Raptor Maps fertilizer trial results for table-stock potato variety in Maine. The median potato weight was 27% higher for 1,100 lbs/acre application.

Variable rate application of fertilizer is increasing in popularity. And it makes sense why. Several variables can change across a field, including soil nutrition and type, yield history, elevation, and seed. And fertilizer isn’t cheap.

Raptor Maps worked with a grower to analyze the effect of fertilizer rate on a table-stock potato variety. One goal was to quantify how pre-plant under-fertilization affects potato size to help him find the “floor,” or minimum fertilizer application rate for his field. Raptor Maps worked with the grower to apply fertilizer in a pattern that would expose a very small area (under 3 acres total) of the field to the lower fertilizer amount. We deployed the Harvest Monitor and analyzed with Raptor Harvest software.

This chart shows a striking difference between the 700 lb/acre and 1,100 lb/acre applications (this is the quantity that was actually sprayed on the field, including the 10x dilution factor). The median (light green bar) potato weight was 2.93 oz for the lower fertilizer amount, and 3.73 oz for the higher fertilizer amount, which is a 27% difference. The p-value (see description here) was less than 0.05, indicating that the result is significant. In this case, 700 lb/acre is clearly below the “floor” of what this grower should be applying.

In this case, we clearly undershot. But maybe 1,100 or 1,000 lbs/acre is the sweet spot. Without rigorous data science, growers understandably err on the side of caution and over-fertilize. Now, tools such as Raptor Harvest allow growers to be their own Extension Service and run comparison tests on their own farms.