Agricultural and Food Science in Finland
477
A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D
Vol. 7 (1998): 477–489.
Effects of fertilisation and irrigation practices on
yield, maturity and storability of onions
Terhi Suojala
Agricultural Research Centre of Finland, Plant Production Research, Horticulture, Toivonlinnantie 518, FIN-21500
Piikkiö, Finland, e-mail: terhi.suojala@mtt.fi
Tapio Salo
Agricultural Research Centre of Finland, Plant Production Research, Crops and Soil, FIN-31600 Jokioinen, Finland
Raili Pessala
Agricultural Research Centre of Finland, Plant Production Research, Horticulture, Toivonlinnantie 518, FIN-21500
Piikkiö, Finland
The study aimed to establish whether a high onion yield and good storage performance could be
obtained with low fertilisation rates if irrigation was applied when necessary. Two-year experiments
investigated the effects of three NPK fertiliser levels (N 50, 100, 125/150 kg/ha), with and without
irrigation, on yield, advancement of maturity, storage losses and shelf life. High fertilisation ad-
vanced maturity but irrigation had no effect. High fertilisation increased yield only in 1996 (5–7%),
but irrigation increased the yield noticeably: by 33.5% in 1995 and 8.5% in 1996. There was no
interaction between fertilisation and irrigation. The low fertilisation optimum is attributed to the
mineralisation of soil nitrogen, as the soil was rich in organic matter. At the low fertilisation level,
plants took up twice as much nitrogen as present in the fertiliser, and with increased fertilisation the
nitrogen uptake increased markedly. The foliage nitrogen content was low, evidently as a result of
late harvesting. Treatments had only a minor effect on the storage performance and shelf life of
onions. The results suggest that fertilisation rates could be reduced in onion production. Irrigation
during warm and dry periods is essential to achieve the maximum yield potential and does not impair
the storage quality of onions.
Key words: Allium cepa, nitrogen, onions, quality, storage
Introduction
At a yield level of 30 t/ha, the recommended ni-
trogen (N) fertilisation for onion in Finland is
© Agricultural and Food Science in Finland
Manuscript received April 1998
A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D
Vol. 7 (1998): 477–489.
100–120 kg/ha (Soil Testing Laboratory of Fin-
land 1997). Phosphorus (P) and potassium (K)
demands are estimated on the basis of soil anal-
ysis. The development of integrated production
methods aims at reducing fertiliser inputs. In the
mailto:terhi.suojala@mtt.fi
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A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D
Suojala, T. et al. Effects of fertilisation and irrigation on onions
experiments of Salo (1998), for example, N up-
take of 120 kg/ha was sufficient for a yield ex-
ceeding 40 t/ha. Aura (1985) found no yield in-
crease in his 4-year experiments when N rates
were raised from 50 kg/ha to 100 or 150 kg/ha.
It is usually suggested that when irrigation
is applied to a field, N rates should be increased
to obtain the maximum gain from the irrigation
(Brewster 1990a). Irrigation can, however, im-
prove the efficiency of fertilisers (Kaila and Elo-
nen 1970, Dragland 1975, Riekels 1977, Sø-
rensen 1996). It is hypothesised that, in soils of
normal fertility, irrigation could ensure high
yields even at a low fertilisation level. The ef-
fects of fertilisation and irrigation practices on
storage performance of onion have often been
contradictory.
Our objective was to find out whether the
fertilisation level could be reduced by optimis-
ing irrigation without a negative impact on the
yield. Also of interest were the effects of fertili-
sation and irrigation practices on the advance-
ment of maturity, storage performance and shelf
life.
Material and methods
Experimental site
Field experiments were performed at the Agri-
cultural Research Centre of Finland, Horticul-
ture at Piikkiö (60°23’N, 22°30’E) in 1995 and
1996. The soil was fine sand, rich in organic
matter (3 – 6% in 1995, 6 – 12% in 1996). Data
on soil P, K, Ca, Mg and Mn extractable in acid
ammonium acetate (pH 4.65) (Vuorinen and
Mäkitie 1955), pH and electrical conductivity in
water suspension, and B by the azomethine-H
method (Sippola and Erviö 1977) are presented
in Table 1. Meteorological data were obtained
from the meteorological station of the unit (Ta-
ble 2). Potential evaporation was measured at
Mietoinen, 45 km from Piikkiö.
Experimental design
The experimental set-up was a split-plot design
Table 1. Chemical properties of soil (0–30 cm).
Year pH Conductivity P K Ca Mg B Mn
10 x mS/cm mg/l mg/l mg/l mg/l mg/l mg/l
1995 6.8 0.9 25 120 1300 100 0.5 4.3
1996 7.1 1.3 44 240 2000 260 0.7 13
Table 2. Meteorological data for 1995 –96 and the average for the period 1961–90 at Piikkiö and Mietoinen
(potential evaporation).
Month Mean temperature (°C) Precipitation (mm/month) Potential evaporation (mm/month)
1995 1996 1961–90 1995 1996 19612–90 1995 1996 1961–90
May 8.6 8.5 9.5 82 64 33 88 97 112
June 17.0 12.8 14.7 58 55 38 111 112 138
July 16.0 14.5 16.4 32 138 77 124 92 125
August 15.7 17.5 15.1 105 32 82 113 96 92
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Table 3. Experimental factors.
Plot 1995 1996
Main plot = irrigation (mm)
N = non-irrigated 0 0
I = irrigated 3 x 15 (13, 19, 26 July) 15 (6 – 7 August)
Subplot = fertilisation
(N-P-K, kg/ha) at planting 30 June sum at planting 18 June 15 July sum
1 50-35-70 0-0-0 50-35-70 50-35-70 0-0-0 0-0-0 50-35-70
2 75-53-105 25-0-25 100-53-130 75-53-105 25-0-25 0-0-0 100-53-130
3 100-70-140 25-0-25 125-70-165 100-70-140 25-0-25 25-0-25 150-70-190
Figure 1. Precipitation, irrigation and plant-available water content in irrigated and non-irrigated plots A. in 1995 and B. in
1996.
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Suojala, T. et al. Effects of fertilisation and irrigation on onions
with irrigation as the main plot factor and ferti-
lisation level as the subplot factor. The treatments
were replicated in four randomised complete
blocks. Details of the experimental factors are
given in Table 3.
Soil water content was monitored by gypsum
blocks at a depth of 15 cm, and irrigation was
applied to irrigated plots when the plant-availa-
ble water content fell below 50% (Fig. 1). Be-
cause it was rainy at the beginning of both grow-
ing seasons and soil water content was high, ir-
rigation was applied only in July in 1995 (45
mm) and in August in 1996 (15 mm). Irrigation
was terminated when the plants started to ma-
ture as indicated by softened pseudostem.
Fertilisation treatments included three levels
of macronutrients. The highest level correspond-
ed to the typical rate of N used by Finnish farm-
ers, which is adjusted according to the season.
Lower levels were set to 50 and 100 kg/ha N. At
the lowest fertiliser level, P and K rates were in
accordance with current recommendations (Soil
Testing Laboratory of Finland 1997) and growth
should not have been limited by either nutrient.
At the lowest level, all fertilisers were applied
before planting and incorporated in the soil; at
the two higher levels, some of the N and K were
given in one or two top dressings.
Crop management
Onion plants of cultivar Sturon were cultivated
from sets transplanted by hand on 15 May 1995
and 20 May 1996. The planting density was 7.5
x 25 cm. The crop was cultivated in five-row
Table 4. Development of maturity of onions.
Treatment Onions with softened pseudostem (%)
1995 1996
25 July 28 July 2 August 8 August 15 August 19 August 22 August 29 August
N1 13 34 79 97 6 35 63 96
N2 33 55 88 100 15 33 69 96
N3 60 79 94 100 20 63 89 99
I1 20 29 66 98 10 34 57 94
I2 16 26 64 100 9 29 62 96
I3 24 52 86 98 10 36 76 98
Mean temperature (°C)
between observation dates
16.9 20.4 16.8 18.8 19.9 17.7
Temperature sum
(°C, base temp. 5°C) from planting
705 740 817 888 737 792 837 926
Probability
Irrigation (I) ns ns
Fertilisation (F) 0.017 0.006
I * F ns ns
Time (T) <0.001 <0.001
I * T ns ns
F * T 0.067 0.012
I * F * T ns ns
N = non-irrigated, I = irrigated
1, 2, 3 = fertilisation levels: N 50, 100, 125/150 kg/ha
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beds in 1995 (444 400 plants/ha) and in four-
row beds in 1996 (426 700 plants/ha). The dis-
tance between beds was 50 cm. Before plant-
ing, the sets were thermally treated at 40°C for
24 h to control downy mildew (Peronospora
destructor) and soaked in isofenphos (Oftanol,
0.1%, Bayer) and thiophanate methyl (Topsin
M, 0.2%, Kemira Agro Inc.) solution to control
pests and diseases. Other plant protection and
cultivation measures were in accordance with
common practice.
Crops were harvested after full maturity (see
Table 4) on 17 and 22 August 1995 and on 4
September 1996. There were two harvests in
1995 due to the uneven maturity in irrigated and
non-irrigated plots. Since there was no differ-
ence in yield between the two harvest dates, the
average yield of the two dates was used in the
data analysis. The yield was measured in the
three inner rows in 1995 and in all four rows in
1996. The harvested area was 4.72 m2 in 1995
and 6.25 m2 in 1996.
After harvest, the bulbs were dried, leaves
attached, in a heated and ventilated greenhouse
at 20–30°C. In 1995, the leaves were removed
after drying, but in 1996, they were not removed
until the storage losses of bulbs were analysed.
Observations and measurements
The development of maturity was monitored by
calculating the proportion of onions with sof-
tened pseudostem. In 1995, the yield was meas-
ured as dried onions after removal of the leaves.
In 1996, the yield included the weight of the dry
leaves, which was 3–4% of the total weight.
In 1995, samples for dry matter and N analy-
ses were taken after artificial drying in the green-
house. In 1996, the samples for N analysis were
taken in January from cold-stored bulbs. The
bulb dry matter content was measured after ad-
ditional drying at 60°C to constant weight. N in
bulbs and foliage was measured by the macro-
Kjeldahl method, which uses copper as a cata-
lyst and potassium sulphate to raise the diges-
tion temperature. After digestion, Kjeldahl-N
was measured with a Kjeltec Auto 1030 Analyz-
er using alkaline distillation of NH
3
. NH
4
was
determined by acidimetric titration (Tecator
1981).
Storage
The bulbs were stored at 0–2°C. The relative
humidity of the store could not be controlled.
Storage losses were analysed three times (on 3–
4 January, 13–14 March and 8–9 May in 1995
and on 14 January, 20 March and 21 May in
1996). Weight loss during storage was measured,
and the bulbs were graded as saleable, rooted,
sprouted or diseased. To evaluate their shelf life,
the saleable onions were stored for a further 4
weeks at 17°C. After the test, the bulbs were once
more graded as they had been after the cold stor-
age. Shelf life was not evaluated in May 1995 as
most of the onions had formed roots during cold
storage.
Statistical analysis
Experimental data were analysed with a mixed
model for split-plot design using the SAS
MIXED procedure (Littell et al. 1996). The mod-
el included fertilisation and irrigation and their
interaction as fixed factors and block, main plot
and subplot errors as random factors. The effects
with F-test probability values above 0.05 were
considered non-significant, and probability val-
ues above 0.10 are not presented. Means of fer-
tilisation treatments were compared by contrasts.
In the data of 1995, four plots that suffered from
standing water after high rainfall were treated
as missing observations. Maturity and storage
data were analysed by repeated measures analy-
sis of variance, with time of analysis or storage
time as the repeated factor. Data on diseased
onions were not analysed statistically if only a
few infected bulbs were infected. The aptness
of the models was checked by residual analyses
which did not indicate any cross departures from
the assumptions of the models.
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Suojala, T. et al. Effects of fertilisation and irrigation on onions
Results
Maturity
Increased fertilisation hastened maturation, and
the proportion of plants with softened pseu-
dostem was greatest in the plots with the high-
est fertilisation (Table 4). Comparison of treat-
ments by contrasts revealed no differences in
maturity between the two lower fertilisation lev-
els. Irrigation seemed to delay maturation in
1995, but the effect was not statistically signifi-
cant. In both years, the air temperature during
maturation was higher than the long-time aver-
age and foliar fall-over proceeded quickly (Ta-
ble 4).
Yield and bulb mean weight
In 1995, the level of fertilisation had no statisti-
cally significant effect on total yield (P=0.644)
or bulb mean weight (P=0.732) (Fig. 2). In 1996,
higher fertilisation resulted in a slightly higher
total yield per area (P=0.051) and a significant-
ly higher bulb mean weight (P=0.026). Accord-
ing to contrasts, the differences were due to the
lower yield and bulb mean weight of the lowest
fertilisation. However, the difference between
the results at highest and lowest fertilisation lev-
els was only 3.3 t/ha (7.2%) in total yield and 13
g (10.0%) in bulb mean weight.
Irrigation increased the yield in both years,
even in 1996 when the field was irrigated only
once with 15 mm of water (Fig. 2). The average
increase in yield due to irrigation was 13.3 t/ha
i n 1 9 9 5 ( P = 0 . 0 2 6 ) a n d 3 . 9 t / h a i n 1 9 9 6
(P=0.032). The increase in bulb mean weight was
26 g (P=0.023) in 1995 and 11 g (P=0.047) in
1996. There were no interactions between irri-
gation and fertilisation in either year. Treatments
had no effect on plant density: hence, the mor-
tality was not affected by fertilisation or irriga-
tion (data not shown).
Figure 2. Total yield and bulb mean weight of onions in 1995 and 1996 at different fertilisation levels (1, 2, 3 = N 50, 100,
125/150 kg/ha). SEM = standard error of the mean.
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Plant dry matter and N uptake
Bulb dry matter content ranged between 14.6%
and 15.3% in 1995 and between 14.3% and 15.6%
in 1996 and was not affected by the treatments.
Neither fertilisation nor irrigation had a clear
effect on the N concentration of the foliage (Ta-
ble 5). Irrigation and fertilisation had a more
marked effect on the N content of the bulb than
on that of the foliage. In both years, irrigated
plants had a lower N content than non-irrigated
plants. Increasing fertilisation increased the con-
centration.
The total N uptake increased with fertilisa-
tion and was not affected by irrigation (Table 6).
Most of the N was localised in bulbs, the amount
of N in foliage being only 3–5 kg/ha in 1995
and 7–9 kg/ha in 1996. In 1995, the total N up-
take was, on average, 111, 140 and 165 kg/ha in
fertilisations 1, 2 and 3, respectively; in 1996,
the corresponding values were 93, 127 and 153
kg/ha. In 1996, there was a significant interac-
tion between fertilisation and irrigation in bulb
and total N uptake, the increase in N uptake be-
tween the two higher fertilisations being less for
the irrigated than for the non-irrigated onions.
Storage losses
Fertilisation and irrigation practices had only
minor effects on storage losses, and in 1995–96,
did not influence the proportions of rooted, dis-
eased or saleable onions. Weight loss was slight-
Table 5. Nitrogen content of foliage and bulb.
Treatment Nitrogen content (% of dry matter)
1995 1996
Foliage Bulb Foliage Bulb
N1 0.76 1.87 0.69 1.21
N2 0.81 2.41 0.73 1.40
N3 0.81 2.30 0.76 1.87
I1 0.86 1.57 0.75 0.93
I2 0.78 1.80 0.71 1.39
I3 0.81 2.27 0.73 1.61
Probability
Irrigation (I) ns 0.037 ns 0.028
Fertilisation (F) ns 0.003 ns <0.001
I * F 0.015 0.064 ns 0.068
N = non-irrigated, I = irrigated
1, 2, 3 = fertilisation levels: N 50, 100, 125/150 kg/ha
Table 6. Nitrogen uptake of plants.
Treatment Nitrogen uptake (kg N/ha)
1995 1996
Foliage Bulb Total Foliage Bulb Total
N1 4 100 104 7 92 99
N2 3 135 138 8 114 122
N3 5 147 152 9 151 161
I1 5 113 118 7 80 87
I2 4 138 141 9 124 132
I3 4 174 178 8 138 145
Probability
Irrigation (I) ns ns ns ns ns ns
Fertilisation (F) ns 0.007 0.009 0.022 <0.001 <0.001
I * F ns ns ns 0.100 0.022 0.014
N = non-irrigated, I = irrigated
1, 2, 3 = fertilisation levels: N 50, 100, 125/150 kg/ha
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Suojala, T. et al. Effects of fertilisation and irrigation on onions
ly less for irrigated than for non-irrigated on-
ions (P=0.087). Losses increased towards the end
of storage, rising from 2.1–2.6% in January to
3.1–3.8% in March and 4.3–5.3% in May. The
proportions of rooted onions were 0%, 18–33%
and 36–49% in January, March and May, respec-
tively; the rates for diseased onions were 0–1%,
2–5% and 14–20% in the same months.
In 1996–97, increased fertilisation decreased
weight loss during storage (Table 7). The occur-
rence of storage diseases was only 0–1% in dif-
ferent treatments and was not analysed statisti-
cally. The major cause of losses was the forma-
tion of new roots, which was affected by fertili-
sation treatment. The effect of fertilisation on
rooting differed with the irrigation treatments,
but the rates for rooted onions were generally
lowest at the lowest level of fertilisation. The
proportion of saleable onions was inversely re-
lated to the proportion of rooted bulbs: in Janu-
ary, 99–100% of the bulbs, but in May, less than
half of the bulbs, were saleable.
Shelf life
In 1995–96, there were no differences in shelf
life results between treatments. Weight loss dur-
ing four weeks’ storage at 17°C was lower in
January (2.7–3.5%) than in March (3.5–4.1%)
(P
time
=0.008). In January, none of the onions
formed new roots, but in March, 5–12% of them
rooted. The proportion of diseased onions was
0–2% in January and 1–6% in March (P
time
=
0.085). Onions did not sprout during shelf-life
tests.
In 1996–97, experimental factors had no di-
rect effect on weight loss or the proportions of
saleable, rooted, diseased or sprouted onions.
Only a few bulbs were infected, and rooted on-
ions were found only in the test that was started
in March (14–46%). Greater weight loss was
observed when the shelf-life test was started af-
ter a longer time in storage (P=0.003), but the
effect of storage time was dependent on fertili-
sation (P
fert*time
=0.005). At the lowest fertilisa-
Table 7. Storage losses in 1996–97 (weight loss: % of weight before storage, others: % of weight after
storage). Percentages of saleable onions are not available for March.
Treatment Weight loss Saleable onions Rooted onions
% % %
Jan Mar May Jan May Jan Mar May
N1 3.2 4.6 6.0 99 41 0 4 57
N2 2.9 4.3 5.4 100 35 0 17 65
N3 2.7 4.1 5.1 100 40 0 3 60
I1 3.3 4.4 5.6 99 57 0 0 42
I2 3.0 4.4 5.4 100 37 0 11 63
I3 2.7 4.2 5.2 100 34 0 4 66
Probability *
Irrigation (I) ns ns ns
Fertilisation (F) 0.007 0.003 <0.001
I * F ns 0.012 0.032
Time (T) <0.001 <0.001 0.001
I * T ns ns ns
F * T ns 0.001 0.054
I * F * T ns 0.017 ns
* Only data for March and May included in analysis
N = non-irrigated, I = irrigated
1, 2, 3 = fertilisation levels: N 50, 100, 125/150 kg/ha
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tion rate, weight loss did not change much in
the course of time (3.9% in January, 3.4% in
March and 4.2% in May), but at the two higher
rates, the increase in weight loss was larger in
May (4.4%) than in January (3.4–3.5%). Towards
the spring, shelf-life tests showed an increase in
sprouting (P<0.001). The effect of time was de-
pendent on fertilisation (P
fert*time
=0.021): at the
lowest fertilisation level, there were 9%, 9% and
28% of sprouted onions in January, March and
May, respectively. At the higher levels, the per-
centages of sprouted onions increased with time
even more: at the second level the proportions
were 9%, 10% and 34% in January, March and
May, and at the highest level 6%, 6% and 40%,
respectively.
Discussion
Yield and maturity
Our results indicate that fertilisation rates could
be reduced in Finnish onion production. In 1995,
increasing the N fertilisation rate over 50 kg/ha
had neither positive nor negative effects on the
yield or storability of onions. In 1996, the yield
increased up to the second level of fertilisation
(100 kg N/ha), in which most nutrients were in-
corporated in soil before planting and an addi-
tional 25 kg/ha N and K was broadcast one month
later, on 18 June. A further top dressing, applied
on 15 July to the plants receiving the highest
fertilisation, resulted in no extra benefit. The
yield benefit from increasing N fertilisation from
50 to 100 kg/ha was only 5.5%.
The lowest rates of P and K were in accord-
ance with Finnish recommendations. Since high-
er P and K applications had no influence on yield,
the present recommendations appear to be ap-
propriate.
The low optimum fertilisation level may have
been due to active mineralisation of N in the soil.
Greenwood et al. (1992) reported mineralisation
rates of 0.56–1.51 kg N/ha d-1 from the Nether-
lands. Finnish soils often have a very high or-
ganic matter content and the potential for min-
eralisation is considerable if the soil tempera-
ture is high enough. The end of the growing pe-
riod in the experimental years was warmer than
the long-time average, so mineralisation presum-
ably satisfied the N demand of onion in mid and
late summer.
Irrigation had a greater impact on yield than
had fertilisation rate. The favourable effect of
irrigation in 1995 is attributed to prolongation
of growth before foliar fall-over. Drought is
known to advance maturation (Riekels 1977,
Henriksen 1984, Mondal et al. 1986). Although
the effect of irrigation on maturation was not sta-
tistically significant here, in 1995 the difference
between irrigated and unirrigated plots was al-
most as great as between fertilisation levels. In
1996, irrigation had no visible or statistically sig-
nificant effect on advancement of maturity since
there was only one application in August.
Onion plants are sensitive to drought during
maximum leaf growth in mid-season, 7–10
weeks after planting (Riley 1989), and at the time
of bulb enlargement (Dragland 1975). In 1995,
a dry and warm period began in the middle of
July, when leaf growth was still active and bulbs
were growing fast. In 1996, when rainfall satis-
fied plant water demand until the end of July,
plants were able to form strong and large leaves,
but the dry and warm weather at the beginning
of August may have interfered with bulb growth.
Pfülb and Zengerle (1990) emphasise that irri-
gation should not be stopped too early, as it is
needed until bulb development is complete and
the skin has started to darken.
When the weather is warm and dry, continu-
ing irrigation in late summer could thus be a way
to prolong growth and ensure maximum yield
potential. If there is a risk of plants not matur-
ing, heavy irrigation should be avoided (Riekels
1977).
Increased fertilisation accelerated matura-
tion, but the difference in time between the low-
est and highest fertilisation rates in achieving
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Suojala, T. et al. Effects of fertilisation and irrigation on onions
80% softened pseudostems was only a few days.
The effect of fertilisation might have been great-
er had the growing season been cooler. Henrik-
sen (1987) found a 9-day delay in harvest time
at 90% leaf fall-over at a fertilisation rate of 0
kg N/ha compared with that of 120 kg N/ha. In-
creasing the fertilisation rate to 180 kg N/ha did
not promote maturity further. In the study of Sø-
rensen (1996), onions grown at a supply rate of
65 kg N/ha showed delayed leaf fall-over rela-
tive to higher N applications.
The timing of N fertilisation is crucial: in
Brewster’s study (1990b) delaying N fertilisa-
tion until 2 August resulted in a significant de-
lay in maturity compared to fertilisation before
sowing. Brewster et al. (1987) found a signifi-
cant positive correlation between the level of N
top dressing and the percentage of thick-necked
onions, and noted that under conditions favour-
ing thick necks (cool summer, slow early growth,
late maturation), high N may aggravate the prob-
lem. In unfavourable years, maturation may be
incomplete in Finland. In view of these and oth-
er results, N fertilisation applied late in the grow-
ing season would seem to delay rather than has-
ten maturation.
Nitrogen uptake
Active mineralisation was shown in plant N up-
take. The total plant uptake of N in 1995 was,
on average, 111 kg/ha at the lowest fertilisation
rate and 165 kg/ha at the highest rate. N uptake
in 1996 was slightly lower: 93 kg/ha and 153
kg/ha at the lowest and highest fertilisations, re-
spectively. At the lowest fertilisation, plants were
therefore able to take up twice the amount of N
that was given in the fertiliser. Increased fertili-
sation caused a marked increase in plant N up-
take, although the yield was little affected. N
uptakes of similar magnitude have been observed
in other studies (Sørensen 1996, Salo 1998).
Irrigation decreased the bulb N content but
did not influence the total plant N uptake. Thus
the decline in N content may be related to the
higher bulb weight in irrigated plots. The bulb
dry matter content was not influenced by irriga-
tion, which is consistent with the results of Drag-
land (1975) and Pfülb and Zengerle (1990).
Most of the N was localised in bulbs, and the
bulb N content increased with the increase in
fertilisation. The low N content of the foliage is
attributed to the late timing of harvest: at the time
of harvest, leaves had already partly senesced,
as was shown by the foliage N content, which
was lower than in the studies of Wiedenfeld
(1986) and Salo (unpublished), in which analy-
ses were made prior to senescence. Nilsson
(1980) observed translocation of N from foliage
to bulb during the maturation period. Transloca-
tion may have continued during artificial drying
of the bulbs at 20–30°C. Despite the potential
translocation, the bulb N content was similar to
that found in other studies (Greenwood et al.
1980, Nilsson 1980, Salo (unpublished)).
Bulb dry matter content was not affected by
the treatments, a finding that is in accordance
with the results of Henriksen (1987) and Maier
et al. (1990).
Storage performance
Fertilisation and irrigation had little effect on the
storage performance of onions. Weight loss dur-
ing storage was slightly affected, but the practi-
cal significance of the differences was negligi-
ble. These results support those of the majority
of earlier studies, which showed no effect of N
fertilisation (Kepka and Sypien 1971, Dragland
1975, Tahvonen 1981, Aura 1985) or irrigation
(Riekels 1977, Henriksen 1984, Pfülb and
Zengerle 1990) on storage losses. Some research-
ers report that high N supply may promote
sprouting during or after cold storage (Riekels
1977, Henriksen 1984), but we did not find any
such effect. Sprouting was not observed during
cold storage, and in shelf life tests only in 1996–
97.
Storage losses were minimal in January but
increased towards the spring, mainly due to the
formation of roots. In 1995–96, storage diseas-
es (mostly Botrytis allii) had spoiled nearly 20%
487
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Vol. 7 (1998): 477–489.
of the bulbs by May, but diseased bulbs amount-
ed to only a few per cent in March. In 1996–97,
bulbs were infected only occasionally.
The high incidence of rooting was probably
caused by the excessively high humidity of the
store. The general recommendation is that rela-
tive humidity should be kept below 70–75% to
suppress rooting and rotting (Komochi 1990). We
could not maintain such conditions in our ex-
periment. Rooting was not observed in analyses
in January, but later in the spring, a substantial
proportion of the bulbs had been spoiled in this
way.
Shelf-life tests in 1996–97 revealed rooted
bulbs only in the test that was started in March.
The reason for this remains a mystery as the con-
ditions during shelf-life tests were similar. One
possible explanation is that rooting did not take
place in May because a large proportion of on-
ions had already formed roots during cold stor-
age, and those that were left for shelf-life tests
were individuals not susceptible to root forma-
tion. In March, when rooting was just beginning
at a low temperature, bulbs whose roots could
not yet be distinguished formed roots very fast
at 17°C.
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Vol. 7 (1998): 477–489.
SELOSTUS
Lannoituksen ja kastelun vaikutus sipulin satoon, sadon valmistumiseen
ja varastokestävyyteen
Terhi Suojala, Tapio Salo ja Raili Pessala
Maatalouden tutkimuskeskus
Tutkimuksen tavoitteena oli selvittää, voidaanko
määrältään ja varastokestävyydeltään hyvä sipulisa-
to saavuttaa vähällä lannoituksella, kun kasvustoa
kastellaan tarvittaessa. Kaksivuotisissa kokeissa tut-
kittiin kolmen lannoitusmäärän (N 50, 100, 125/150
kg/ha) ja kastelun vaikutusta sadon määrään, valmis-
tumiseen sekä varasto- ja kauppakestävyyteen. Sadon
valmistuminen nopeutui lannoitusta lisättäessä, mutta
kastelu ei vaikuttanut tuleentumiseen. Runsas lannoi-
tus lisäsi satoa vain vuonna 1996 (5–7 %). Sen si-
jaan kastelu lisäsi satoa selvästi: 33,5 % vuonna 1995
ja 8,5 % vuonna 1996. Kastelun ja lannoituksen vä-
lillä ei havaittu yhdysvaikutusta. Alhaisen lannoitus-
optimin oletetaan johtuvan typen mineralisoitumises-
ta maassa. Pienimmällä lannoitustasolla kasvit pys-
tyivät ottamaan kaksinkertaisen typpimäärän lannoi-
tuksessa annettuun määrään verrattuna. Lannoituksen
lisääminen lisäsi huomattavasti kasvien typenottoa.
Lehtien typpipitoisuus oli pieni, minkä oletetaan joh-
tuvan myöhäisestä sadonkorjuusta. Käsittelyiden vai-
kutus sipulin varasto- tai kauppakestävyyteen oli vä-
häinen. Tulosten mukaan sipulin lannoitusta voidaan
alentaa, jolloin ravinnepäästöt ympäristöön vähene-
vät. Kastelu on välttämätöntä kuivien jaksojen aika-
na täyden sadon saamiseksi, eikä se heikennä sipu-
leiden varastokestävyyttä.
Title
Introduction
Material and methods
Results
Discussion
References
SELOSTUS