Soil sampling paddocks with a history of precision placement of fertilizer requires special planning. Some elements, such as soil P, remain very close to where they were placed in the soil. Random sampling could give falsely high or low test results depending on how the sampling pattern intersects with the placement pattern.
A sample taken from a band location will differ sharply from one taken only a few inches away. Soil concentrations of these nutrients will vary, with patterns matching the band spacing at which they were applied.
Soil sampling should follow the same process as normal, except for the pattern of collecting samples within sampling zones. Give careful attention to how many samples are collected from in-the-row and between-the-row. The guidelines differ depending on whether the band locations are known or not.
If the band locations are known:
USA research suggests these soil sampling ratios of in-the-band to between-the-band, depending on the row width:
• 76 cm row width 1:20 (1 sample in-the-band, 20 between-the-band)
• 61 cm row width 1:16
• 30 cm row width 1:8
Some practitioners simply avoid sampling in-the band.
If the band locations are unknown (but widths are known):
Use a paired sampling approach: one sample consisting of cores taken at random, and the second consisting of cores each taken at a distance of half the band spacing from each of the first cores, perpendicular to the direction of the bands. The greatest deviation from the ‘true’ soil test level occurs when the band location is over-sampled, so the sample with the lower soil test level is most likely to be representative. This approach was validated in a 1990 paper published in Soil Science Society of America Journal.
Article by Rob Norton, IPNI with technical input from Abigail Jenkins, NSW DPI & James Easton, CSBP.
This article first appeared in 2015.