R0A RJCUTURAL RESEARCH & EDUCATION CENTER
J''IFAS, UNIVERSITY OF FLORIDA
13138 LEWIS GALLAGHER ROAD
0 Florida DOVER, FL 33527
Dover AREC Res arch Report DOV1988-2 July 1988
SOIL TEST PROC DURES TO PREDICT STRAWBERRY FRUITING RESPONSE WHEN
ALL SOLUBLE D SLOW-RELEASE FERTILIZER IS APPLIED PRIOR TO
E. E. lbregts, C. M. Howard, and C. K. Chandler1
Sandy soils, s ch as those used for strawberry production in
Florida, are ge erally subject to nutrient leaching, which occurs
because of exc ss rainfall or irrigation. Thus the nutrient
status of a s ndy soil can be changed quite drastically by
rainfall or ir igation in a short period of time. To reduce
leaching, many rops are grown on raised beds to permit the plant
roots and the a plied fertilizer to be above the surface water.
When polyethylee mulch is added, the possibility of leaching as a
result of rai fall or irrigation is reduced. However,
considerable le ching can still occur with raised beds and
polyethylene mu ch. This occurs if the water table rises to
saturate part o all of the bed or large amounts of water enter
through the plan ing holes. In either case, as the water drains,
portions of the soluble fertilizer move with it and out of the
root zone. Le ching can be further reduced if part of the
fertilizer nitro en is derived from a slow release source (sulfur
coated urea, IBD osmocote, etc) and/or if it is positioned in
the bed to avoid most if not all of the excess water movement.
Generally, with he two row strawberry bed, the volume of the bed
least likely to each is in the bed center near the soil surface.
Soil testing as well as tissue testing have long been used to
indicate the fer ility of the soil and nutritional status of the
plant. Soil tests have been used for several decades on the
heavier soils (s ils which have some silt and clay) to indicate
the soil nutrient needs to produce high yields of a given crop.
Also soil tests re used regularly now on the sandy soils of
Florida to deter ine crop needs; however, leaching during the
cropping season f equently negates the soil test.
Tissue tests can inform growers only of the present nutritional
status of plants. A soil test that could give growers the status
Professor of Soi Scientist, Professor of Plant Pathology, and
Asnifsftant P-ofao-pr Plant RnoAor, rAepocti.vely.
of the soil fe utility as it was at sampling and as it will likely
be in the fore eeable future would be a helpful tool, especially
if the analysis can be done easily and quickly. This is assuming
no excess lea thing occurs. With this in mind, trials were
conducted in which one of the goals was to find the most reliable
procedure which also produces the most useful results. Remember
that these soil tests are only for cultural systems in which all
of the fertiliz r is applied prior to mulching and one-half of the
fertilizer mixt re is slow-release nitrogen (sulfur coated urea,
IBDU, osmocote, etc) and one-half is soluble nitrogen (NH4NO3,
urea, KNO3, (NH )2S04, etc).
MATERIALS AND METHODS
Since nitrogen 's usually the most limiting nutrient because of
leaching, it wa the only nutrient varied in the trials. Nitrogen
fertilizer deri ed from sulfur coated urea and ammonium nitrate
was applied at ates of 100 and 200 lbs nitrogen/acre the first
year with 'Tufts' and 'Dover' cultivars. The second year
'Chandler' plants received 67, 133, and 200 lbs nitrogen/acre.
Phosphorus was a plied at 50 lbs/acre as P 0 and potassium at 200
lbs/acre as K2 each season. During both seasons beds were
fumigated with m thyl bromide chloropicrin (MC-2) at 400 Ibs/acre
and mulched wi h black polyethylene. Plants were set in
mid-October eac season. Fruit were harvested twice weekly
through April, g aded, counted, and weighed.
To determine the best method of soil sampling from the plant bed,
three methods we e used. These methods included: a) 5 soil cores
equally spaced a ross the bed with one core coming from fertilizer
band, b) soil co es that included the fertilizer band and adjacent
4 inches on either side, and c) a cross-section across the entire
bed, the width f the section was about one inch wide. All
samples were tak n to a depth of 7 inches. Saturated extracts
were obtained and the electrical conductivities determined. These
values were then correlated to the fertilizer rate and the plant
fruiting response The cross-section method of sampling gave the
Samples were take from each replicate at plant establishment, at
fruit harvest, at mid-season, and near end of season in April.
Saturated extract were obtained from the soil samples, and the
electrical condu tivities were determined. The electrical
conductivities gi e a measure of the soluble salts present soil
(See Table 1). T ese are generally expressed in millimhos/cm. A
saturated extract from a sandy moist soil with a reading of one
millimhos/cm is e ual to about 1400 ppm of soluble salts in the
Regression ana yses were conducted, and those equations with a
high correlati n of soil soluble salts to yield response are
presented in able 1. These electrical conductivities are a
tentative sugge tion. The electrical conductivities in Table 1
indicate the am unt of fertility needed at an early period of the
season so that sufficient nutrients will be available at a later
period. The electrical conductivities obtained in these
experiments wer fairly consistent in their predictions. A soil
sample taken in November for April fruit yield indicates that an
electrical cond activity in November of between 4 and 5 is needed
if sufficient f utility is to be maintained to the end of season.
If a soil sample is taken in February, then for April yield, a
reading of great r than 1.6 is needed in February.
The electrical c nductivity of the soil volume in which the plant
roots are locate should remain below 2 (2800 ppm) because plants
will be adverse y affected. If all of the plant roots are
confined to this concentration or greater of soluble salts, fruit
production will uffer and foliage burn will begin to appear. It
is for this reas n that a large portion of the fertilizer should
be banded in the bed center just below the soil surface. Plant
roots should be allowed to grow to the fertilizer band and not be
planted into it. Research has shown that as the root approaches
the fertilizer b nd it adjusts to the increased salts. Portions
of the root system will grow into the fertilizer band if the salt
concentration is not too great. The plant will not be adversely
affected as long as the soil moisture does not change radically
and portions of t e root system are in a low salt volume of soil.
Using slow release nitrogen or potassium, will help to reduce the
amount of soluble salts in the bed and decrease the possibility of
Data given in Table 2 are suggested optimum electrical
conductivities or soil soluble salts needed at various times
during the growing season. The higher levels in November are
required to provi e the crop with fertilizer later in season. If
the crop is to be fertilized as the season progresses than
conductivities of ess than one would be adequate.
The determine ion of the electrical conductivities (soil soluble
salts) of a trawberry field can be easily accomplished by one
person. First, divide the field into areas of no more than 5
acres. Be s re the fertilizer rate, source, and application
method are th same within the 5 acres. In addition, the soil
type, the surf ce and the internal drainage of the soil as well as
the irrigation rate, timing and method should also be similar.
Any arpa treat d differently should be sampled separately.
The next step is to select at random 5 to 8 areas within the 5
acres to be s mpled, and obtain a soil sample in each of these
areas. When s mpling the bed, take a cross section of the bed (a
one inch wide slice across the bed) to a depth of 6-8 inches.
Place the 5 t 8 samples in one container and mix well. Remove
one quart of t is soil for analysis. To determine the electrical
conductivity, dd water to the soil while mixing it until the soil
is saturated. The soil is saturated if a trace of water, after
thorough mixin is present at the bottom of the container.
Vacuum the w ter from the soil, and read the electrical
conductivity o the water with an electrical conductivity meter.
If a portable eter is used in the field it must be calibrated
with a machine using the above procedure. If all slow-release or
all soluble fe tilizer is applied to the bed, the values in the
table may not b valid.
Under normal circumstances and with a 50-50 mixture of
slow-release (I DU, sulfur coated urea, osmocote, etc) and soluble
fertilizer nitr gen sources, (NH NO3, urea, KNO (NH ) SO4, etc),
an electrical c nductivity of a bed slice less than 0.75 indicates
the soil will s on be unable to supply the strawberry plants with
sufficient nut ients. If this occurs in the last month of
harvest, it is f little concern. However; if this occurs prior
to the last m nth of harvest, plants may require additional
fertilizer for ptimum yields.
Optimum electric l conductivities or soil soluble salt levels are
suggested for th strawberry plant bed during the fruiting season.
These suggested evels correlated well with the yield response and
are a starting oint for growers in evaluating the fertility of
plant beds during the fruiting season. Sampling of the bed must
follow the procedures described and the analysis of the soluble
salts must be p operly determined to obtain meaningful results.
In addition, the fertilizer sources must he as described.
Table 1. IRegression equations of marketable fruit yield vs electrical conductivities
(and soluble salts, ppia) of soil solution for various harvest periods and
soil sampling dates.
I"arvest s ling
period saling Cultivar Equationz mnilliros cr ? Significance
April Noveaber Der ver Y=88 + 190S 22S2 4.2 5830 0%
April February Dover Y=319 + 2343S 72032 1.6 2240 1%
Season February Tufts Y=12144 + 19928S 6143S2 1.6 2240 1%
April February Tufts Y=3030 + 8380S 24442 1.7 2380 1%
April Noverber Tufts Y=1788 + 667S 7632 5.0 7000 5%
April Noverber Clandler Y=6102 + 28032 4.0 5600 1%
.arch Novamber Chandler Y=4706 + 1460S 3.6 5040 1%
Season Noveiber Chandler Y=17123 +483S2 4.1 5740 5%
April January Chandler Y=7018 29S + 736S2 2.4 3360 1%
Z7= yield in Ib/ac, and S = electrical conductivities of saturated paste extract.
Electrical conductivities vwhici produced highest yields as determined by the equations and yet were still within
ranges represented by the data.
The optimrmi conductivities are valid only for a cultural system which applies all fertilizer prior to mulching
and a saturated paste extract is used. Also fertilizer nitrogen must be derived one-half each frao slow release
nitrogen fertilizer (sulfur coated urea, IBDU, osmocoate, etc) and one-half from soluble nitrogen sources (irO3,,
urea, S3' 4)2SO, etc).
Table 2. Sug-estec electrical conductivities (or ppn) in soil slice needed
to produ maximum strawberry yields through April.
soil soluble salts
Soil sampling d te conductivity PIPE
November 4 to 5 6500
January 2 to 3 3500
February 2 2800
Hiarch 1.5 to 2 2500
April 0.75 to 1 1200
ZTese suggested optum levels are valid only for a cultural system which
applies all fertilizr prior to mulching and a saturated paste extract is
used. Also fertiliz nitrogen must be derived one-half each from slow
release nitrogen fe ilizer (sulfur coated urea, IBDU, osmocde, etc) and
one-half soluble nit ogen sources (NHI4NO3, urea, KNO3, (NH4)2804, etc).
The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
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site maintained by the Florida
Cooperative Extension Service.
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