/-, AGRICULTURAL RESEARCH AND EDUCATION CENTER
IFAS, UNIVERSITY OF FLORIDA
13138 LEWIS GALLAGHER ROAD
DOVER, FL 33527
Dover AREC Research Report DOV-1991-5 August, .199
Potash Fertilization for Overhead Sprinkler L :rr
Irrigated Strawberry in the Fruiting Field
1 ji, 2 7 192
E. E. Albregts and C. M. Howard
INTRODUCTION .. Fl
Potassium (K) is an essential element for the production of strawberry
fruit. To produce about 2500 flats of fruit per acre, requires about 23
Ibs/acre of K to grow the plant and another 33 Ibs/acre to produce the
marketable and cull fruit (Albregts and Howard, 1980). Thus, a total of
about 50 to 60 Ibs/acre of K is needed to grow the crop. Fresh fruit
contain about 0.15 to 0.20 % K and are 85 to 95% water.
Potassium is supplied to the soil mainly through commercial fertilizer,
crop residues, and manures. Crop removal, leaching, and erosion are the
principal reasons for loss of K from sandy soils. For strawberry,
removal by leaching is probably the most important reason for a low K
level in Florida sands, and leaching can be rapid with excess rainfall
or irrigation (Volk, 1964). If polyethylene mulch is included in the
cultural regime as with strawberry, then some protection is provided
against nutrient leaching. In addition, the placement of the fertilizer
will affect nutrient movement in the plant bed (Albregts and Howard,
1984). Fertilizer placed in the bed center near the soil surface is
less likely to leach than when placed elsewhere in the bed. Results of
studies to determine optimum K rate have varied. Bradfield et al (1975)
in England increased fruit yields with increased rates of K from
flowering through fruiting. Yield increases in England were due to more
flowering, but in North Carolina increased rates of K gave no yield
response (Lineberry and Collins, 1942). In Florida, Locascio and Saxena
(1967) with Ona and Kanapaha fine sands obtained no positive yield
response with rates from 0 to 180 Ibs/acre K derived from either
potassium chloride (muriate of potash), potassium nitrate or potassium
sulfate. The prefertilization K soil test concentrations were between
37 and 45 ppm with an ammonium acetate pH 4.8 extractant. Only the K
level and the titratable acidity of the fruit increased with increased K
rates (Saxena and Locascio, 1968). No yield response to K with Ona and
Blanton fine sands was found in another Florida study (Dennison and
Hall, 1956). The prefertilization soil test concentrations in that
Professors of Soils and Plant Pathology, respectively, with the Univ. of
Florida, IFAS, Agricultural Research and Education Center, Dover, FL
study were 18 and 94 ppm with an ammonium acetate, pH 4.8 extractant.
The purpose of these studies was to evaluate different K rates on the
fruiting response of Florida and California developed strawberry
The experiments were conducted during the 1983-84 season with 'Dover'
and 'Tufts' cultivars and during the 1985-86 season with the 'Dover' and
'Chandler' cultivars. These experiments evaluated potassium chloride
(muriate of potash) rates of 0, 50, 100, 150, and 200 Ibs of K/acre on
strawberry fruit production and quality. The prefertilization Mehlich I
extractant K levels in the top six inches of the soil were 23 and 30 ppm
the first and second seasons, respectively. Plots were 4 ft x 12 ft
with four replications in all experiments. In addition to the potassium
applied, 2000 Ibs/acre of a 10-2-0 (NPK) with micronutrients were also
applied. One fourth of all fertilizer was incorporated into the bed and
the remainder was banded in the bed center 1.5 inches below the surface.
In September of each season, beds were fumigated with a 98% methyl
bromide and 2% chloropicrin mixture, applied at 400 Ibs/acre and mulched
with black polyethylene. Transplants were set in mid-October each
season. Plants were irrigated with overhead sprinkler irrigation as
needed. 'Dover' leaves were evaluated for K concentrations at various
times during the season.
Because of a December freeze in 1983, fruit harvest was delayed until
February 1984. All fruit were harvested twice weekly, graded, counted,
and weighed. Marketable fruit were those free of rot, not misshapen,
and weighed 10 g or more.
'Dover' responded to K fertilization both seasons (Table 1). During
both seasons, total marketable fruit yields increased with 'Dover' to
about the 150 Ibs per acre rate of K. In addition, March and April
marketable yields of 'Dover' also increased, but not always
significantly so during both seasons, with increased fertilizer K to at
least 150 Ibs K/acre. Declining yields related to a K deficiency are
most likely to occur in the latter part of the season as a result of
inadequate fertilizer applied or from leaching. In contrast, 'Tufts'
and 'Chandler' did not give significant yield responses to K rates,
except for February and March the first season and February the second
season. Even though the April and seasonal marketable fruit yields of
'Tufts' did not significantly increase, the increase in yield would have
more than paid for the additional fertilizer applied to the higher rate
treatments during the first season up to a 100 Ib/acre rate. The 50
Ib/acre rate would have done the same for 'Chandler' the second season.
However, the increased yields were not significant and may not occur
often enough in the future to be beneficial to the grower.
During the first season, the average fruit weight of 'Tufts' and 'Dover'
decreased in March and for the season as K rate increased (Table 2).
During the second season this did not occur with either cultivar. Cull
yield did not vary with K rate. Potassium deficiency symptoms were
displayed in February by 'Dover' plants both seasons but not by 'Tufts'
or 'Chandler'. Potassium fertilization of strawberry is usually not
recommended-in California because of normally adequate supplies of K in
the soil (Voth et al, 1961, Voth 1974). In these experiments the soil K
levels before fertilization were 23 and 30 ppm, respectively for the 2
seasons before and evidently insufficient for optimum yield for 'Dover'
without high rates of fertilizer K. If a grower's soil test indicates
soil K ppm in the upper 30's or higher with the Mehlich I soil
extraction test, a small trial with lower and higher K rates than
currently used would be helpful in determining their fertilizer K
Leaf K for Dover in these experiments decreased from flowering
(December) through harvest (February and March) (Table 3). The optimal
leaf K concentration has been given as 1.2 to 1.5% during fruiting. In
some previous experiments, strawberry leaf K during fruiting varied from
1.06 to 1.5% and a fruit yield response was not obtained, (Bould, 1964,
Molino et al, 1981, Locascio and Saxena, 1967, and Smith and Childers
1960). Thus, K concentrations of the most recently mature leaves above
1% during fruiting will probably not result in a reduction in fruit
yield. However, a leaf K concentration of 1% at the beginning of the
fruiting season may result in insufficient leaf K and lower yields.
Leaf K decreases quite rapidly during the fruiting season. As noted in
this experiment, K leaf levels of 'Dover' in some treatments were below
1% by February and March each season and yields were reduced. Potassium
leaf contents were not determined for the California cultivars, but
Ulrich et al (1980) indicated that leaf K concentrations at or below 1%
during flowering and fruiting would reduce fruit yields.
Plant size did not appear to be affected by K rates. However, 'Dover'
plants became lighter green in color as the K rate decreased. The
lower K rates used in this study resulted in K deficiency symptoms (see
Ulrich et al, 198U) in the last two or three months of the season.
Since most plants have reached the maximum size by February, sufficient
K may have been available to 'Dover' plants for plant growth, but not
enough for fruit production. The following suggestions may help in
minimizing fertilizer costs for all nutrients in addition to K.
1. Have soil tests of your fields conducted at least every 2 to 3
years. One soil test for a field may not be sufficient. Take soil
samples separately in each area of the field where the crops are
doing very well, average, or poorly. In addition, take separate
soil samples in.areas where elevation, soil type, or soil color
changes. Considerable research in other areas of the United States
has demonstrated that fertilizer needs vary greatly within the same
field. One fertilizer mix for a field may be an expensive way to
fertilize. Equipment is available in some areas of the U.S. to
fertilize every few feet with a different fertilizer.
2. Be sure that whoever does the soil analyses also has correlations of
fertilizer rates to crop response for your soil. Without
correlations the soil test has little value.
3. Use cultural practices such as mulch, proper fertilizer source and
placement, effective and efficient water rates, and proper timing
of water application so as to reduce leaching.
4. Capture the residual fertilizer at the end of the crop season with a
second crop or a cover crop. A cover crop reduces fertilizer needs
and adds organic matter to the soil.
5. Determine the nutrient status of the crop often enough during the
season so you can respond to the needs of the crop. This can also
give you more information about your previous soil tests and
present soil nutrient status as well as other soil problems that may
exist, such as, a soil too wet, too dry, too cold, and unexpected
6. Most California cultivars will probably respond to rates of
fertilizer K at 100 Ibs/acre or less. However, each grower should
experiment with K fertilizer rates to find the most economical K
fertilizer rates for their farm.
Albregts, E. E., and C. M. Howard. 1980. Accumulation of nutrients
by strawberry plants and fruit grown in annual hill culture. J.
Amer. Soc, Hort. Sci. 105:386-388.
Albregts, E. E., and C. M. Howard. 1984. Strawberry production in
Florida. FLa. Agr. Expt. Sta. Res. Bul. 841.
Bould, C. 1964. Leaf analysis as a guide to the nutrition of fruit
crops. V. Sand culture N, P, K, Mg experiments with strawberry
(Fragariae Spp.). J. Sci. Fd. Agric. 1964. 15:474-487.
Bradfield, E. G., D. Bonatsos, and J. F. Strickland. 1975.
Potassium nutrition of the strawberry plant. Effect of potassium
treatment and of the rooting media on components of yield and
critical leaf potassium concentrations. J. Sci. Fd. Agric.
Dennison, R. A., and C. B. Hall. 1956. Influence of NPK and lime
on the growth and yield of strawberries. Proc. Fla. Sta. Hort.
Lineberry, R. A., and E. R. Collins. 1942. Fertilizing
strawberries in North Carolina. North Carolina Agr. Expt. Sta.
Locascio, S. J., and G. K. Saxena. 1967. Effects of potassium
source and rate and nitrogen on strawberry tissue composition and
fruit yield. Proc. Fla. Sta. Hort. Soc. 80:173-176.
Molino, M. D., R. Riestra, and S. De La Peuenta. 1981. Nitrogen
and potassium nutrition of strawberry. Creciminto y production de
fruto. An. Edafologia y Agrobrologia 40:955-966.
Saxena, G. K., and S. J. Locascio. 1968. Fruit quality of fresh
strawberries as influenced by nitrogen and potassium nutrition.
Proc. Amer. Soc. Hort. Sci. 92:354-362.
Smith, C. R., and N. F. Childers. 1960. Controlled phosphorus,
potassium, and magnesium studies with strawberry. Proc. Amer.
Soc. Hort. Sci. 75:360-366.
Ulrich, A, M. A. E. Mostafa, and W. W. Allen. 1980. Strawberry
deficiency symptoms: A visual and plant analysis guide to
fertilization. University of California publication No. 4098.
Volk, G. M. 1964. A suggested approach to the use of rainfall
record as an aid in estimating leaching losses and fertilization
needs. Soil and Crop Sci. Soc. Fla. Proc. 24:254-259.
Voth, V., E. L. Proebsting, Sr., and R. S. Bringhurst. 1961.
Response of strawberries to nitrogen in southern California.
Proc. Amer. Soc. Hort. Sci. 78:270-274.
Voth, V. 1974. Materials and methods for strawberry fertilization.
Proc. California Fertilizer Conference. Volume 20d:44-49.
Table 1. Effect of K rate on 'Dover', 'Tufts', and 'Chandler'
marketable strawberry fruit yield (flats/acre) during two seasons.
K-rate Marketable fruit yield (flats/acre)
(1bs/acre) December January February March April Total
592 967 83
533 1108 113
467 1292 141
417 1633 150
733 1316 150
NS ** NS
,P = 0.01).
ZRelationship tested by regression analysis ( ,P = 0.05;
Table 2. Effect of K rate on marketable average fruit weight of
'Tufts', 'Dover', and 'Chandler' strawberry cultivars during two
K-rate 1 Average fruit weight (g/fruit)z
(Ibs/acre ) December January February March April Season
z28.35 gram (g) = one ounce.
YRelationship tested by regression analysis (*, P 0.05; **, P = 0.01).
Table 3. Linear regression of K rate on 'Dover' strawberry leaf K
during two seasons.
K-rate 1983-84 1985-86
(Ibs/acre) December February. March December February
Leaf K concn (% dry wt.)
0 1.85 0.88 0.54 1.29 0.87
50 1.84 1.01 0.65 1.63 1.10
100 1.96 1.29 0.85 1.90 1.53
150 1.93 1.46 0.96 1.93 1.68
200 2.05 1.57 1.34 1.97 1.81
2 ** *** *** *** ***
R 0.28 0.76 0.72 0.62 0.85
Regression significant at or 0.1%, respectively.
Regression significant at 1 or 0.1%, respectively.
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