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Effect of N and K on greenhouse tomato Page 1
EFFECT OF NITROGEN AND POTASSIUM SUPPLY ON YIELD AND TISSUE
COMPOSITION OF GREENHOUSE TOMATO
M.P.N. Gent
Department of Forestry and Horticulture
Connecticut Agricultural Experiment Station
PO Box 1106, New Haven, CT 06504 USA
Keywords:Lycopersicon esculentum Mill, ammonium, nitrate
Abstract
Does yield of greenhouse tomato benefit from supplemental nitrogen (N) and
potassium (K) supplied in amounts greater than taken up by the plants? To answer this
question, yield and fruit and leaf tissue composition were compared for tomato plants
grown in rock-wool medium and supplied with sufficient N and K, or with N and/or K
supply increased by about 30% over the control. In 1999, supplemental N in the form of
NH4NO3 decreased yield, a trend that became more obvious as the season progressed. The
K supply had no significant effect. In 2000, supplemental N in the form of Mg(NO3)2
increased early yield and fruit size. This effect disappeared later in the season. The different
response to supplemental N in the two years may be due to the effect on vegetative tissue
of the form of nitrogen supplied. An NH4NO3 supplement increased N in leaf or petiole
tissue, more than an Mg(NO3)2 supplement. Supplemental N did not affect composition of
the fruit. Supplemental K increased N and K in leaf or petiole tissue. It did not affect K in
fruit tissue but did decrease calcium in fruit in 1999.
1. Introduction
There are regional differences in the fertilizer combinations recommended for
greenhouse tomato production. These recommendations differ in concentration of
nitrogen, particularly during the fruit production stage, and also in the ratio of nitrogen (N)
to potassium (K) (Table 1). The higher values are for fruit production, excepting the
recommendation from England is highest at flowering. It is not clear which is the optimum
recommendation for current production methods in the northeast USA. Although many
growers in this region use greenhouses to extend the production season for tomatoes, few
use hydroponics to gain more control of crop growth and yield.
Table 1. Fertilizer recommendations for greenhouse tomato.
Location, root medium N P K Reference
Michigan, sand culture 200 50 365 Wittwer and Honma, 1979
England, nutrient film 150-200 300-500 Graves, 1983
Florida, rockwool, 70-150 120-200 Hochmuth, 1990
Colorado, hydroponics 73-123 50 163 Hydrogardens, 1994
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The concentration of nutrients can be quite dilute and still support rapid growth, as
long as the ratio of the nutrients is well matched to the crop requirements. In England,
tomato plants produced fruit when fed a wide range of nutrient concentrations from 10 to
320 mg N L-1 or 20 to 375 mg K L-1, as long as nutrients were supplied in the correct
proportion (Massey and Winsor, 1980). In Hawaii, as little as 28 mg NO3-N L-1 in the
nutrient solution resulted in a concentration of NO3 in petiole sap that was sufficient for
maximum yield (Coltman, 1988). Concentrations higher than 30 mg NO3-N L-1 are almost
always used in commercial practice because excess NO3 is generally not deleterious for
tomato. When tomato was fed excess NO3, it was all taken up in a spring planting in Japan,
but in fall, a high NO3 supply increased NO3 in petiole sap to the point where it decreased
fruit set and yield (He et al., 1999).
The form and concentration of N fertilizer affects organic acids in plant tissue, and
this may result in an interaction of effects of N and K nutrition. Increasing the NO3 supply
from 4 to 200 mg N L-1 doubled the concentration of organic acids such as malate, and
increased K in leaf tissue of tomato (Kirkby and Knight, 1977). Tomato plants fed NO3
had 3-fold more organic acids in leaves compared to plants with mixed NO3 : NH4
nutrition (Martinez et al., 1994). The import of NO3 or organic acids into the fruit during
growth requires import of K or some other counter-ion to maintain ionic balance. The
supply of K may be more critical for fruit production than for vegetative growth.
Potassium supply affected calcium uptake and blossom end rot in greenhouse tomato (Bar-
Tal and Pressman, 1996). The concentration of K in field soil affected uneven ripening
(Picha and Hall, 1981; Hartz et al., 1999).
An N- or K-supplement, supplied in amounts or concentrations greater than that
taken up by plants, may not benefit yield and fruit quality of greenhouse tomato. To test
this hypothesis, I examined the response of greenhouse tomato plants grown in rock-wool
medium and supplied low or high concentrations of N and K in the nutrient solution. Yield,
and fruit and leaf tissue composition were compared for plants grown with sufficient N and
K, or with added N- and/or K-supplements to increase the concentrations by about 30%
over the control.
2. Materials and Methods
2.1 Treatments
The experiment was conducted in greenhouses at Lockwood Farm, Hamden, CT
(Lat. 42 N Long. 73 W). The greenhouses were 4.4 x 17 x 2.5 m high and covered with a
double-wall, inflated clear-polyethylene cover. In each greenhouse there were four rows of
rock-wool slabs (Talent, Agrodynamics, East Brunswick, NJ) that corresponded to four
nutrient treatments. Each row had an independent supply manifold to feed solutions that
differed in N and K to the plants during fruit production. In 1999, the N-supply was
supplemented with NH4NO3. The proportion of NO3 to NH4-N was 0.97 in low-N, and
0.80 in high-N treatments. The low-N and high-N treatments had 95 and 145 mg N L-1 in
the nutrient solution. The K supply was supplemented with K2SO4. The low-K and high-K
treatments had 140 and 190 mg K L-1 in solution. Injection of the N and K supplements
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were time-shared with injection of Mg(SO4). Each supplement reduced the fraction of time
for injection of Mg(SO4) by 0.20. All solutions also contained a complete nutrient
formulation (3-15-26 tomato and lettuce formula, Hydrogardens, Colorado Springs, CO)
and Ca(NO3)2. These nutrients were supplied at the same concentration in all treatments.
The final concentrations in dilute solution were 40 mg P and 130 mg Ca L-1. Sulfate varied
from 50 to 75 mg S L-1 due to the K2SO4 supplement, and Mg varied from 35 to 42 mg L-
1. Due to the poor response to supplemental N in 1999, the N supplement was changed to
Mg(NO3)2 in 2000. The low-N and high-N treatments had 105 and 130 mg N L-1 in
solution. More than 0.97 of the N was NO3 in both treatments. In 2000, the K supplement
was changed to K2CO3 because of its higher solubility. The low-K and high-K treatments
had 130 and 180 mg K L-1 in dilute solution. The complete nutrient formulation and
Ca(NO3)2 were supplied at the same rates in 2000 as in 1999. In 2000, the Mg varied from
35 to 54 mg L-1 due to the N supplement. In both years, all plants were watered with the
low-N low-K nutrient solution until fruit production commenced. The nutrients were
supplied each time plants were watered. The frequency of watering was adjusted according
to plant size and light integral, so nutrients were not depleted, and adequate water
remained in the root zone.
2.2 Plant material and culture
Twelve cultivars of greenhouse tomato (Lycopersicon esculentum Mill) were
grown in each year. This report is based on results from the six cultivars grown in both
years: Buffalo, Cabernet, Cobra, Dynamo, Match and Trust. Differences among cultivars
are not reported here. In 1999, seeds were germinated under controlled conditions on 18
March, transplanted to 10-cm rock-wool cubes on 19 April, and set at the final spacing on
rock-wool slabs on 20 May. Fruit production and fertilizer treatments commenced on 20
July, and the final harvest was on 27 September. Two greenhouses were used and there
was only one sub-plot of each cultivar by treatment combination. For the 2000 season,
seeds were germinated on 30 Dec 1999, transplanted to 10-cm rock-wool cubes on 4
February, and set at the final spacing on rock-wool slabs on 23 February 2000. Fruit
production and fertilizer treatments commenced on 12 May, and the final harvest was on
15 August 2000. Four greenhouses were used and there were two replicate sub-plots of
each cultivar by treatment combination. Each of the four rows in each greenhouse had 12
slabs planted with 6 sub-plots, consisting of four plants of one cultivar. Treatment and
cultivar locations were randomized within each and among greenhouses. Plants were
pruned to a single stem and supported by string. Flower trusses were pruned to 4 or 5
fruit. Fruits were harvested at 4- to 5-day intervals as they ripened. The total yield, fruit
size, and nature of defects of the fruit were recorded. Fruit and leaf tissue were sampled
for nutrients two or more times in each year. Methods for nutrient analysis were as
described previously (Gent and Ma, 2000).
2.3 Analysis
Analysis of variance was conducted separately for data in 1999, and for early and
total yield in 2000. Six cultivars grown in two years were included in analysis. Main effects
were N supply and K supply and cultivar. All interaction effects, except N x K, were
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included in the error term. Cultivar effects are not reported here. ANOVA was conducted
using the general linear model in SYSTAT (SPSS Inc., Chicago IL).
3. Results
3.1 Yield and fruit quality in 1999
Total yield per plant varied among the treatments in 1999. Supplemental N in the
form of NH4NO3 decreased yield (Table 2). This trend became more obvious as the season
progressed (Fig 1). The K supply had no significant effect. The weight per fruit was
decreased in the high-N treatment, particularly in combination with high-K. The planting
was late in 1999, the summer was hot and sunny, and there was no shade cloth on the
greenhouses. This combination resulted in poor fruit quality. The principle cause of non-
marketable fruit was cracked skin. The fraction of fruit with cracked skin was 0.41, 0.38,
0.46, and 0.40, for plants fed low-N/low-K, high-N/low-K, low-N/high-K, and high-
N/high-K solutions, respectively. Plants fed low-N or high-K had a greater fraction of fruit
with cracked skin. Green shoulder or uneven ripening was seen in 0.19 of the fruit, and
blossom end rot in 0.04 of the fruit. There was a low concentration of Ca in fruit picked in
1999 (see below), which may have been responsible for these defects. About 0.15 of the
fruit were too small to be marketable. Except for the cracked skin, the frequency of defects
did not differ among the treatments.
3.2 Yield and fruit quality in 2000
Supplemental N in the form of Mg(NO3)2 increased early yield by 30 June 2000.
Early yields were 2.3, 2.7, 2.5, and 2.8 kg/plant for plants fed low-N/low-K, high-N/low-
K, low-N/high-K, and high-N/high-K solutions, respectively. There was no effect of added
K in the form of K2CO3. Supplemental N increased fruit size. The fruit were 107, 119, 111
and 118 grams, for tomatoes picked from plants fertilized with low-N/low-K, high-N/low-
K, low-N/high-K, and high-N/high-K, respectively. Supplemental K did not affect fruit
size. Most of these treatment effects disappeared later in the season (Fig. 2). By the final
harvest on 15 August 2000, the treatments did not affect total yield or fruit size (Table 2).
The environment in 2000 differed from that in 1999 because the planting was much earlier,
and the summer was cool and wet. A 30% shade cloth was applied to the houses on 13
June 2000, and the doors were removed to increase ventilation. The fraction of fruit with
cracked skin, green shoulder, or blossom end rot was relatively low in 2000, 0.16, 0.04,
and 0.01, respectively. The principal cause of unmarketable fruit was a small size; 0.21 of
the total number of fruit had this defect. The frequency of defects did not differ among the
treatments.
3.3 Tissue composition
Supplemental N did not affect fruit composition in 1999. Supplemental K lowered
the already low level of Ca in fruit, but had no effect on K in fruit (Table 2). Supplemental
N affected most of the nutrients in leaves (Table 3). It increased N but decreased Ca and
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Mg. Supplemental K did not increase K in leaves, but it did increase K in petioles. Neither
N- or K-supplements affected fruit composition in 2000, except for an interaction of
effects of N and K on Mg. The fruit picked in 2000 had more than twice the Ca than the
fruit picked in 1999 (Table 2). The concentration of other elements did not change
between years. In 2000, both N- and K- supplements increased N in leaves (Table 3). The
increase in Mg in leaves was due to the Mg(NO3)2 used as the N-supplement. A high-K
supply increased K in leaves. Overall, the concentrations of N and Ca in leaves were higher
in 1999 than in 2000.
4. Discussion
In these experiments, providing supplemental nitrogen, in excess of that taken up
by the plants, was of temporary benefit to yield of greenhouse tomato when supplied in the
form of NO3, but it was not beneficial when supplied in the form of NH4. The rate of
production without supplemental N was similar across years, but the yield response to N-
supplements differed due to the form of N supplied. For instance, the low N-regimes
produced similar yields of about 3.0 kg/plant on 30 Aug 99 and 12 July 2000, when yields
differed by more than 0.5 kg between the two years under-high N regimes, (Figs. 1 & 2).
There was a benefit to yield due to supplemental N early in picking in 2000 that
disappeared by the end of the season, while the deleterious effect of supplemental N in
1999 accumulated throughout fruit production. In 1999, the N supplement was NH4NO3,
which increased the fraction of N as NH4 to 0.20 of the total N. In 2000, the N supplement
was supplied as Mg(NO3)2, so more than 0.97 of the N was supplied as NO3. In other
studies, increasing the fraction of NH4-N up to 0.25 of the total N increased vegetative
growth of tomato, but fruit yield and weight were lowered by any NH4 (Hartman et al.,
1986) and NH4 lowered Ca and Mg uptake and increased blossom end rot (Hohjo et al.,
1995).
Providing supplemental potassium was of marginal benefit to yield of greenhouse
tomato, only in conjunction with no N-supplement. The K-supplement did not have a
significant effect on total yield in either year, but the trend was for higher production with
high-K under low-N supply. Such a trend was seen in the middle of the picking season in
both years. In 1999, supplemental K lowered the concentration of Ca in fruit. Bar-Tal and
Pressman (1996) noted that a high concentration of K in solution lowers Ca uptake in
tomato and increases the frequency of blossom end rot. Although K-supplement increased
cracking of the skin of tomato fruit in 1999 in the present study, it had no effect on uneven
ripening or blossom end rot. Increasing the rate of K fertilization reduced blotchy ripening
of fruit picked from field-grown plants (Picha and Hall, 1981). In this case K and Mg in
fruit were little affected by the K applied. In another field study, uneven ripening was
inversely related to K application rate (Hartz et al., 1999).
The most dramatic difference in tissue composition between the two years of this
study was the amount of Ca in fruit. The concentration in 1999 was less than half that in
2000, 1.2 and 3.2 mg Ca g-1, respectively (Table 2). In vegetative tissue, the difference in
Ca was relatively subtle and in the opposite direction from the change in the fruit. A
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decrease in transpiration, due to shading and generally cooler weather in 2000 compared to
1999 may have accounted for this dramatic change in composition of the fruit.
Acknowledgements
I thank Mr. Michael Short for assistance with cultivation and harvesting tomatoes and
analysis of plant tissue.
References
Bar-Tal A. and Pressman E., 1996. Root restriction and potassium and calcium solution
concentrations affect dry matter production cation uptake and blossom end rot in
greenhouse tomato. J. Amer. Soc Hort. Sci. 121: 649-655.
Coltman R.R., 1988. Yields of greenhouse tomatoes managed to maintain specific petiole
sap nitrate levels. Hortscience 23: 148-151.
Gent M.P.N. and Ma Y.Z., 2000. Mineral nutrition of tomato under diurnal temperature
variation of root and shoot. Crop Science 40: 1629-1636.
Graves C.J., 1983. The nutrient film technique. Horticultural Reviews 5: 1-44.
Hartman P.L., Mills H.A.and Jones J.B., 1986. The influence of nitrate, ammonium and
element concentration in Floradel tomato. J. Amer. Soc. Hort. Sci. 111: 487-490.
Hartz T.K., Miyao G., Mullen R.J., Cahn M.D., Valencia J. and Brittan K.L., 1999.
Potassium requirements for maximum yield and fruit quality of processing tomato. J.
Amer. Soc. Hort. Sci. 124: 199-204.
He Y.Q., Terabashi S., Asaka T. and Namiki T., 1999. Effect of restricted supply of nitrate
on fruit growth and nutrient concentrations in the petiole sap of tomato cultured
hydroponically. J. Plant Nutr. 22: 799-811.
Hochmuth G., 1990. Design suggestions and greenhouse management for rockwool
vegetable greenhouses in Florida. SSVEC-41, U. of Florida, Gainesville, FL, 47 pp.
Hohjo M., Uwata C., Yoshikawa K. and Ito T., 1995. Effects of nitrogen form, nutrient
concentration, and calcium concentration on the growth, yield, and fruit quality in
NFT tomato plants. Acta Horticulturae 396: 145-152.
Hydrogardens. 1994. Hydrogardens Catalog 94D. Hydrogardens, Inc. Colorado Springs
CO, 96 pp.
Kirkby E.A. and Knight A.H., 1977. Influence of the level of nitrate nutrition on ion
uptake and assimilation, organic acid accumulation, and cation anion balance in whole
tomato plants. Plant Physiology 60: 349-353.
Martinez V., Nunez J.M., Ortiz A., Cerda A., 1994. Changes in amino acid and organic
acid composition in tomato and cucumber plants in relation to salinity and nitrogen
nutrition. J. Plant Nut. 17: 1359-1368.
Massey D. and Winsor G.W., 1980. Some responses of tomatoes to nitrogen in
recirculating solutions. Acta Horticulturae 98: 127-137.
Picha D.H. and Hall C.B., 1981. Influences of potassium, cultivar and season on tomato
greywall and blotchy ripening. J. Amer. Soc. Hort. Sci. 106: 704-708.
Wittwer S.H. and Honma S., 1979. Greenhouse Tomatoes, Lettuce and Cucumber.
Michigan State University Press, East Lansing, MI, 225 pp.
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Table 2. Effect of supplemental N and K on yield and composition of tomato fruit.
Year Yield Weight Conc. per g dry weight
Treatment per plant per fruit K Ca Mg
kg g mg mg mg
1999
Low-N Low-K 4.5 140 26.9 1.3 1.4
High-N Low-K 4.1 134 26.4 1.3 1.2
Low-N High-K 5.0 154 26.8 1.2 1.4
High-N High-K 4.2 122 27.4 1.0 1.3
Significance
N-supply * * ns ns ns
K-supply ns ns ns * ns
N x K ns ns ns ns *
2000
Low-N Low-K 6.1 130 26.6 3.1 1.9
High-N Low-K 6.5 134 27.2 3.2 2.1
Low-N High-K 6.3 134 26.9 3.5 2.0
High-N High-K 6.4 131 24.7 3.0 1.9
Significance
N-supply ns ns ns ns ns
K-supply ns ns ns ns ns
N x K ns ns ns ns *
Figure 1.
Cumulative yield of
tomatoes in 1999 as
affected by
supplemental N and K.
0
1
2
3
4
5
6
7
7/1 7/15 7/29 8/12 8/26 9/9 9/23
Yield per plant, kg
Low-N Low-K
High-N Low-K
Low-N High-K
High-N High-K
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Figure 2.
Cumulative yield of
tomatoes in 2000 as
affected by
supplemental N and K.
Table 3. Effect of supplemental N and K on composition of tomato leaves.
Year Conc. per g dry weight
Treatment Total N K Ca Mg
mg mg mg mg
1999
Low-N Low-K 42.2 18.8 6.9 3.2
High-N Low-K 46.9 19.3 5.9 2.9
Low-N High-K 44.6 19.4 7.2 3.4
High-N High-K 49.1 19.7 5.8 3.0
Significance
N-supply *** ns * **
K-supply ** ns ns ns
N x K ns ns ns ns
2000
Low-N Low-K 40.2 18.4 6.5 2.6
High-N Low-K 41.1 17.3 6.2 2.9
Low-N High-K 41.6 19.3 5.9 2.7
High-N High-K 42.5 18.5 5.9 3.1
Significance
N-supply * ns ns **
K-supply ** ns ns
N x K ns ns Ns ns
0
1
2
3
4
5
6
7
5/15 5/29 6/12 6/26 7/10 7/24 8/7
Yield per plant, kg
Low-N Low-K
High-N Low-K
Low-N High-K
High-N High-K
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