Nitrogen and Water Effects on Yield, Quality and Tissue
Nitrate Concentration in Subsurface Trickle
Irrigated Melons
J.W. Pier, T.A. Doerge, J.L Stroehlein and T. McCreary
Abstract
Rising water costs and concern for groundwater contamination are encouraging growers
to improve irrigation and fertilization efficiency. The objectives of this study were to
determine water and fertilizer nitrogen (N) rates leading to optimum yield and harvest
quality and to develop a plant tissue test to aid in melon nitrogen fertilization. In 1990, a
field experiment consisting of a complete 3x3 factorial arrangement of optimum, sub- and
super- optimum rates of urea ammonium- nitrate and water applied through a subsurface
trickle irrigation system to cantaloupe, honeyloupe and watermelon was conducted at the
Maricopa Agricultural Center. Petioles were sampled from the youngest mature leaf
beginning at the early runner stage and then weekly until first harvest. Petiole nitrate
concentrations were determined using a high pressure liquid ion chromatograph. Har-
vested melons were weighed and graded for marketability and soluble solids were
determined. Petiole nitrate levels were highly responsive to N fertilizer treatments and
accurately quantified visual observations of crop N status. Petiole nitrate results also
indicated that later fertilizer split applications occurred after the point of maximum plant
uptake. Tensiometer readings suggested that the highest rate of water application led to
deep percolation and nitrate leaching where nitrogen fertilizer was excessive. Watermelon
showed the clearest yield response to the water and nitrogen treatments. Honeyloupe
responded well to high water but poorly to higher nitrogen application rates. Cantaloupe
yields responded best to higher nitrogen and medium water levels.
Introduction
Melons are an economically important crop in Arizona. They rival cotton in terms of gross returns per unit
land area. Vegetable crops such as melons require large quantities of water and nitrogen, relative to
other agronomic crops, to attain optimum yields. In situations where large inputs of water and fertilizer
nitrogen are being applied there is a risk of nitrate leaching and subsequent ground water contamination.
The anionic nature of nitrate means that it moves with the wetting front as the deep percolated water
moves down the profile.
In Arizona, melons are normally grown using furrow irrigation and deep percolation is common. Melons
are also grown on light textured soils which have limited water holding capacity. Frequent irrigations are
necessary to avoid water stress thus compounding the deep percolation problem.
Drip irrigation provides frequent, light applications of water to meet crop water needs thereby reducing
the chance of deep percolation. Nitrogen fertilizer can be applied through drip tubing which is placed
in the densest portion of the crop root zone where it will be most available. Since water is applied daily,
59
provide nitrogen to the crop when demands are greatest. Methods for determining crop water and
nitrogen requirements during the growing season are essential in order to better schedule irrigation and
fertilization.
The objectives of this study were: 1) to determine the usefulness of tensiometers and plant tissue nitrogen
tests as feedback mechanisms for use in scheduling irrigation and fertilization of three melon varieties
and; 2) to determine recommended quantities of water and nitrogen to apply to the melon varieties in
order to achieve optimum yield.
Materials and Methods
In 1990, a field experiment consisting of a complete 3x3 factorial arrangement with four replicates of
optimum (130 kg N /ha, 0.41 m water), sub- (40 kg N/ha, 0.21 m water) and super- optimum (270 kg N/ha,
0.63 m water) rates of urea ammonium -nitrate and water applied through a subsurface trickle irrigation
system to Laguna cantaloupe, Gallicum honeyloupe and Mirage watermelon was conducted at the
Maricopa Agricultural Center in southern Arizona. Individual plots consisted of 15.4 m of row using a 2
m inter -row spacing. Seeds were hand planted every 30 cm within rows for a plant population of 16667
plants /ha.
Before planting, sudangrass was planted then harvested and removed from the field to reduce residual
soil nitrogen. Flood irrigation was used to help leach remaining soil nitrogen out of the profile. The
ammonium and nitrate levels in the surface 60 cm of soil was 0.4 ppm NH4 N and 6.8 ppm NO3-N. Drip
tubing (Twin Wall IV, 0.5 gal /min /100 ft, Chapin Watermatics) was placed directly below the seed line at
a depth of 20 cm on the south side of the east -west oriented melon beds.
Following planting, tensiometers were installed at 30 and 60 cm depths in the optimum nitrogen (N2)
treatment for all water rates. Tensiometer readings were taken prior to the daily irrigation events twice
weekly. All plots were watered uniformly for 42 days after planting in order to establish the melon
seedlings. Water treatments began at the early runner stage. The amounts of water applied were based
on a consumptive use curve for early season cantaloupe developed by Erie et al. (1982) and arbitrary
target soil water potentials. Five bee hives were placed in close proximity to the experiment just prior to
flowering to insure uniform, early pollination. Petioles were sampled from the newest fully developed
leaves at weekly intervals beginning at early runner stage through first harvest. Fertilizer was applied as
urea -ammonium nitrate and phosphoric acid through the drip system according to the dates in Table 1.
There were seven harvests during the period from June 19 to July 10. Cantaloupes and honeyloupes
were harvested at full slip or if melons were rotten or split. Watermelons were harvested when the tendril
associated with the fruit stem had just senesced. Each melon was weighed and graded for marketability
at the time of harvest. Sugar content was measured from one representative marketable melon from each
plot using a refractometer.
Results and Discussion
A comparison of the water applied over time with the consumptive use curve of Erie et al., Figure 1.,
shows a close agreement with the optimum water treatment and the cantaloupe water requirement. A
prolonged period of above average temperature from 73 to 100 days after planting (DAP) required
additional water application above the historical consumptive use curve. The high temperatures also
hastened the time to maturity and the water was cut back after 100 DAP to insure fruit with high sugar
content at harvest.
Tensiometer readings for cantaloupe at 30 and 60 cm are shown in Figure 2. The data have been
smoothed with a moving average operator for presentation purposes. Low values of tension are moist
60
soil conditions and high values indicate dry soil. A tension of 150 cm was arbitrarily chosen as the target
soil moisture at the 30 cm depth. One can see the coincidence of tensions at 50 cm and 100 cm tension
for 30 and 60 cm depths during the establishment period (0 - 42 DAP) and the separation of tensions after
water treatments were begun. The 60 cm depth tensions are dryer than the 30 cm depth for both the
medium and low water rates. However, two weeks after the beginning of the water treatments, the
tensions at 60 cm are the same as at the 30 cm depth. Studies have shown that crop roots proliferate
in the immediate vicinity of drip tubing and do not tend to grow much beyond a small radius around the
tubing. Therefore, it is unlikely that there is any significant uptake of water or fertilizer below 60 cm depth
and thus deep percolation and nitrate leaching is evident at the high water rate.
The results of the petiole nitrate -N analyses for cantaloupe, honeyloupe and watermelon are seen in
Figures 3 - 5, respectively. Overall, there was a good response of the petiole nitrate values to nitrogen
fertilizer application rates with the exception of 56 DAP when all three fertilizer rates gave low tissue nitrate
values. The most likely explanation for the drop in tissue nitrate at 56 DAP is rapid vegetative growth
following the onset of early runner stage at 42 DAP and a phosphorus deficiency which was corrected
prior to 56 DAP which accelerated growth at the expense of tissue nitrate. The petioles showed a rapid
recovery after application of nitrogen and phosphorus fertilizer at 56 DAP. The tissue nitrate then dropped
or remained the same one week after the third application of nitrogen at 63 DAP for the medium and high
nitrogen application rates. This may have been due to resumption of vegetative growth after the plants
had recovered from the deficient nitrogen state along with development of fruit. The low N rate showed
increased tissue nitrate as this was the only other nitrogen applied to this treatment other than the initial
application one month after planting. Vegetative growth resumed in the low nitrogen treatment after 70
DAP and this can be seen in the drop in tissue nitrate after this date.
The effect of different water application rates appears to be a lowering of tissue nitrate levels as the water
application rate increases. This can be attributed to increased vegetative growth and therefore increased
demand for nitrogen at higher water rates. Leaching losses of NO3 -N in the high nitrogen treatments (N3)
when deep percolation occurs could also explain this observation. Honeyloupes appear to have the
highest levels of tissue nitrate followed by cantaloupe and watermelon. Except for the low nitrogen
application rate, the watermelon petiole nitrate levels were low and fairly constant until the last two
nitrogen applications when the nitrate values rose rapidly. This rapid increase suggests that nitrogen had
been applied in excess of plant needs.
In retrospect, the nitrogen fertilization schedule did not precisely track the nitrogen needs of the melons.
This was due to fertilization two weeks past the onset of a rapid increase in nitrogen uptake at the early
runner stage. Tissue samples should be taken at the 3-4 leaf stage to guide early season N applications.
In addition, pre -plant soil phosphorus testing should.be performed to guide pre- or at planting phosphorus
fertilization.
Fertilizer scheduling was based on petiole nitrate values analyzed from one repetition using a nitrate anion
selective electrode. The procedure for the electrode assumes that plant tissue chloride levels are low and
therefore there is little interference of chloride in the detection of nitrate. However, subsequent analyses
of petiole samples using high pressure liquid ion chromatography indicates a negative relationship
between nitrate and chloride (data for chloride not shown); when nitrate is low, chloride is high and vice
versa. Therefore, one might observe higher nitrate values than actually exist due to high levels of chloride
in the tissue sample.
Marketable melon yields for cantaloupe, honeyloupe and watermelon are shown in Figures 6-8,
respectively. The triangles represent means of the observed data for the nine water x N treatments. The
response surface was fit to the data by SAS PROC RSREG.
Cantaloupe showed a maximum yield response to the optimum water rate at all three nitrogen levels. This
is not surprising since the water rates were based on the cantaloupe consumptive use curve. The
maximum yield occurred with the highest nitrogen rate. This is most likely due to delayed nitrogen
61
applications as mentioned previously. It is important to identify where the yield curve begins to level off
with increasing nitrogen rates since it is beyond this point where further nitrogen application could lead
to potential for nitrate leaching. For low applications of water, yield leveled off after 130 kg N /ha No clear
deflection point can be determined for the higher water rates.
Honeyloupe shows a great deal of yield variation with water and fertilizer treatments. The response
surface has only a fair fit to the data The yield increases with increasing water application rate but drops
or levels off past the middle nitrogen application rate. The honeyloupe was susceptible to root knot
nematode which severely damaged the vines in the high nitrogen -high water treatments. This disease
may have effected the data Another explanation for low honeyloupe yields at higher nitrogen rates could
be the indeterminate nature of the plant. The increased nitrogen was used for excess vegetative growth
at the expense of fruit production.
Watermelon shows the clearest yield response to the nitrogen and water treatments. Yield variability was
low and there was a good fit of the response surface to the data Melon yields increased in response to
greater water and nitrogen rates. Marketable yields for watermelon were twice that of either cantaloupe
or honeyloupe.
Water had the greatest effect in determining fruit sugar content. The optimum water rate had a
significantly higher sugar content than the low or high water rates for cantaloupe. For honeyloupe, the
high water rate showed higher fruit sugar content than the medium and low water rates though the overall
model was not significant. Neither nitrogen nor water had an effect on watermelon sugar content.
Honeyloupe had the highest sugar content, 11 %, followed by watermelon, 10 %, and cantaloupe, 9.7 %.
Conclusions
Tensiometers and plant tissue analyses are beneficial diagnostic tools for determining crop water and
nitrogen needs. Good melon yields can be achieved with limited environmental damage when the
information provided by tensiometers and plant tissue tests is used to schedule water and N applications.
Melon petiole nitrate levels are highly responsive to nitrogen treatments and can efficiently be used to
check if a preplanned N fertilization schedule is on target.
At this point in time, it is not possible to recommend ranges of fertilizer nitrogen and water for optimum
crop yield. One of the main problems lies in the definition of optimum yield. In the past, optimum yield
has been equated with maximum yield with no regard for environmental impact. As awareness of the real
costs of over - watering and over - fertilization are realized, optimum yield must be redefined in terms of both
efficiency and the environment. Adequate yields can be obtained at water and nitrogen application rates
that reduce the risk of groundwater contamination. However, more information pertaining to the point
where nitrogen applications become excessive must be gathered for these crops.
References
Erie L.J., O.F. French, D.A. Bucks, and K Harris. 1982. Consumptive Use of Water by Major Crops in the
Southwestern United States. U.S.D.A., Conservation Res. Rep. No. 29., 42 p.
62
Table 1.
Fertilization schedule.
Uniform
Date
DAP
P2O5
____ ___________ _____________
Apr 17
26
May 10
49
May 17
56
May 22
61
May 24
Jun 4
_____
Hi N Rate
kg/ha
10
20
30
0
30
60
63
30
60
90
77
0
30
90
40
130
270
45
45
Total N
Total P2O5
Med N Rate
Low N Rate
90
63
Cumulative Consumptive Use
0.7
o
n
0.6
s
U
0.5
p
0.4
t
e
U
Low Rate
+ Med Rate
--
HI Rate
- Erie
0.3
0.2
Water treatments begin......
s
e
0.1
m
o
10
17
24
31
38 45 52 59 66 73 80 87 94 101 108
Days After Planting
Figure 1.
Comparison of cumulative water applied with historic cantaloupe consumptive use.
64
Tensiometer Readings
a
800
Laguna Cantaloupe 30 cm
Tension cm
700
-9- Low Rate
600
- - Med Rate
-e-- HI Rate
500
400
300
200
100
o
26
40
33
54
47
68
61
75
82
96
89
103
Days After Planting
Laguna Cantaloupe 60 cm
800
Tension cm
-- Low Rate
700
-4- Med Rate
600
-e- HI Rate
500
400
300
200
100
o
26
33
40
47
54
61
68
75
82
89
96
103
Days After Planting
Figure 2.
Tensiometer readings during growing season for cantaloupe at: a) 30 cm depth and; b)
60 cm depth.
65
77
84
63
70
77
84
- 40kgN/ha
+ 130kgN/ha
56
63
70
77
High Water
Days After Planting
49
* 270kgN/ha
0
42
56
0
42
49
10
10
Medium Water
70
15
15
Low Water
63
20
20
Days After Planting
56
25
25
Days After Planting
49
30
30
Laguna Cantaloupe
Petiole Nitrate -N
84
42
42
49
0
56
63
70
77
84
+ 130kgN /ha
84
40kgN /ha
77
Medium Water
70
Low Water
63
Days After Planting
56
Days After Planting
49
10
20
30
40
66
63
70
77
High Water
Days After Planting
49
* 270kgN /ha
0
42
Gallicum Honeyloupe
Petiole Nitrate -N
84
P
0
42
20
ti
P
N
42
0
49
56
63
70
77
84
+ 130 kg N / ha
84
40 kg N / ha
77
Medium Water
70
Low Water
63
Days After Planting
56
Days After Planting
49
10
20
30
56
63
70
77
High Water
Days After Planting
49
* 270 kg N / ha
0
42
10
20
30
40
40
40
3 30
0
N
50
50
50
Mirage Watermelon
Petiole Nitrate-N
84
MARKETABLE YIELD
LAGUNA CANTALOUPE
API P
#.11114Wis
go
70
Aft.44t *%*
50
30
0 429
NO'
0
A TER
APPLieU
0 .207
Figure 6.
ho
tPr4
Marketable yield of Laguna Cantaloupe for nine water by N treatments.
69
MARKETABLE YIELD
GALLWCUM HONEYLOUPE
4011k.
m
A
ka_\
80
e
w,
1.7\
o 14
270
IV
Figure 7.
NRATEKG`NA
Marketable yield of Gallicum Honeyloupe for nine water by N treatments.
70
MARKETABLE YIELD
MIRAGE WATERMELON
v ,.
em,
.e .
LSD 0.06
/ ##
1 I II II I I..
.i
.
*1k
Ai
i ##
Ar
, ,4
N
St\
k
a I I IL
JITI IL
0
270
130
N RATE KG f
Figure 8.
\
Marketable yield of Mirage Watermelon for nine water by N treatments.
71
HA