PCC MarchApril Final Draft

Figure 1. The GreenSeeker held above a recently headed small grain and
displaying the NDVI value. Values range from 0 to 1 (i.e. less-green to very
green plants).
Figure 3. Small grain leaf inserted in the sampling area of the
at LEAF showing a chlorophyll reading in the lower right corner
of the display while the user’s back shades the device.
Figure 2. A field in Solano County where three N-rich reference zones are visible at tillering
using NDRE captured via drone (left), but not visible to the naked eye (right).
Continued from Page 9
form of N. Using the SNQT when N
fertilizer decisions are being made will
help to narrow a range of fertilizer rates
appropriate for that field. More information
on using the SNQT in small
grains, including a sample protocol
and demonstration video, is available at
smallgrains.ucanr.edu/Nutrient_Management/snqt/.
Over the past several
years, UCCE agronomists have developed
a strong relationship between the
value measured using the SNQT and an
estimate of fertilizer N equivalence.
Crop reflectance can be measured
using a number of tools, including
handheld devices, drones and satellite
imagery. Common indices that
result from measurements of canopy
reflectance are normalized difference
vegetation index (NDVI) and normalized
difference red edge index (NDRE).
These indices represent measurements
of light reflected from the crop canopy
at key wavelengths indicative of plant
vigor. Relative differences in vigor
among plants in the same field can be
captured by comparing canopy reflectance
measurements like NDVI and
NDRE. We have been using handheld
devices, drones and satellite imagery
at our demonstration sites to compare
crop reflectance values in the N-rich
zones and the broader field.
One of the tools we are using is the
GreenSeeker by Trimble Agriculture.
This is a hand-held NDVI meter (See
Figure 1) that emits light and detects
how much is reflected from the crop
canopy in the red and infrared wavelengths.
The GreenSeeker’s canopy
measurement indicates how well the
plants are growing and covering the
soil with greenness. This information
about vigor is important early in the
crop’s growth because it indicates the
ability of plants to support grain production
and yield potential.
We are obtaining similar information
as from the GreenSeeker by measuring
NDRE with a five-band multispectral
camera (MicaSense RedEdge-MX)
mounted on a drone (DJI Matrice
M200 V2). NDRE is similar to NDVI
but replaces the reflectance from the
red wavelength with reflectance from
the red edge wavelength. Because the
drone is able to capture data from hundreds
of feet above the ground, it allows
us to measure a large area quickly and
under conditions when entering the
field is not possible. Figure 2 depicts
side-by-side images from a field in
Solano County where N-rich reference
zones were implemented during the
2019-20 season.
Another device we are using to monitor
plant N is the atLEAF CHL by FT
Green LLC, which is a chlorophyll
meter that measures light absorbed
by a single leaf (Figure 3). Like
the GreenSeeker, it also emits and
detects light. The atLEAF CHL, however,
measures how much light passes
through a single leaf instead of measuring
reflected light. This information
becomes increasingly valuable as an
indicator of whether or not the crop
has sufficient N as it begins heading out
and filling grain.
Step-by-step instructions for using
both the GreenSeeker and atLEAF
CHL in small grains are available at
ucanr.edu/blogs/blogcore/postdetail.
cfm?postnum=42903.
Since plant N is strongly related to
plant greenness and chlorophyll
content, measurements of NDVI,
10 Progressive Crop Consultant March / April 2021