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Use of Soil Moisture Sensing and Irrigation Scheduling for Pepper Production

 

By Lincoln Zotarelli, Assistant Research Scientist and Michael D. Dukes, Associate Professor, UF/IFAS, Agricultural and Biological Engineering Department

Pepper is an economically important vegetable for Florida. With a crop value of 209 million dollars, the average acreage annually planted with bell peppers in Florida is 18,500 acres (FASS, 2006; USDA, 2006). Similar to other vegetable crops, pepper production is typically intensely managed with high inputs of fertilizer and irrigation water, which increases the risk of groundwater nitrate contamination. Nitrate contamination in Florida is likely the result of a combination of factors such as N application in excess of crop demand, excessive irrigation and sandy soils. To address N pollution concerns, various best management practices have been suggested, such as the use of plastic mulch to improve irrigation water use efficiency (IWUE) and reduce rainfall-induced N leaching losses (Romic et al., 2003). Plastic mulching in combination with drip irrigation and frequent injection of nutrients can be used in the irrigation system (fertigation) to enhance productivity and reduce volume of irrigation water. Irrigation management plays an important role on vegetable crop production. The use of frequent but low water application volumes is superior to the more traditional scheduling of few applications of large irrigation volumes (Locascio, 2005). Because the former systems may be viewed as labor intensive and/or technically difficult to employ, automated irrigation systems which make use of soil moisture sensing devices may greatly facilitate the successful employment of low volume-high frequency irrigation systems for commercial vegetable crops (Muñoz-Carpena et al., 2005; Dukes et al. 2006). For example, Dukes and Scholberg (2003) reported a 50% reduction in water use when using a soil moisture sensor-based automated irrigation system for bell pepper as compared to a once daily manually irrigated system without affecting yield. The use of improved irrigation scheduling techniques has been shown to greatly increase irrigation water use efficiency and thus reduce irrigation water requirements by as much as 30-100%. With more efficient water use, fertilizer is also retained in the effective root zone longer and growers can attain maximum yields at much lower N-fertilizer application rates. As a result, better irrigation scheduling techniques will not only provide substantial water savings but can also greatly reduce potential N-leaching losses and thus minimize water quality impacts. 

This project focused on the evaluation of different soil moisture threshold setting of commercially available soil moisture-based (SMS) irrigation controllers. The SMSs were used on plastic mulched green bell pepper production, and their potential to increase water use efficiency and crop yield were evaluated. A field experiment was implemented to determine the effects of water application rates and irrigation scheduling on crop yield, water use efficiency, crop growth, fruit quality, water leached and nitrogen leached.

Experiments were conducted at the Plant Science Research and Education Unit (PSREU) near Citra, FL. Green bell peppers were cultivated in the spring of 2005, 2006, 2007, and 2008 on plastic mulch using a bed spacing of 6 feet (Fig. 1). The soil has been classified as Candler sand and Tavares sand, which contains 97% sand-sized particles, with soil field capacity of 10-12% of volumetric soil moisture content and the soil water saturation point of 30-35% of soil volumetric water content (VWC).

Figure 1. Twin rows of green bell pepper just after transplanting.

The area was rototilled and raised beds were constructed at 6 ft bed centers. Beds were fumigated (80% methyl bromide, 20% chloropicrin by weight) at a rate of 543 lbs ac-1 after placement of both drip tape and plastic mulch in a single pass.  “Brigadier” bell peppers were transplanted 10 days after fumigation. Weekly fertigation rates for N, K, Mg were based on IFAS recommendations (Olson et al., 2005). Weekly N-fertilizer applications rates corresponded to 186 lb ac-1of N as calcium nitrate. The irrigation treatments were regulated by the commercial RS500 (Fig. 2) soil moisture sensor (SMS) controller manufactured by Acclima, Inc. (Meridian, ID).  The RS500 unit controls irrigation application by bypassing time clock initiated irrigation events if soil moisture was at or above a preset threshold of 4-12% volumetric water content (VWC) depending on irrigation treatment (Table 1).

Figure 2. Acclima RS500 soil moisture controller (left) and Acclima RS500 soil moisture probe (right).

Table 1. Bell pepper irrigation treatments using soil moisture sensor (SMS) irrigation and 2 hours fixed time irrigation (TIME).
Irrigation \ Year	2005	2006	2007	2008
SMS threshold
4%	N/A	N/A	N/A	X
8%	X	X	X	N/A
10%	X	X	X	X
12%	X	X	X	X
Reference
TIME	X	X	X	X

For all soil moisture sensor controllers, a sensor was installed at a 45 degree angle between two plants that measured the soil moisture in the upper 6 inches of the bed. Timed irrigation windows were specified as five possible events per day, starting at 8:00 am, 10:00 am, 12:00 pm, 2:00 pm, and 4:00 pm for 24 minutes each (totaling 2 hr/day total). As a reference treatment, a time-based irrigation treatment (TIME) was set for one fixed 2 hr (approx. 80 gal/100 ft/day) irrigation event per day.

In the first 15-20 days after transplanting all treatments received application of approximately 75 gal/100ft/day (~0.20 inches/day). After the pepper plants were well established, the irrigation treatments were initiated. Lower sensor thresholds generally reduced irrigation application. The rate (gal/100ft/day) was calculated considering total number of days from the transplanting until the last harvest minus establishment period divided by the volume of irrigation water applied during the corresponding period. Overall irrigation application was 15.2; 22.4; 29.3; 43.7 and 63.5 gal/100ft/day for irrigation treatments with threshold of 4%; 8%; 10%; 12% and TIME, respectively. The TIME treatment received the highest volume of irrigation water on average water volume of 4,894 gal/100ft. The TIME irrigation treatment was applied in one single event per day. This irrigation schedule was intended to mimic a type of schedule commonly used by producers.

One of the challenges that will be encountered in the application of SMS technology to large commercial areas will be the adaptation of the appropriate number of soil moisture sensors to a give field area. In 2006, due to the cross communication between 8% and 10% treatments  which were connected to the same irrigation timer, resulted in over irrigation of these treatments, the problem was fixed 45 days after the transplanting. However, in commercial field settings this would not be an issue since only one setting would be used for the entire irrigation zone. Another challenge observed during execution of pepper experiments was the probe position and installation for these particular sandy soils in a raised bed, where the soil hydraulic conductivity is very high. Differences in probe installation and local differences related to the irrigation drip position, which associated with the individual probe error which is typically on the order of 1-2% VWC could result in minor differences between the threshold set points. A third challenge to be considered in our research is the determination of the real set point (threshold) for SMS, in face of the accuracy of the instruments we have available. For example, the performance of treatments of 10% and 12% were very similar to each other over the course of the growing seasons.  The thresholds of 4% and 8% tended to bypass a high number of irrigation events as expected. The performance of the treatments at the 12% threshold at consistently corresponded to a soil moisture level of 12% v/v. A possible explanation for this is because 12% VWC is slightly above soil field capacity point for this soil, which is approximately10%. In addition, the effective field capacity can change year after year due to differences in raised bed construction.  As expected, this treatment bypassed relatively few irrigation events each season.  Overall, the treatments initiated irrigation events at soil moisture levels (measured by TDR) of 8.5%; 10.1%, 12.0% and 11.9% for thresholds of 4%, 8%, 10% and 12%, respectively, averaged across all seasons. 

Marketable pepper yields ranged between 515 and 1129 cartons/ac during the four years of study (Table 2). In 2005 and 2008, the treatment with threshold of 12% VWC achieved the highest marketable yield, however similar to the TIME irrigation treatment. The lowest yields were observed in the spring of 2006, which was due to the over irrigation of the treatments. The increase in overall pepper yield in 2008 compared to 2005 and 2007 was attributed to several combined factors. The volume of irrigation applied was lower in the last year compared to 2005 and 2007, which certainly reduced the NO3-N leaching. The use of SMS with reduced water rate resulted in a higher water use efficiency by pepper, in other words, these treatments yielded more with less water.

Table 2. Marketable yield (28 lb carton per acre), irrigation water applied (gallon per 100 ft of drip line) and irrigation water use efficiency (lb of marketable yield per irrigation water applied) of green bell pepper cultivated in spring of 2005 to 2008 in Citra, FL.
Irrigation Treatment	2005			2006			2007			2008
	Mkt. Yield cart/ac	Irrig. applied gal/100ft	IWUE lb/gal	Mkt. Yield cart/ac	Irrig. applied gal/100ft	IWUE lb/gal	Mkt. Yield cart/ac	Irrig. applied gal/100ft	IWUE lb/gal	Mkt. Yield cart/ac	Irrig. applied gal/100ft	IWUE lb/gal
4%	-	-	-	-	-	-	-	-	-	842 c	1,174	0.27a
8%	-	-	-	504	2,579	0.07a	941	2,376	0.15a	979 b	1,710	0.22b
10%	810	1,739	0.18a	482	4,782	0.04b	871	2,521	0.13b	-	-	-
12%	944	3,057	0.12b	415	4,579	0.03b	734	3,825	0.07c	1,129a	2,000	0.21b
TIME	734	4,796	0.06c	545	5,216	0.04b	817	4,622	0.07c	1,110a	4,941	0.09c

Figures 3 and 4 show the soil moisture content as measured by TDR probes and the occurrence of scheduled irrigation events and rainfall during given periods throughout the growing season. After each scheduled irrigation event, there is a noticeable increase in soil moisture content. The degree to which the soil moisture content increases, however, is dependent upon the irrigation treatment.  For example, all of the soil moisture sensor based treatments irrigated for short periods of time and resulted in a relatively small increase in soil moisture, consequently decreasing the volume of percolate (water moving below the root zone). The fixed TIME treatment; however, irrigated for a longer time period resulting in a relatively larger increase in soil moisture.  This spike in soil moisture appears to only be temporary, as the irrigation water rapidly drains down, ultimately bringing the soil moisture content back to where it was before the event in a relatively short period of time. This rapid spike in soil water content indicates that the soil water content as measured by the TDR probes rapidly reaches a point above the soil water holding capacity and the water starts to percolate down to deeper soil layers, explaining the higher percolate values due to fixed time daily irrigation events (TIME) compared to the other treatments. On the other hand, irrigation water from the soil moisture sensor based treatments resulted in relatively steady soil moisture content over time, because irrigation water was distributed across multiple irrigation events according to the soil moisture threshold.

Figure 3. Daily irrigation event for May 26 to June 2 of 2008 with scheduled irrigation events during early reproductive phase of green bell peppers.

 

Figure 4. Volumetric water content (VWC) in volume of water per volume of soil measured at 0-15 cm depth for June 23 to June 29 of 2006 with scheduled irrigation events and rainfall during harvest of green bell peppers. Horizontal lines indicate range of soil moisture contents where the soil moisture sensor control systems allowed irrigation events.

In conclusion, the results for pepper trials seem to confirm the previous findings from other projects that use of sensor based irrigation control techniques can greatly reduce water requirements by up to half as much without compromising pepper yield. For our experimental soil conditions, the best results in terms of water savings were obtained with set point of 10-12% VWC. Sensor based irrigation technology have been improved in the past year, in order to adapt the commercial controller designed for residential irrigation to an agricultural field with raised beds in sandy soils. In our case, the controllers and probes were adapted to our research site conditions.  However, proper placement of the sensor as well as the use of multiple sensors for relatively large irrigation blocks on farms will need to be investigated.