Thesis (PhD(Agric) (Agronomy))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: The use of capacitance water sensors for the scheduling of irrigation for hydroponic tomato and
cucumber crops grown in coir was investigated in a series of laboratory and glasshouse experiments
in the Free State province of South Africa.
Laboratory experiments in a climate controlled chamber were conducted to accurately calibrate
ECH2O capacitance sensors, models EC-10 and EC-20, in coir with an improved calibration
procedure. Water content predictions by the coir-specific calibration and manufacturer’s calibration
equations were compared to actual water content measured from mass loss of the coir sample. The
manufacturer’s calibration equation indicated a poor accuracy of prediction, which mostly
underestimated the volumetric water content, compared to the near perfect prediction of the
coir-specific calibration of individual sensors. A rapid calibration procedure for EC-10 and EC-20
sensors was proposed to reduce the calibration time of the sensors and promote their commercial
use for irrigation management in coir. The accuracy of prediction by the rapid calibration procedure
for the plant available water content range was high for both EC-10 and EC-20 sensors and allowed
for the compensation for variation between sensors.
Glasshouse studies aimed to characterise the water retention and ability of coir to supply water to
greenhouse tomato and cucumber crops through the continuous monitoring of medium water
content in small and large growing bags with the EC-10 and EC-20 capacitance sensors during a
drying cycle, compared to well-watered plants. Stages of crop water stress were identified and,
based only on the plant’s response to the drying cycle, it was suggested that water depletion can be
allowed to the point of mild water stress for both greenhouse tomato and cucumber crops, which can
be detected by soil water sensors. In a second series of glasshouse experiments, the identified stages
of crop water stress were used to determine and apply depletion levels in coir and compare this
irrigation strategy to a well-watered treatment for greenhouse cucumber and tomato plants, with
regard to the water balance components, yield and water use efficiency for different bag sizes.
Results indicated that irrigation was successfully managed to the pre-determined water depletion
levels for cucumber and tomato plants in coir, through the use of in situ calibrated capacitance
sensors. For both crops the depletion of water varied between bag sizes, indicating that various bag
sizes require different irrigation management strategies. Scheduling to the highest pre-determined by 124 L m-2 in the small and 240 L m-2 in the large bags for
cucumbers and 427 L m-2 in the small and 487 L m-2 in the large bags for tomato plants, compared
to the well-watered treatments. Yields achieved by the greenhouse tomato plants in the large
growing bags and cucumber plants in the small and large bags were maintained or improved when
scheduled to the highest depletion level (approximately 60% available water content) compared to
the well-watered treatment. The combination of reduced irrigation and improved or maintained
yields resulted in improved water use efficiencies (based on irrigation and transpiration) for the
highest depletion level compared to the well-watered treatments. In all glasshouse experiments the
well-watered treatment resulted in luxury water use by the plants.
Finally, a study was conducted in order to compare crop water stress of greenhouse cucumber and
tomato plants under luxury water supply and cyclic water deficit conditions. The comparison was
based on the transpiration ratio and yield, while the use of capacitance sensors was evaluated for
irrigation scheduling in coir for both crops. Transpiration data indicated that cucumber and tomato
plants subjected to luxury water supply experience water stress earlier than plants subjected to
cyclic water deficit conditions, irrespective of bag size. Results also indicated that irrigation
scheduling according to water depletion levels in small bags is not yet recommended for greenhouse
tomato and cucumber plants grown in coir, until further research is conducted. Scheduling to water
depletion levels in large bags is, however, justified by the improved or maintained yields of the
greenhouse cucumber and tomato plants. The estimated depletion levels for large bags beyond
which yield are reduced was at 85% for tomatoes and 70% for cucumbers.
In conclusion, the results clearly indicated that the use of capacitance sensors in large growing bags
improves irrigation management of hydroponic cucumbers and tomatoes in coir by eliminating
over-irrigation and improving water use efficiency. More research is needed before a conclusion can
be made regarding irrigation scheduling with capacitance sensors in small growing bags. / AFRIKAANSE OPSOMMING: Die gebruik van kapasitansie water sensors vir besproeiingskedulering van tamatie en komkommer
plante wat hidroponies in kokosveen gegroei is, is ondersoek in ‘n reeks laboratorium en glashuis
eksperimente in die Vrystaat provinsie van Suid Afrika.
Laboratorium eksperimente is uitgevoer in ‘n klimaat beheerde kas om ECH2O kapasitansie sensors,
modelle EC-10 en EC-20, akkuraat te kalibreer vir kokosveen deur ’n verbeterde kalibrasie
prosedure. Waterinhoud voorspellings deur die kokosveen spesifieke kalibrasie en die vervaardiger
se kalibrasie vergelykings is vergelyk met die werklike waterinhoud wat gemeet is deur die
kokosveen monster se massaverlies te monitor. Akkuraatheid van voorspelling deur die
vervaardiger se kalibrasie vergelykings was swak en het meestal die volumetriese waterinhoud
onderskat in vergelyking met die byna perfekte voorspelling deur die kokosveen spesifieke
kalibrasie van individuele sensors. ’n Vinnige kalibrasie prosedure vir die EC-10 en EC-20 sensors
is voorgestel om die kalibrasie tyd te verkort en die kommersiële gebruik van die sensors vir
besproeiingsbestuur in kokosveen aan te moedig. Die akkuraatheid van voorspelling deur die
vinnige kalibrasie prosedure, binne die grense van plant beskikbare waterinhoud, was hoog vir beide
EC-10 en EC-20 sensors, terwyl die prosedure ook voorsiening maak vir variasie tussen sensors.
Glashuis studies is uitgevoer om die water retensie en vermoë van kokosveen om water te voorsien
aan tamatie en komkommer gewasse in kweekhuise, te karakteriseer. Dit is bereik deur die
mediumwaterinhoud van klein en groot plantsakke deurlopend te monitor met behulp van die EC-10
en EC-20 kapasitansie sensors gedurende ’n uitdroging siklus, en dit te vergelyk met ’n waterryke
behandeling vir elke gewas waarvolgens die plante agt keer per dag besproei is. Fases van gewas
waterstremming is geïdentifiseer en, volgens die reaksie van die plant tot die drogingsiklus, is dit
voorgestel dat wateronttrekking toegelaat kan word tot die punt van matige waterstremming wat
aangewys kan word deur kapasitansie water sensors vir beide kweekhuis tamatie en komkommer
gewasse. In ’n tweede reeks glashuis eksperimente is die geïdentifiseerde fases van gewas
waterstremming gebruik om onttrekkingsvlakke vir kokosveen te bepaal en toe te pas as
besproeiingskeduleringstrategie vir kweekhuis komkommer en tamatie plante. Toegepaste vlakke is
vir elke gewas vergelyk met ’n waterryke behandeling ten opsigte van die waterbalans komponente,
opbrengs en watergebruiksdoeltreffendheid in verskillede sakgroottes. Resultate het aangedui dat
besproeiing suksesvol bestuur is tot die voorafbepaalde wateronttrekkingsvlakke vir komkommer entamatie plante in kokosveen, deur gebruik te maak van in situ gekalibreerde kapasitansie sensors.
Die onttrekking van water deur beide gewasse het verskil tussen klein en groot sakke, wat aangedui
het dat verskillende sakgroottes verskillende besproeiingsbestuur strategieë vereis. Skedulering tot
die hoogste voorafbepaalde onttrekkingsvlak het, in vergelyking met die waterryke behandelings,
besproeiing verminder met 124 L m-2 in die klein en 240 L m-2 in die groot sakke vir komkommers,
en 427 L m-2 in die klein en 487 L m-2 in die groot sakke vir tamatie plante. Opbrengste van
kweekhuis tamatie plante in die groot plantsakke en komkommer plante in die klein en groot sakke
is gehandhaaf of verbeter deur skedulering tot die hoogste onttrekkingsvlak (ongeveer 60% van
beskikbare water inhoud), in vergelyking met die waterryke behandeling. Die kombinasie van
verminderde besproeiing en verbeterde of gehandhaafde opbrengste het gelei tot verbeterde
watergebruiksdoeltreffendheid (besproeiing en transpirasie) vir die hoogste onttrekkingsvlak, in
vergelyking met die waterryke behandelings. In al die glashuis eksperimente het die waterryke
behandeling gelei tot oorvloedige watergebruik deur plante.
’n Finale studie is uitgevoer om gewas waterstremming van kweekhuis komkommer en tamatie
plante wat onderwerp is aan oorvloedige watervoorsiening deur agt keer per dag te besproei en
sikliese watertekorttoestande, te vergelyk. Die vergelyking is gebaseer op die transpirasie
verhouding en opbrengs, terwyl die gebruik van kapasitansie sensors vir besproeiingskedulering in
kokosveen vir beide gewasse geëvalueer is. Transpirasie data het aangedui dat komkommer en
tamatie plante wat onderwerp is aan oorvloedige watervoorsiening vroeër waterstremming ervaar as
plante wat onderwerp is aan sikliese watertekorttoestande, ongeag van die sakgrootte. Resultate het
aangedui dat besproeiingskedulering volgens wateronttrekkingsvlakke vir klein sakke nog nie
aanbeveel kan word vir kweekhuis tamatie en komkommer plante alvorens verdere navorsing
gedoen is nie. Skedulering tot wateronttrekkingsvlakke vir groot sakke word egter geregverdig deur
die verbeterde of gehandhaafde opbrengste van kweekhuis komkommers en tamaties. Die beraamde
laagste onttrekkingsvlakke vir groot sakke wat nie opbrengs betekenisvol sal beïnvloed nie is 85%
vir tamaties en 70% vir komkommers.
Ten slotte dui die resultate duidelik daarop dat die gebruik van kapasitansie sensors in groot
plantsakke besproeiingsbestuur van hidroponiese komkommers en tamaties in kokosveen verbeter
deur oorbesproeiing uit te skakel en die watergebruiksdoeltreffendheid te verbeter. Meer navorsing
is nodig alvorens ’n gevolgtrekking gemaak kan word ten opsigte van besproeiingskedulering met
kapasitansie sensors in klein plantsakke.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1086 |
| Date | 12 1900 |
| Creators | Van der Westhuizen, Rykie Jacoba |
| Contributors | Agenbag, G. A., Van Rensburg, L. D., Deckers, Ir. S., University of Stellenbosch. Faculty of Agrisciences. Dept. of Agronomy. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | Unknown |
| Type | Thesis |
| Rights | University of Stellenbosch |
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