will be the same as the 2.5-acre grids but with better placement. 

Soil survey maps are fairly accurate most of the time. However, many times, small changes within a soil type were not discovered or mapped. Also, the changes from one soil type to another do not occur on a given line; rather, there are transitional zones between different soils. Many soil survey maps have soil changes in error of 50 feet to 200 feet. 

Variable rate prescriptions for nitrogen, planting, lime, and fertilizer are created using the soil information. If the soil surveys are incorrect, the variable rate prescriptions will not accurately address the variability within the field. 

A few years ago, Veris Technologies introduced a machine that could measure soil electrical conductivity (EC) on the go. EC is a measurement that provides an indication of soil layers, soil texture, and water-holding capacity. EC readings are taken at two levels: 0”-12” (shallow reading) and 0”-36” (deep reading). The EC data will define the soil variability. Also, the EC data enables the precise creation of management zones for predetermining soil sample points and the creation of all variable rate prescriptions (please refer to Figures 3 and 4). 

In comparison, Figures 3 and 5 are not very similar. Figure 3 illustrates the soil variability much better than the soil survey map. 

The management zones created from the EC data can be used to pull soil samples strategically. Strategic sampling is the most accurate method for determining nutrient variability. Also, the predicted values between sample points will have a high degree of spatial dependence. 

The soil pH within fields is highly variable. In the past, several soil sampling methods have been used to account for the pH variability. However, the pH can vary as much within a 2.5-acre grid as it does throughout the whole field. Keep in mind that pH is the first variable that must be addressed for proper nutrient management. Nutrient availability highly depends on soil pH (Figure 6). For most row crops, the ideal pH is 6.3-7.0. If pH is too high or low, nutrient availability will be an issue, or toxic levels of some nutrients could kill the plants. 

Until recently, taking a high volume of soil samples was not economically feasible because the cost would likely be greater than the benefit. However, soil pH measurements can be taken on the go with the Mobile Sensor Platform (MSP) manufactured by Veris Technologies. The MSP is calibrated every day using a pH solution of 4 and a pH solution of 7. Also, a few samples are pulled and sent to the lab just to verify the MSP-pH. 

​The number of pH samples taken by the MSP depends on the speed and transect width. Usually, about 10 samples per acre are taken. That will allow growers to have 25 times the information about pH levels than previous methods. 

Figure 7 is the pH from the lab samples, and Figure 8 is the pH readings from the Veris MSP. As you can see, there is variability that one sample per 2.5 acres cannot indicate. 

Defining the pH variability with the Veris MSP will enable growers to apply lime much more accurately than ever before. As stated earlier, the correct pH will enable optimum uptake of all other nutrients and therefore, higher yields. 

Figure 9 is the lime recommendation based off of the 20 samples sent to the lab. Figure 10 is the lime recommendation derived from the Veris MSP. Figure 10 is a more defined and accurate prescription. The extra information will not always lead to lower amounts of lime needing to be applied, but it will help balance the pH so you can get higher yields out of your soils. Also, the information will enable you to make better management decisions on other inputs. 

Please notice the variable rate seeding prescription. This prescription was created using the Veris Electrical Conductivity.