5e-08-monitoring soil water content for irrigation scheduling carambola

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MONITORING SOIL WATER CONTENT FOR IRRIGATION SCHEDULINGIN A CARAMBOLA ORCHARD IN A GRAVELLY LIMESTONE SOIL

RASHIDAL-Y

AHYAI

AND

B

RUCE

S

CHAFFER

1

F

REDERICK

S. D

AVIES

. , capacitance probes,neutron probe, tensiometers

Multisensor capacitance probes, tensiometers, and aneutron probe were used for assessing soil water content forscheduling irrigation in an 8-year-old carambola (

Averrhoa

carambola

L.) orchard in Krome very gravelly loam soil insouth Florida. Four irrigation treatments were applied whensoil water content reached four different moisture set pointsexpressed in terms of percentage of field capacity as deter-mined with multisensor capacitance probes. The tensiometersand neutron probe gave a good estimation of absolute soil wa-ter content. The use of tensiometers was limited to a maximumsoil water tension of 20 cbar due to air entry into the water col-umn of the tensiometer and water column discharge. The useof a neutron probe by growers is not practical because its ra-dioactive source requires health and safety monitoring, and itis also labor intensive. Soil water content determined automat-ically and continuously with multisensor capacitance probesand computer software designed for irrigation scheduling canbe a practical method of irrigation scheduling in gravelly lime-

stone soils. However, capacitance probes are relatively expen-sive, labor intensive to install and maintain and gave variablereadings of absolute water content among sensors in thesame treatment. However, the rate of soil water depletion wasconsistent among probes. Since irrigation scheduling withmultisensor capacitance probes is based on the rate of soilwater depletion rather than the absolute soil water content,this method may be an effective tool for scheduling irrigationin orchards with Krome very gravelly loam soil. To achievethis, the pre-set soil water depletion rate at which to irrigatemust be related to plant vigor, growth and yield.

There are approximately 100 ha of carambolas in Florida(J. H. Crane, University of Florida, personal communication),of which 46 ha are in Miami-Dade County (Degner et al., 2002).

A sweet-type, Arkin, is the leading commercial carambola cul-tivar in Florida (Campbell, 1989; Crane, 1989; Crane, 1994;Lamberts and Crane, 1990; Nez-Elisea and Crane, 1998).

Scheduling irrigation is vital for commercial carambolaproduction in south Florida where most of the annual rainfaloccurs during the summer months. In 2001 86% and in 200279% of total rainfall occurred during the summer between Mayand October (Fig. 1). During the winter months, irrigation isessential to compensate for the lack of rainfall. Irrigation is alsorequired to compensate for the lack of water between unevenlydistributed rainfall events within a month. Excessive soil watercontent (Joyner and Schaffer, 1989) and drought (Ismail andNoor, 1996; Ismail et al., 1996; Salakpetch et al., 1990) haveadverse effects on carambola growth and yield.

The soil of the Miami-Dade County, where carambola iscultivated, is composed primarily of calcium carbonate (Degner et al., 1997) and classified as Krome very gravelly loamThis is a very shallow, mineral soil with a high pH of 7.4-8.4(Noble et al., 1996). This soil is extremely low in organic matter and commercial farming largely depends on fertilizer applications (Degner et al., 2002). The high demand for

fertilizer coupled with excessive irrigation creates a potentiafor agrochemical leaching into the groundwater (MuozCarpena et al., 2002; Zekri et al., 1999). In addition to reducing potential agrochemical leaching, scheduling irrigation toapply only the amount of water required by the plant shouldincrease grower returns by reducing fertilizer and water inputs, and improving plant growth and yields.

A survey by Li et al. (2000) in 1998 showed that 73% oftropical fruit growers in Miami-Dade County schedule irrigation based on the frequency and quantity of rain. The per-centage has declined to 64.3% in 2002 according to a morerecent survey by Muoz-Carpena et al. (2003). Monitoring osoil moisture for irrigation scheduling has increased to include 48.8% of the respondents to the 2002 water-use survey

(Muoz-Carpena et al., 2003) compared to only 15% in 1998(Li et al., 2000). According to the 1998 survey, methods of soimoisture determination included tensiometers, capacitanceprobes, digging and squeezing soil, and the feel and appearance of the soil (Li et al., 2000). The variability of durationand frequency of irrigation was high among tropical fruitgrowers, which highlights the need for a better understanding of irrigation requirements of these crops. With overheadhigh-volume sprinklers, irrigation was applied from one tothree times per week for one to 12 h per application (Li et al.2000). For microsprinklers, the frequency of operation wasvariable from 0.5 to 7.5 h per application and from one to seven applications per week. The amount of water applied pertree ranged from 110 to 2302 L with overhead sprinklers, 19

to 341 L per tree with microsprinklers, and from 7.6 to 45 Lper tree for drip irrigation (Li et al., 2000). The variability inresponses to irrigation quantities and frequencies used bygrowers can be attributed to the lack of basic quantitative information.

Tensiometers, neutron probes, and capacitance probesdirectly or indirectly monitor soil water content and are oftenused for irrigation scheduling. Tensiometers measure soisuction or matric water potential rather than soil water content (Richards, 1942; Smajstrla and Harrison, 1998). A soilwater retention curve must be established to determine the

The authors thank Drs. Jonathan Crane, Yuncong Li and Rafael Muoz-Carpena for critical review of this manuscript. The authors also thank AngelColls for assistance with installation of the capacitance probe system and Os-vany Rodriguez for orchard maintenance. This research was supported bythe Florida Agricultural Experiment Station, and approved for publication asJournal Series No. N-02362.

1

Corresponding author.


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soil water content that corresponds to the water matric poten-tial in order to estimate soil water content with a tensiometer.

The neutron probe is a portable compact unit that is easyto operate and volumetric soil water content can be obtainedinstantly at different depths of the soil profile. Neutronprobes are considered more accurate than other soil waterstatus monitoring devices for irrigation scheduling (Evett andSteiner, 1995; Mostert and Hoffman, 1996). The main com-

ponent of a neutron probe is a fast neutron source encased ina protective shield and an electronic counting scaler whichare connected by an electric cable that is also used to lowerthe probe into an access tube to determine soil moisture con-tent throughout the soil profile (Chanasyk and Naeth, 1996).Neutrons with a high energy are emitted by a radioactivesource, such as americium 241/beryllium, into the soil andare slowed down by collisions with nuclei, primarily hydrogenatoms (Gardner and Kirkham, 1952). The density of the re-sultant cloud of slow neutrons is a function of the soil watercontent (Chanasyk and Naeth, 1996). The number of fastneutrons that are slowed is detected and measured as a countrate per unit time (Gardner et al., 1991). The count rate isconverted to volumetric water content using a calibration

curve. Neutron probes measure soil volumetric water contentas the percentage of water per volume of soil.Capacitance probes measure the soil water content based

on the dielectric constant of the soil mixture (Paltineanu andStarr, 1997; Phene et al., 1990; Wu, 1998) a concept that wasfirst proposed for soil monitoring by time domain reflectom-etry (TDR) (Topp et al., 1980). The dielectric constant of thesoil is composed of the dielectric constants of water (80.4),soil particles (3-7) and air (1) (Paltineanu and Starr, 1997;Robinson and Dean, 1993; Starr and Paltineanu, 1998; Wu,1998). Since the dielectric constant of the soil particles and

air are small and relatively constant compared to that of water, changes in the dielectric constant of the soil are a measure of the change in soil water content. The volumetric watercontent can be expressed either as a percentage or a depth owater (mm of water/10 cm of soil) (Nez-Elisea et al., 2001Paltineanu and Starr, 1997; Starr and Paltineanu, 1998).

The objective of this study was to evaluate and comparetensiometers, multisensor capacitance probes, and a neutron

probe for accurately assessing soil water content for irrigationscheduling in a carambola orchard in Krome very gravellyloam soil.

Materials and Methods

The experiment was conducted in an orchard of 8-yearold Arkin carambola trees grafted onto open-pollinatedGolden Star rootstock at the Tropical Research and Education Center in Homestead, Fla. Trees were spaced at 4.5 mwithin rows and 6.1 m between rows.

Low-tension tensiometers (0 to 40 cbar) (Model LT; Irrometer Co., Inc., Riverside, Calif.) were calibrated prior to installation using a calibration vacuum chamber (Smajstrla and

Pitts, 1997) to ensure that the water column in the tensiometerwas air free, that there were no leaks, and to synchronize gaugereadings among tensiometers. One tensiometer was installed60 cm from the trunk of each of the three replicate trees ineach of the four treatments (defined in the following section)Tensiometers were installed at a depth of 10 cm below the soilsurface. Prior to installation, a hole was made in the soil, slightly larger in diameter than the tensiometer. A slurry, preparedwith sieved Krome very gravelly loam soil mixed with water, waapplied to the tensiometer hole to ensure that the ceramic cupof the tensiometer was in contact with the soil (Nez-Elisea

Fig. 1. Total monthly rainfall and evapotranspiration (ET) during 2001 and 2002 in Homestead, Fla. Source: Florida Automated Weather Network, IFASUniversity of Florida, Gainesville.


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et al., 2001). Tensiometers were maintained regularly in thefield using a hand-pump and water was refilled whenever itdrained from the tensiometer tube.

For neutron probe measurements, one 85-cm long poly-vinyl chloride (PVC) access tube was installed at 60 cm fromthe trunk of each of the three replicate trees in each of thefour treatments. The neutron probe (Model 503DR, Camp-bell Pacific Nuclear, Inc., Martinez, Calif.) was placed in theaccess tubes and lowered to 10, 20, 30, and 50 cm depths be-low the soil surface. Neutron probe readings were taken fromcounts per 16 s at each depth.

A multisensor capacitance probe system (EnviroSCAN,Sentek PTY Ltd., Kent Town, Australia) was used to automati-cally and continuously measure soil water content in eachtreatment. Prior to installation, data-loggers were configuredin the laboratory following the procedure described by Pal-tineanu and Starr (1997). Sensors were normalized to air andwater counts by placing the probe in a tube surrounded by wa-ter. One probe was installed inside a 74 cm-long PVC accesstube at 60 cm from the trunk of each of the three replicatetrees in each of the four treatments. Four sensors were placedin each probe at 10, 20, 30, and 50 cm below the soil surface.The PVC tubes were installed with a motorized drill and a slur-ry was then added to the hole. The slurry consisted of 2:1:1 byvolume of calcareous rock, cement, and water to prevent airgaps from forming between the tubes and the surrounding soil(Nez-Elisea et al., 2001). The sensors in each probe wereconnected to a data-logger powered by a 12-volt batterycharged with a solar panel. Data were recorded every 30 min-utes and downloaded from the data-logger to a portable laptopcomputer and graphs of soil water depletion rates at each soildepth and location were created with the EnviroScan software.

Trees were divided into four irrigation treatments basedon the field capacity determined with the multisensor capaci-tance probes. Treatments were based on the rate of soil waterdepletion between the field capacity and the permanent wilt-ing point of carambola trees (as measured with multisensorcapacitance probes in a preliminary experiment). The treat-ments were: 100-92%, 91-89%, 88-86%, or 85-83% soil waterdepletion below field capacity. Treatments were randomly dis-tributed in a completely randomized design with three repli-cations per treatment. When the soil water content reachedthe treatment range, trees in the treatment were irrigated us-ing microsprinklers (discharge rate = 89 Lhr

-1

) for one hour,which brought the soil water content to above field capacity.

Data from the three instruments were compared and ana-lyzed by linear and nonlinear regression and correlation tests.

Results and Discussion

A soil water retention curve was developed using the vanGenuchten (1980) model:

where

is the matric potential (suction or water tension)

r

is the residual water contents, and

s

is the saturated watercontent, and

, , and are fitting parameters directly depen-dant on the shape of the

(

) curve. Parameters (i.e.

r

,

s

,

,

, and ) for the van Genuchten model for soil water reten-tion curve of Krome soil were obtained from the relationshipbetween matric potential (

) measured by tensiometers andvolumetric water content (

) determined by neutron and ca-

pacitance probes (Table 1). Matric potential measured withtensiometer fitted the van Genuchten model better when volumetric soil water content was measured with a neutronprobe (r

2

= 0.42) than with multi-sensor capacitance probes(r

2

= 0.35) (Figs. 2 and 3, respectively). The fairly weak rela-tionship between soil water tension and volumetric soil watercontent in Krome very gravelly loam soils can be attributed tothe inaccuracy of tensiometer readings above a suction of 20cbar and heterogeneity of very gravelly Krome soils (NezElisea et al., 2001). Tensiometers installed at 10 cm below thesoil surface were not effective at a tension of above 20 cbar be

cause air entered into the suction cup through the large poresin the gravel and produced inaccurate measurements or waterdischarged completely from the tensiometer. In a previousstudy, tensiometers installed at a 10 cm depth in Krome verygravelly loam soil in south Florida were successfully used toschedule irrigation of tomato (Li et al., 1998). In contrast, aprevious study in tropical fruit orchards with the same soishowed that tensiometers were not effective in accurately estimating soil water potential at a depth of 30 cm below the soilsurface (Nez-Elisea et al., 2001). The difference betweenthe usefulness of tensiometers in the vegetable field and fruitorchard may have been a result of significantly larger soil particles in fruit orchards. In south Florida, vegetable fields arerock-plowed and repeatedly disked to break up the top layerof the soil (Colburn and Goldweber, 1961). Nez-Elisea et al(2001) reported that the lack of effectiveness of tensiometersin tropical fruit orchards at a depth of 30 cm below the soilsurface was attributed to the rockiness of Krome soil at thatdepth where 71 to 73% of the soil was gravel compared to 26%to 38% at in top 10 cm. Similar relationship between tensiometer readings and capacitance probes readings were also ob-served in the present study. In a laboratory measuredgravimetric water content and soil suction, Muoz-Carpena etal. (2002) reported that at soil suction above 10 cbar, watercontent in Krome very gravelly loam soil is relatively insensi-tive to tension changes, thus large changes in soil water tension reflect small changes in actual soil water content. In thepresent study, soil suction above 10 cbar resulted in variabletensiometer readings and discharge of the water column atabove 20 to 30 cbar. Thus, tensiometers are not very useful formonitoring soil water content for irrigation scheduling in carambola orchards in Krome very gravelly loam soil.

Volumetric soil water content determined with the neutron probe and capacitance probes were positively correlatedat all depths (Fig. 4). However, the correlation was not veryhigh, probably due to differences in the principles of operation of the devices, variability in soil microclimate around theaccess tubes, and the larger sphere of influence (volume ofsoil measured by the probe) measured by the neutron probethan for each capacitance sensor. Neutron probe measure

( ) r

s

r

( ) 1 ( )n+[ ] m+=

Table 1. Fitted parameters of van Genuchten model (1980) used to describesoil water retention curve of Krome soil where soil water content wasmeasured in a carambola orchard using neutron probe and multisensorcapacitance probes.

Parameters Neutron probe Capacitance probes

s

r

0.47580.2260.0292.670.625

0.47580.2290.05982.410.585


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ments close to the soil surface are not reliable (Chanasyk andNaeth, 1996; Cole, 1989; Gardner et al., 1991) due to escapeof fast neutrons to the atmosphere (Cuenca, 1989) as a result

of the height of the sphere of influence of neutron probes(approximately 20 cm; Chanasyk and Naeth, 1988). This ex-plains the overestimation of soil water content observed inthis study (Fig. 2) compared to values estimated by Muoz-Carpena et al. (2002) for the same soil. Of the three soil watermonitoring devices we tested, variability among readings atdifferent locations at the same depth were the lowest with theneutron probe [coefficient of variation (CV) = 11%] com-pared to tensiometers (CV = 17%) or capacitance probes (CV= 33%). Although neutron probe readings were less variablethan those obtained from capacitance probe and tensiome-ters, its use is less practical for irrigation scheduling due to theradioactive source of fast neutrons which can cause healthhazards if the neutron probe is not used properly. Moreover,

neutron probes operation and transportation require permitsand continuous monitoring for radiation leakage. Specificcalibration is required for accurately assessing actual volumet-

ric soil water content. The use of a neutron probe to monitorsoil water content in Krome soil is labor intensive because irequires installation of several access tubes using motorizeddrill and the manual taking of readings over a large area at different depths in the soil profile. Furthermore, the initial cosof the instrument is very high compared to tensiometers.

Several studies have shown that volumetric soil water content determined with multisensor capacitance probes was highly correlated with volumetric soil water content determinedgravimetrically (Paltineanu and Starr, 1997) in Mattapex silloam fine soil, in silty clay loam soils (Ould Mohamed et al.1997), in fine sand soils of Florida (Morgan et al., 1999), and inweathered heterogeneous soils (Wu, 1998). For citrus orchardin Candler fine sand soil, multisensor capacitance probes provided accurate indications of soil moisture content which wauseful for irrigation scheduling (Fares and Alva, 2000). It alsohas been proven a viable method of measuring soil water content for irrigation scheduling in avocado and Tahiti lime orchards in south Florida (Nez-Elisea et al., 2001; Nez-Eliseaet al., 2000; Zekri et al., 1999). However, in the present study

there were often variable readings among sensors at the samedepth receiving the same irrigation treatment. Inaccurate measurements of volumetric water content by capacitance probeswere reported by Hanson and Peters (2000) in silty loam andsilty clay soils in California, and Evett and Steiner (1995) in Amarillo fine sandy loam soil. Tomer and Anderson (1995) concluded that for coarse textured soils, changes in water contenare difficult to detect using capacitance probes. Although noproven, we speculate that increased capacitance probe variability in Krome soil over time could be attributed to temperatureextremes from the winter to the summer, soil wetness and drying cycles, long-term change in soil chemical and physical properties, and soil initial disturbance caused by trenches wheretropical fruit trees are planted in Krome very gravelly loam soils

Air gaps and disturbance of soil around the probe may lead toa change in soil bulk density that can produce measurement errors (Paltineanu and Starr, 1997). Throughout this study, lightning had a devastating effect on electrical connections andcapacitance sensors in carambola orchards even when the system was surge protected and a grounding rod was installed. Atimes, capacitance sensors briefly produced erroneous readings, such as sudden decreases in soil moisture readings, spikesor discontinuous readings. However, the system generally recovered from these temporary disturbances. Variability of capacitance probe readings in this study can also be attributed to

Fig. 2. Relationship between volumetric soil water content measured witha neutron probe and soil water tension measured with a tensiometer in a car-ambola orchard in Krome very gravelly loam soil. The response line was fittedusing van Genuchten (1980) equation (r2 = 0.42).

Fig. 3. Relationship between volumetric soil water content measured withmultisensor capacitance probes and soil water tension measured with a ten-siometer in a carambola orchard in Krome very gravelly loam soil. The re-sponse line was fitted using van Genuchten (1980) equation (r2 = 0.35).

Fig. 4. Relationship between soil water content (%) as measured by multisensor capacitance probes and a neutron probe in Krome very gravelly loamsoil (r2 = 0.35).


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a low battery or a damaged solar panel and condensation of wa-ter vapor in the probes that adversely affected the sensor read-ings. To reduce water vapor condensation in access tubes, silicagel desiccants were frequently replaced but due to the largetube, this may not be sufficient to maintain a dry climate insideeach probe.

A major advantage of irrigation scheduling using capaci-tance probe is that it is based on the rate of soil water deple-tion rather than on the absolute water content. Therefore, itreflects the change of soil water content over time and alsoplant activity, specifically evapotranspiration (Zekri et al.,

1999). Using Sentek EnviroSCAN software, the data arepresented graphically and options are provided for the userto divide the display into separate zones to manage soil watercontent. Multisensor capacitance probe systems can be effec-tive tools for irrigation scheduling if the cost of operation andmaintenance is financially justified.

In addition to tensiometers, neutron probes, and multi-sensor capacitance probes, there are several other methods ofmonitoring soil water status that include dielectric moisturesensors, volumetric water content measurement devices, andwater suction devices that can be used for irrigation schedul-ing. Several of these devices are currently being tested for usein Krome very gravelly loam soils at the Tropical Researchand Education Center in Homestead, Fla. Additionally, treevigor, growth, and productivity need to be correlated with soilwater content for accurate irrigation scheduling in orchards.

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