This is the sixth column in a series on phosphorus (P) management proficiency. Last week I wrote about the importance of controlling and consistently pushing the soil probe to the appropriate depth. The UNL soil test interpretation and fertilizer recommendation based on a 0-8 inch sample for P. Additionally, collecting enough subsamples or cores to make up a composite sample is very important in getting a good estimate of soil test phosphorus (P).
A composite soil sample for nutrient analysis is composed of numerous subsamples or soil cores collected with a soil probe that is then mixed, and a representative portion is poured in the soil bag to the recommended fill line. The number of recommended cores that makes up one soil sample is dependent on soil variability and the size of the area. The starting point is 15 to 20 cores per soil sample, with the sample area being 20 acres or less. Mix the 15 to 20 cores as best you can and then pour a representative portion (not all used) in the bag up to the marked fill line.
In areas of Saline, Jefferson, and Gage counties with broad and relatively flat landscapes and with previous soil sample results showing low variability in soil test phosphorus, 40 acres would still be the maximum area one soil sample should represent. On a quarter section or 160 acres, an agronomist or farmer should collect 4 to 8 soil samples at the very least, which means at least 60 to 120 pushes of the soil probe.
Deviations from these sampling recommendations include fields with a history of manure or high rates of banded P fertilizer, since that creates more variability in soil test phosphorus. In these cases, 25 to 30 cores should be collected. Strategies for taking those 25 to 30 cores for strip-tilled fields is 1 core in the band and 3 cores between the bands based on research in Illinois. If the bands move year-to-year or are unknown, then transact sampling across one row with a core every 3-inches apart, or about 10 cores has been suggested by North Dakota State University (pages 3-4). For fields sampled on 2.5-acre grids, it is still recommended to collect at least 10 cores to capture variability in soil test P.
By now, you are wondering why we have to take so many cores. Phosphorus variability is high at both the microscale (2 ft) and macroscale (10 acres). Cornell researchers in a 2010 study reduced the effects of microscale variation by using 6 cores to make a composite sample within a two foot diameter point in the field. They took 60 point samples via an 80 foot grid within 10 acres. They then treated a point sample as a proxy for one core in simulating how many cores are need when sampling a 10-acre area for soil test P. There was an 80% probability that 20 cores was a good enough estimate of the actual soil test P average.
In summary, do not take skimp on the number of subsamples or cores that make up a soil sample! Contact me with questions or suggest topics for me to write about in regards to phosphorus management at or 402-821-1722. Know your crop, know your tech, know your bottom line at croptechcafe.org.
