• Matt Ruark
  • Jamie West
  • Dept. of Soil Science, Univ. of Wisconsin-Madison
Project Media

Previous research (Wang et al., 2015) on snap bean response to N provided
interesting results, but it is unclear if the results are applicable to all fields. This
previous research was conducted in Plover, WI with high yielding DelMonte varieties.
Results suggested that 100 lb-N/ac was the optimal N rate (20 lb-N/ac in starter and 80
lb-N/ac in-season) when yields are greater than 9 ton/ac. However, typical yields for
snap bean are in the 4-5 ton/ac range (personal communication with processing crop
agronomists), which may not require 100 lb-N/ac (current UW recommendations are 60
lb-N/ac for yields up to 6.5 ton/ac). In addition, the previous research also indicated that
for non-nodulating varieties (i.e. varieties that do not allow root infection of rhizobium,
and thus do not directly obtain N fixed from the atmosphere), had an N utilization
efficiency of 68% when 100 lb-N/ac was applied. For nodulating varieties (in this case
the high-yielding Del Monte varieties) additional analysis using 15N stable isotope
concentrations was necessary to determine the true removal efficiency as it is unknown
how the addition of N fertilizer will inhibit the amount of N that is fixed. Preliminary
analyses of these results indicate that the 100 lb-N/ac rate completely inhibits N fixation
in snap beans. Now, it may seem counter-intuitive, but this is actually beneficial for
water quality. It means that the applied N is replacing the N fixed by the atmosphere
and is actually well-utilized in the system. If applying N fertilizer did not completely
inhibit N fixation, then much of the N that was applied would not be used and thus
leached to groundwater. However, 100% inhibition of N fixation occurred at the 100 lb-
N/ac rate, with lower N rates inhibiting a small percentage of N fixation. Now, if more
commonly used varieties require less N inputs (in the 50 to 80 lb-N/ac range) it is
important to know what the true N use efficiency is as less N on lower yielding varieties
may be less efficient than more N on higher yielding varieties. With all of the issues
concerning nitrate concentrations in the Central Sands, we know little about the actual
fate of N (or at least the utilization of applied N) in snap bean production systems.