Soil sampling has adapted and evolved over time. As genetics, crop nutrition, equipment and precision ag technologies have matured, so has the need for more advanced soil sampling in the field. From the beginning of soil test labs in North America in the 1940s, and ’50s, agronomists have generally pulled a single soil sample for each field. As we progress into higher-yield systems, it is important to understand soil sampling options that are available today and the level of detail that higher-frequency soil sampling can provide.
Whole-field composite sampling stays true to its name. Generally, eight to 12 cores are randomly pulled from a field, mixed in a bucket, and sent to the lab as a single representative sample for that field. This option is truly an average of nutrients over an entire block of land (5 to 500 acres). The question that needs to be asked is this: Does the nutrient analysis reported by the soil test lab accurately represent each and every part of that field? The answer is “No.” Soil nutrients may vary a great deal across a field depending on soil type, rainfall, microbial activity, temperature, and fertilizer applications to name a few.
Management zone sampling identifies areas of the field that are similar and groups them for sampling accordingly. These zones may be determined by experience of the agronomist sampling the field and communication with the grower. A more analytical approach entails using digital-soil-type maps, aerial imagery, or multiple years of yield data to delineate the zones.
For an agronomist, topography and field history play a huge role in deciding how to carve these areas out. This can vary by region and soil type, but generally, three to six zones can be identified for any given field. It’s always good for an agronomist and a grower to talk and compare notes before they go out and define management zones and begin pulling samples.
Grid sampling is the most detailed form of soil sampling. This sampling procedure divides the field into equal blocks so that all areas of the field are equally represented. The most widely adopted grid size is 2.5 acres, but other sizes may be used as well. This type of sampling does a great job of building a “baseline” of soil test values.
The results of this advanced soil sampling procedure provide a detailed understanding of the variability of nutrients in a soil and across the field. Grid sampling also provides a roadmap for subsequent sampling to monitor your crop nutrient budget. It is commonly suggested to conduct grid soil sampling every four years and use less detailed sampling methods every year between these more advanced sampling years.
Different options, of course, mean different costs. But all growers should understand the level of options available and consult their agronomists for what makes sense across their operations, and their fields.
Curt Woolfolk is a Product Development Specialist for The Mosaic Company. He leads the on-farm research and development program in North America, which includes testing Mosaic premium products, as well as new product concepts in pre-commercial stages.