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English Walnut
Production on Marginal Soils John P. Edstrom Objective: The objective of this trial is to investigate the feasibility of producing
English walnuts on substandard soils (Class II-IV) using drip irrigation, slip
plow tillage, frequent fertilization and a tight hedgerow planting system. Methods: Nursery grafted trees were planted in March of 1986 as a hedgerow, 12 feet between trees in the row and 18 feet between the rows, giving 202 trees per acre in a north-south row orientation. Prior to planting, half of the plot was slip plowed to a depth of 5-6 feet at intervals of 10 feet in a north and south direction. The soil comprises two types as classified by the U.S. Department of Interior Bureau of Reclamation: the majority of the area consists of the Kimball series (Class III-IV), a gravelly sandy loam underlain by a dense clay layer at depths of 18 to 36 inches, and about 30% of the area consists of the Arbuckle series (Class II), a deep gravelly sandy loam. Four treatments were replicated six times in two soil treatment blocks; one area was slip plowed and second area was unmodified. The treatments were Howard ctv. on Northern California Black rootstock, Howard on Paradox Hybrid rootstock, and Chandler ctv. on Northern California Black, and Chandler on Paradox Hybrid. Howard trees grafted onto Paradox were unavailable at the time of planting, so Paradox seedlings were planted for this treatment and then grafted to the Howard cultivar in April 1987. Cisco and Franquette ctvs. trees were planted as pollenizers in the perimeter row surrounding the two acre trial area . The trees are drip irrigated using evapotranspiration (ETc) calculations based on the nearby CIMIS weather station, and received monthly nitrogen applications via drip lines. Initially the trees were irrigated with a single drip line with four 4 liter per hour emitters per tree. In response to unacceptable vigor in the early years, a second drip hose was added in 1991, which doubled the number of emitters per tree and increased the wetted soil area. The drip lines were positioned 4 feet from the trunks on each side of the tree rows compared to the single hose placed directly beside the row. Potassium sulfate, K2S04 is applied beneath the emitters at 2 lbs. per tree (600 lbs. per acre.) yearly after harvest. The trees were pruned by hand for the first 3 years to develop a fruiting
wall using a modified central leader framework. Beginning in the 4th dormant
season both the east and west sides of the hedgerows were mechanically hedged at
4 feet from the trunk and also topped to remove about one half of the past
seasons top shoot growth. This regime continued yearly until March of 1993. At
that time yearly alternate side mechanical hedging began by pruning only one
side of the hedgerow. All east sides of the hedgerows were dormant pruned one
year, and, all west sides the next. Moderate mechanical topping was continued
yearly as needed to stimulate shoot growth up to the desired tree height of
18-20 feet. Since 1995 only selected plots with trees shorter than the desired
height have been topped (mainly trees on NC Black). Thus, the more vigorous
plots, which had achieved the desired height, received no further topping since
the 9th dormant season.
Results & Discussion: Rootstock Tree Size Measurements taken in September, 1999 (TABLE I) show larger trunk size for both varieties when grown on Paradox compared to NC Black. These differences are dramatic and easily seen each year during the development of the orchard. Records kept on mechanical topping also support these measurements as topping was needed only on NC Black trees after 1995 to stimulate tree growth to the desired height. Paradox rooted trees had achieved the desired height by that time. Production figures show a big yield advantage for the Paradox Hybrid rooted
trees compared to production on NC Black rootstock. This holds true for both
varieties under both soil conditions. Also notable is the consistency of this
Paradox advantage over eleven consecutive years 1989-1999 (Table 2). Variety Yields for both varieties were the same in 1999. Howards have generally
outyielded Chandlers in most years of this test despite our Chandler production
being among the highest in the state for a Chandler hedgerow. The Howard variety
is known to yield better in response to alternate year hedging while Chandler
prefers longer intervals between hedging for maximum yield. However, our hedging
practice has been to cut one side in each year compared to the more commercial
practice of cutting both sides in alternate middles. This allows more even light
penetration into every middle each season verses alternating seasons of more
intense radiation into alternate middles. Here, both adjacent sides of the
fruiting walls receive more consistent light. Whether this could help explain
the high yield of Chandler in this hedgerow location is unknown. But, Chandlers
in this case still grow for only two seasons before hedging removes fruit wood.
Slip Plow Effects The effect of slip plowing in this trial is unclear. The plowed areas are not randomized within the non-plowed plots, so only rough comparisons can be made from this side by side test. Limited soil investigations done in 1996 with backhoe pits showed deeper and more extensive rooting beneath slip plowed trees. The slip plow successfully mixed these two layers in continuous channels down the tree rows. However, trunk growth measurements and average yields show no advantages to the slip plowed treatment. It should be noted that soil conditions vary significantly within the trial area, in general this area is Class II-IV soil. About one third of the test area consists of the Arbuckle series, a gravelly, sandy loam to a depth of 3-6 feet. The Kimball series covers the remaining two thirds of the area with a similar sandy loam texture on the surface but only to a depth of 18 - 36 inches where a dense clay layer limits deeper rooting. Lack of tree response to slip plowing may be due to a combination of factors:
use of high frequency (2-3 times/week) drip irrigation, monthly fertilization
and simply the small tree size in this hedgerow planting. Also, one third of the
area was without stratification so soil mixing should not be beneficial in that
section of the test. And finally, current slip plow recommendations include a
diagonal pass of the plow in addition to the main direction. Here the machine
traveled only North-South. But, again our visual observations of the rootzones
showed increased rooting in the plowed area but still yield was not improved. Microirrigation Shoot growth and canopy development slowed to unacceptable levels in the test
area during the 4-5th growing seasons. Apparently irrigating with a single drip
hose resulted in over saturation of the limited rootzone, reduced soil oxygen
levels and limited tree vigor. (Almonds irrigated this way at Nickels have not
shown this problem, apparently, walnuts are more demanding of soil oxygen.) A
second drip line was added during the 6th growing season which doubled both the
water application rate and the wetted soil area. This allowed a switch to
alternate day or longer irrigation intervals. Shoot growth increased greatly and
returned to acceptable levels following this change. Other The row width used here has proven too close for commercial use. Under these
soil conditions, row spacing should be increased to 20 feet for Howard and 22
feet for Chandler to avoid machinery difficulties. Distance down the row is less
important in a fruiting wall, 12-15 feet would seem appropriate.
Summary: This trial continues to produce high walnut yields under marginal soils, comparable to yields obtained in hedgerows planted to prime soils. Thirteen years of field evaluation of walnuts on marginal soil has concluded that 1) yields of 3.5 tons/ac (in-shell) are attainable under these substandard soil conditions 2) Paradox hybrid rootstock outyields Northern California Black by 30%, 3) kernels of high commercial quality can be produced, and 4) slip plow soil modifications may not improve tree growth, yield or crop quality in drip irrigated walnut hedgerows.
TABLE 1
TABLE 2
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Send mail to jlschmierer@ucdavis.edu with questions or comments about this web site.
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