Application
frequency ranged from one to four applications in 2013, but the
number of applications was reduced in 2014 based on ambient
weed populations and 2013 results. A hand-weeded weed-free
control and a weedy control plot were included in each replicate
block. This design resulted in 72 plots in 2013 (4 grit types 4
application frequencies þ 2 controls in 4 blocks) and 44 plots in
2014 (3 grit types 2 possible application frequencies þ 2 controls
in 4 blocks [the experiment was originally designed for 3 application
frequencies, but crop canopy cover and low ambient weed
seedbank abundance eliminated the need for a third application;
thus, 3 plots were pooled with 2 plots resulting in twice as many
2 experimental units compared with the 1 and control treatments]).
Each plot was 3.72 m2 (1.22 m wide 3.05 m long) and
included five tomato plants spaced 0.61 m apart or nine pepper
plants spaced 0.33 m apart. Experimental plot size was limited in
part by the amount of abrasive grit available because greensand
fertilizer and soybean meal had to be hand-sieved to remove particles
greater than 0.85 mm.
2.2. Site and crop management
The experimental sites were roto-tilled in early spring, and
raised-beds were shaped (approximately 0.61 m bed tops) and
spaced 1.83 m apart measuring from the row centers. Drip tape
irrigation line with 15 cm emitter spacing was laid down the
middle of each raised-bed and beds were covered with 0.025 mm
black polyethylene mulch film. Eight-week old greenhouse grown
crop seedlings were transplanted into 10 by 15 cm planting holes in
the plastic mulch on 6 June 2013 and 19 May 2014. Tomatoes were
trellised vertically using the basket-weave method on 24 June 2013
and peppers received no physical support.Weeds between rows of
plastic mulch were controlled with mowing and hand-weeding.
Crops were drip-irrigated regularly to maintain soil moisture in
the top 7.5 cm of the profile above 20% volumetric soil moisture.
Crop nutrition was supplemented during the growing season with
water soluble fish emulsion delivered via drip irrigation lines.
Plants were sprayed with a copper sulfate solution to manage
fungal pathogens. All cultural practices followed USDA National
Organic Program guidelines.