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 NO3-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 NO3-N leaching for vegetable crops. The application of fertilizer above the irrigation drip in SDI system should further reduce NO3-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 P2O5 fertilizer was incorporated into the beds at a rate of 100 lb ac-1. 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-1 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 ECH2O 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-1) 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 of N 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-1, 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-1. 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 NO3-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 NO3-N concetration in the leached volume. The lower N-load leaching and lower NO3-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-1) 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 of N 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 NO3-N leaching by 5 to 35 kg ha-1 and 7 to 56 kg ha-1 for N application rates of 220 and 330 kg ha-1, 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.
Photo: Left: overview of lysimeters installation. Center: leachate collection. Right: overview of experimental conditions.