Agriculturally
Speaking
- By RUSSELL C.KING
County E»t*n*ion Chairman
Leaching is the
movement of nutrients
below normal rooting
depth due to excess water
moving (percolating)
through the root zone.
Although leaching
generally has not been a
problem since the 1978
season, recent heavy rains
in 1982 have caused
concern among tobacco
growers in some areas.
Leaching is most likely
to occur on deep, coarse
soils with good internal
drainage. These soil
conditions enhance both
water penetration into the
soil as well as its
movement through the
fertilized zone. On these
soils, most nutrients,
except phosphorus, may be
leached to some extent.
Nitrogen, however, is the
nutrient most likely to be
leached in amounts great
enough to reduce yield and
quality of cured leaf if
adequate amounts are not
replaced. Leaching
adjustments for nitrogen
may include some
replacement of potassium,
magnesium, and/or sulfur
if soil reserves and
previous fertilizer
applications of these
nutrients are known to be
low.
Determining the correct
amount of nitrogen to
replace for leaching losses
is one of the most difficult
and risky tasks in tobacco
production. If uncorrected,
nitrogen deficiency will
result in low yields of pale,
thin-bodied tobacco which
has poor demand on the
warehouse floor. If over
corrected, excess nitrogen
will delay maturity and
reduce curability, which
usually results in more
tobacco in the lower
quality unripe (K, S) and
immature (G,GK) grades,
as well as more sucker
growth.
Water usually can move
more freely and in larger
quantities through deeper
topsoils. Too, a portion of
the leached nitrogen will
accumulate just above or
in the upper portion of the
clay layer. Since the mass
of tobacco roots normally
occurs in the upper 12-14
inches of soil, the deeper
the clay is below the
normal rooting depth, the
smaller the amount of
leached nitrogen that will
be absorbed by the roots.
Crop age is included in
the guide because tobacco
plants absorb a greater
proportion of their needed
nutrients as they become
older. Since only those
nutrients remaining in the
soil can be leached,
excessive rainfall during
the first several weeks
after transplanting, i.e.
before substantial nutrient
absorption has occurred,
usually causes more
leaching than the same
amount of rainfall later in
the growing season. Too,
as the plants get larger,
their leaves form a canopy
which sheds some of the
water to the row middles,
thus reducing the amount
of water passing through
the fertilized zone. In
setting up the leaching
guide, the assumption was
made that the crop was
making normal growth
when leaching occurred.
However, if the crop has
grown unusually slow, the
replacement percentage
may be increased slightly
(about 5 percentage
points).
An estimate of water
that enters the soil and
ultimately percolates
through the root zone is
necessary for the
adjustment procedure to
be reasonably reliable. The
amount of rainfall alone is
usually not a good
indication of how much
leaching has occurred.
Factors such as soil
texture, soil slope, crust
formation, amount and
duration of rainfall, and
the amount of water
already present in the soil
must be considered.
Generally, maximum
water penetration occurs
when slow rains fall on flat,
dry, coarse and deep soils
which have been recently
cultivated. Minimum
water penetration occurs
when hard, fast rains fall
on sloped, finer-textured
and, shallow soils which
are crusted.
Only water that enters a
soil in excess of its water
holding capacity will cause
leaching. Most medium
textured soils typically
used to flue-cured tobacco
production will hold about
1 inch of water in the upper
12 inches. Coarse sands
may hold only 0.5 inches,
while more clayed soils
may hold 1.5-2.0 inches.
Therefore, even if the
same amount of water
were to enter these soils
under dry conditions,
percolation and thus
leaching would be greatest
in the sand because it has
the lowest water-holding
capacity.
Without question, a
reliable estimate of
percolated water is the
most difficult of the three
leaching factors to
determine. Unfortunately,
a method which
incorporates the many
factors that influence
percolation has not been
developed. However, this
can be done reasonably
well by growers who have
experienced similiar
rainfall conditions on their
land in past years. An
invaluable tool in making
leaching adjustments is an
up-to-date rainfall record
which shows both the
amounts of daily rain and
estimates of how much of
each rain entered the soil.
Leaching is usually not a
serious problem on soils
with tight clay or possibly
a hardpan within 12 inches
of the soil surface. This is
because percolation
through the root zone is
restricted. If it becomes
saturated, oxygen
starvation and then root
decay will occur unless the
saturated condition is
alleviated. Usually,
yellowing of the plants is
accompanied by partial or
complete flopping. This is
a good indication that
leaching losses are
minimal. Although some of
the nitrogen may be moved
down to the clay, causing a
temporary deficiency, it
will be absorbed later as
root growth resumes.
Adding extra nitrogen in
these situations often
results in an overfertilized
crop.
The rainfall for this week
in Warren County was 2.3
inches.
Texture
1-11
Exterior
Plywood
Siding
Deep parallel grooved
panels add dimen
sional interest to your
home's exterior wWi
I1mL| aLaiiain AAA
•igni ana vnaaow con
trast. PIUS ghts
wptfb protection I
FALKNER'S
BUILDING SUPPLY
Um Our Court 41 Garnttt St Entrance
HENDERSON. N.C.