A Horticultural Sciences Department Extension Publication on Vegetable Crops
Eat your Veggies!!!!!
Issue No. 543 March 2009
Irrigation Strategies to Minimize Nitrate Leaching for
Drip Irrigated Tomatoes
By: Lincoln Zotarelli, Assistant Research Scientist and Michael D. Dukes, Associate
Professor, UF/IFAS, Agricultural and Biological Engineering Department
Many tomato growers in Florida apply most of the N-fertilizer via injection in the drip lines
during the growing season; however, excessively high N-fertilizer/irrigation rates greatly
increase the risk of nitrate leaching. Nitrogen fertilizer may be injected in the microirrigation
systems using a number of different nitrogenous compounds including urea, ammonium, or
nitrate forms. Regardless of the applied compound, under conditions that prevail in the
southeastern USA most of the soil N is rapidly converted into N03-N which is more
susceptible to leaching. Actual N distribution in the soil depends on N source and application
rates, crop removal capacity, and water displacement below the active root zone (irrigation).
As the nitrate tends to accumulate toward the boundary of the wetted volume, the use of
irrigation strategies that limit the wetted volume in the root zone may improve water and
nitrogen use efficiency, as well as reducing nitrate leaching. In the past 20 years,
technological advances in soil moisture sensing have provided growers feedback on
optimization of irrigation scheduling in order to optimize irrigation water applied. Compared
to manual irrigation treatments with one or two irrigation events per day, irrigation amounts
were reduced by approximately 50% while maintaining similar yields. In addition, subsurface
drip irrigation (SDI) has also shown to be an efficient technique to control irrigation
scheduling when controlled by sensor based methods. The increase of irrigation water use
efficiency should inherently minimize N03-N leaching for vegetable crops. The application of
fertilizer above the irrigation drip in SDI system should further reduce N03-N leaching by
maintaining nutrients in the root system. The objective of this study was therefore to identify
suitable irrigation scheduling methods and drip irrigation system configurations to reduce
irrigation water application.
Field experiments were conducted during the spring of 2005, 2006, and 2007 at the University
of Florida, Plant Science Research and Education Unit, near Citra, FL. The soil has been
classified as Candler sand (97% of uncoated sand) in the upper 3 ft of the profile, with a field
capacity in the range of 0.10-0.12 (v v-1) at the 0-1 ft depth. Raised beds (1 ft height) were
constructed with 6 ft distance between bed centers. Granulated P205 fertilizer was
incorporated into the beds at a rate of 100 lb ac- Beds were fumigated after placement of
both drip tapes and plastic mulch in a single pass 13 days before transplanting. Irrigation was
applied via drip tape. Tomato transplants (Lycopersicon esculentum Mill. var. "Florida 47")
were set in the first week of April of 2005, 2006, and 2007. Weekly fertigation consisted of
injecting dissolved fertilizer salts into fertigation lines (Fig. 1). All plots received 200 lb ac-
of N as calcium nitrate, 220 lb ac-1 of K as potassium chloride, 10 lb ac-1 of Mg as magnesium
sulphate. The irrigation treatments were differentiated by their arrangement of drip irrigation
lines. The treatments were identified as surface drip irrigation (SUR), whereby both irrigation
and fertigation drip lines were positioned on the soil surface. The second treatment was
identified as subsurface drip irrigation (SDI, Fig. 2), with the irrigation drip line positioned at
6 inches below the raised bed surface while the fertigation drip line was positioned on the soil
surface. For both treatments, irrigation events were controlled by a University of Florida
developed Quantified Irrigation Controller (QIC) system, which included an 8 inch long
ECH20 probe (Decagon Devices, Inc. Pullman, WA) to monitor soil moisture. Probes were
inserted vertically in order to integrate the soil water content in the upper 8 inches at 2 inches
from the irrigation drip line. The QIC irrigation controllers allowed a pre-programmed timed
irrigation event if measured soil water content was below a volumetric water content (VWC)
value of 0.10 m3 m-3 during one of five daily irrigation events, with each potential irrigation
event lasting 24 min. Based on these readings up to a maximum of five irrigation events could
occur per day totaling 2 hr. A reference treatment employed a fixed time-based irrigation
(TIME) featuring one fixed 2-h irrigation event per day. Similar to the SUR treatment, the
TIME treatment used twin lines, one for irrigation and one for fertigation. The twin line setup
was due to the experimental convenience; however, a commercial grower would most likely
have one drip line for irrigation and fertigation at the surface.
The volumetric water content of the top soil of the production beds was monitored by
coupling time domain reflectometry (TDR) probes with a datalogger (CR-10X). Soil moisture
probes were placed in the beds at two subsequent soil layers which recorded soil moisture
values. The upper probe was inserted at an angle in order to capture soil moisture in the top 10
inches of the profile and the lower probe was inserted vertically below the upper probe
recording soil moisture between 10 and 22 inches. Zero tension drainage lysimeters were
located 2.4 ft below the surface of the bed (Photo). The drainage lysimeters were constructed
out of 55 gal capacity drums that were cut in half lengthwise with a length of 33 inches, a
diameter of 22 inches, and a height of 10 inches. A vacuum pump was used to extract the
leachate accumulated at the bottom of the lysimeter. The leachate was removed weekly one
day prior to the next fertigation event by applying a partial vacuum (4-6 psi) using 5 gal
vacuum bottles for each drainage lysimeters (Photo). Total leachate volume was determined
gravimetrically and subsamples collected from each bottle were analysed for NO3-N and thus
total N loading rates could be calculated.
The final harvest occurred on 84, 86 and 90 days after transplanting (DAT) in 2005; 2006 and
2007, respectively. The harvested area consisted of a central 30 ft long region within each
plot. Marketable weight was calculated as total harvested weight minus the weight of culls.
Irrigation water use efficiency (WUE) expressed in lb of fruits per 100 gal of irrigation water
applied was calculated by taking the quotient of the marketable yields (lb ac-') and the total
applied seasonal irrigation depth (gal ac-1). Maximum biomass accumulation was evaluated
by harvesting one representative plant per treatment replicate at final harvest. Tissue material
was analyzed for total Kjeldahl N. Plant N accumulation was calculated by multiplying
weights of stems plus leaves and fruit tissue by the corresponding N concentrations. Nitrogen
use efficiency (NUE) was defined as N uptake by the plants divided by the total amount ofN
supplied from weekly fertigation.
A soil moisture sensor controller was programmed to bypass irrigation if the probe read soil
moisture at or above the set threshold (10%) at the beginning of an irrigation window. During
the crop season, programmed irrigation events were skipped which significantly reduced the
amount of water applied to soil moisture sensor (SUR and SDI) based treatments. The volume
of irrigation increased in order SUR < SDI < TIME (Fig. 3). The SUR treatment received an
average of 16, 38 and 42 gal 100 ft-1 day-', in 2005, 2006 and 2007, respectively. The
corresponding average irrigation rates for the SDI treatment were 37, 55 and 67 gal 100 ft-1
day-'. Use of the SDI system resulted in higher water application, even though soil moisture
content thresholds were the same for both the SUR and SDI treatments. The water savings for
SDI compared to TIME treatments ranged from 7 to 42% (Fig. 3). These values are very low
when compared to the potential water savings of SUR treatment (67-216%).
Soil Moisture and Water Percolation
The soil moisture content as measured by TDR probes had a noticeable increase in soil
moisture after each irrigation event throughout the growing season for SUR and TIME (data
not shown). Soil moisture sensor based irrigation treatments irrigated for short periods of time
which resulted in a relatively small increase in soil moisture, consequently decreasing the
volume of percolate (Fig. 4). This was true for both the SUR and SDI treatment, which
received a higher volume of water than SUR (Fig. 3). On the other hand, the TIME treatment
was irrigated for a longer time period which resulted in very pronounced soil moisture
fluctuations, resulting in substantial percolation below the root zone (Fig. 4). The relatively
large irrigation events on the TIME treatment compared to the other treatments promoted not
only excessive water percolation but also nitrate movement below the root zone (Figs. 4 and
5). In terms of soil water availability to plants, the TIME treatment initially may provide more
favorable growth conditions since the soil remains wetter, thus reducing potential water stress.
However, excessive water percolation also may reduce N retention and crop N supply and
thereby reduced yield for tomato (Table 1). By comparison, irrigation water from the SUR
and SDI treatments produced relatively constant soil moisture values over time, as irrigation
water was distributed across multiple irrigation events according to the soil moisture threshold
and thus crop water demand.
Nitrate Leaching and Tomato Yields
The TIME treatment resulted in the most leaching. Cumulative N03-N leaching values were
55; 43; and 9 lb ac-1 for TIME treatment, in 2005, 2006 and 2007, respectively. The single
high volume daily application of the TIME treatment is likely the cause of the appreciable
drainage and NO3 leaching below the rootzone. By comparison in 2005 and 2006, the SUR
and SDI treatments reduced NO3 leaching on the order of 90%, representing a total load of 4
to 6 lb ac-1 of N (Fig. 5). In 2007, the overall leached volume for SDI and SUR treatment was
slightly higher than previous years; however, the N-loads below the root zone were drastically
reduced (Fig.5) due to the reduction in the N03-N concentration in the leached volume. The
lower N-load leaching and lower N03-N concentration were directly related to irrigation
treatments, which was associated to the tomato yields.
Irrigation treatments had an important impact on WUE and tomato yield (Table 1). The use of
soil moisture sensors increased marketable tomato yield 69-80% in 2005; 20-26% in 2006 and
11-21% in 2007 when compared to the TIME treatment (Table 1). There was no significant
difference on tomato marketable yield for SDI and SUR treatments, in 2005 and 2006.
However, in 2007, SUR treatments out-yielded SDI treatments (3,176 vs 2,876 boxes ac-1).
Except in 2005, when unfavorable growth conditions hampered plant growth, tomato yield
obtained in these experiments were in the range of those reported in the literature for sandy
soils in Florida. The increase in tomato yield in 2006 and 2007 compared to 2005 was
attributed to the higher volume of irrigation applied in 2006 and 2007 and weather conditions.
During 2006 and 2007, favorable weather conditions characterized by lower temperatures,
humidity, and precipitation occurred during the reproductive phase.
Irrigation Water and N-fertilizer Use Efficiency
The use of different drip position arrangements significantly affected the IWUE and yield
(Table 1). The treatment ranking for WUE was as follows: SUR > SDI > TIME. The TIME
treatment had a lower WUE value (7-17 lb 100 gal-') due to the high irrigation rates applied,
and also due to the lower marketable yield (Table 1). Nitrogen fertilizer use efficiency for the
200 lb ofN ac-1 applied was significantly higher with the use of soil moisture sensor to
control irrigation either with SDI or SUR treatment. The NUE increased with the increase in
yield level. The NUE values ranged between 33% and 47% in 2005; 42% and 64% in 2006;
and 57% and 74% in 2007. The TIME treatment showed consistently lower values of NUE,
which was related to the higher irrigation volumes being applied, resulting in increased N
dilution and displacement thus reducing N uptake efficiency.
Soil-moisture sensor based irrigation systems in tomato significantly reduced the applied
irrigation on tomato, with the surface drip irrigation (SUR) controlled by soil moisture sensor
(SMS) treatment resulting in 67%-216% less irrigation water applied compared to fixed time
irrigation (TIME) treatments. Corresponding reductions in irrigation water application for
subsurface drip irrigation controlled by soil moisture sensor (SDI) were 7%-42%. In addition,
yield on tomato was also increased 11%-26% for SMS-based treatments compared to the
TIME treatment, in 2006 and 2007, when weather was not a yield limiting factor. Water use
efficiency was superior when surface drip and fertigation were used. Soil-moisture sensor
based irrigation systems in tomato significantly reduced crop water requirements, water
percolation, and nitrate leaching. Nitrogen use efficiency (NUE) and the total plant N
accumulation were higher for SMS-based surface drip irrigation (SUR) and subsurface drip
irrigation (SDI) production systems compared to fixed time irrigation (TIME) treatments.
Similarly, SUR and SDI successfully reduced N03-N leaching by 5 to 35 kg ha-' and 7 to 56
kg ha-1 for N application rates of 220 and 330 kg ha- respectively. It is concluded that
appropriate use of SDI and/or sensor-based irrigation systems can allow growers to sustain
profitable yield while reducing irrigation application in low water holding capacity soils.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Weeks after transplanting
Fig. 1. Weekly NO3-N fertilizer applied as CaNO3 by fertigation
for fresh market tomato.
* Fertigation Drip
* Iiigation Diilp
12 24 inches
24 36 inches
f Fertigation Wet Zone "Bulb"
I Irrigation Wet Zone "Bulb"
Fig. 2 Schematic diagram of water and solute displacement patterns in SDI system
combined with surface fertigation. Note that irrigation does not result in
fertilizer displacement below the root zone concentrate in the top 12 inches.
6000 -- SDI ZUUO A
,6000o 2006 B
5. 3000 -
'S i i o o
6000ooo 2007 C
0 10 20 30 40 50 60 70 80 90 100
Days after transplanting
Fig. 3. Cumulative irrigation after initial plant establishment as affected by
different irrigation scheduling methods during the 2005, 2006, 2007
tomato growth season.
20 40 60 80 100 0
20 40 60 80 100
Days after transplanting
Fig 4. Cumulative leachate volume of drainage lysimeters in the spring of 2005, 2006 and 2007. Different letters
indicate differences at the 95% confidence level. Error bars represent standard error from the mean, n = 4.
0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100
Days after transplanting
Fig. 5. Cumulative NO3-N leaching in the drainage lysimeters for irrigation treatments under N-rate of 200 Ib ac"
during spring 2005, 2006 and 2007. Error bars represent standard error from the mean, n = 4.
0 20 40 60 80 100 0
Table 1. Total and marketable yield, nitrogen use efficiency (NUE) and water use efficiency
(WUE) as affected by irrigation treatment (I) at N-rate of 200 Ib ac-1.
Yield WUE NUE
Season Irrigation Total Marketable
25 Ib box ac-1 Lb 100 ga %
SDI 1,253 a 1,085 a 12.9 b 47.8a
SMS 1,317 a 1,188 a 35.1 a 39.3 b
FTI 881 b 667 b 6.9 c 33.5 c
SDI 2,056 ab 1,870 a 15.5 b 62.4 a
SMS 2,591 a 2,280 a 27.8 a 64.2 a
FTI 1,756 b 1,470 b 8.7 c 41.6 b
SDI 2,934 b 2,876 b 18.7 b 68.2 ab
SMS 3,262 a 3,176 a 32.0 a 74.4 a
FTI 2,851 b 2,805 b 17.0 b 56.9 b
t Means within columns followed by the same lowercase letters are not significantly different (P <
0.05) according to Duncan's multiple range test for irrigation treatments within same season.
Photo: Left: overview of lysimeters installation. Center: leachate collection. Right:
overview of experimental conditions.