Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Soilless production of crops relies on the addition of high concentrations of nutrients with the irrigation
water. The drained nutrient solution should be re-used to reduce the risk of pollution and to increase
the water- and nutrient use efficiency of the system. Besides the risk of pathogen build-up, one of the
main impediments of a wider application of this method is the frequent analysis required to maintain
optimum nutrient concentrations and ratios in the rootzone. Yield reductions may be caused by an
unbalanced nutrient solution.
Alternatively the addition level of nutrients can be calculated through the use of nutrient uptake models
that simulate the change in the re-circulated nutrient solution. To simulate crop water and nutrient
demand necessary for model based regulation it was necessary to quantify the key factors affecting
nutrient uptake by plants.
The nutrient solution concentration and ratios between the macro-nutrients affected the uptake of
water and nutrients. The total nutrient uptake per root dry weight increased and more specifically the
nitrate (NO3
-), phosphate (H2PO4
-), potassium (K+) and sulphate (SO4
2-) uptake increased with an
increase in nutrient solution electrical conductivity (EC) from 0.8 to 4.0 mS cm-1 while water uptake
decreased. Except for Ca2+ uptake there was no correlation between nutrient and water uptake.
Nutrient uptake can thus not be calculated based on water uptake. Instead a mechanistic high-affinity
Michaelis-Menten based model can be used to estimate macro-nutrient uptake (Un, mg m-2 hr-1).
Water and nutrient uptake was also affected by the solar radiation levels. Since nutrient uptake is
related to the growth rate, solar radiation levels can be expected to influence nutrient uptake. The
uptake of all ions increased with an increase in the solar radiation levels and for NO3
-, K+ and H2PO4
-
the uptake rate was higher at higher nutrient solution concentrations. The Michaelis-Menten based
model was adjusted to incorporate the effect of solar radiation levels on nutrient uptake. Water uptake
(Wu, L m-2 day-1) was simulated as a function of crop transpiration and crop leaf area using a linear
regression model, but since leaf area development was affected by solar radiation levels this was
additionally incorporated into the estimation of the leaf area index (LAI).
The composition of the nutrient solution also affected the biomass allocation of the crop which can again affect nutrient use as well as the fruit yield. There was also a direct effect of nutrient solution composition on fruit yield and quality with higher EC’s resulting in smaller fruit but an increase in fruit
dry matter %, total soluble solids (TSS), titratable acidity (TA) and lycopene content.
The results in this thesis make a valuable contribution to our understanding of the effect of nutrient
availability (concentration and ratios) and nutrient requirement for growth (solar radiation levels) on
nutrient uptake. Incorporating these into nutrient uptake models resulted in the development of a
handy tool to simulate changes in composition of re-circulating nutrient solutions ultimately resulting in
an improvement of the water and nutrient use efficiency of soilless systems. / AFRIKAANSE OPSOMMING: Die grondlose verbouing van gewasse is afhanklik van toediening van voedingselemente teen hoë
peile in die besproeiingswater. Die voedingsoplossing wat dreineer moet hergebruik word om die
risiko van besoedeling te verminder en ook om die water en nutriënt verbruik doeltreffendheid van die
sisteem te verbeter. ʼn Ongebalanseerde voedingsoplossing kan ʼn verlaging in opbrengste veroorsaak.
Benewens die risiko van patogene wat opbou, is die gereelde analises nodig word vir die handhawing
van optimale nutriënt konsentrasies en verhouding tussen elemente in die wortelsone een van die
hoof faktore wat ʼn meer algemene gebruik van die metode verhoed.
Alternatiewelik kan die nutriënt toedieningspeile bereken word deur voedingstof opname modelle en
simulasie van die verandering in water en nutriente wat dreineer. Om ʼn model gebaseerde
reguleringsmetode daar te stel was dit nodig om die belangrikste faktore wat nutriënt opname
beïnvloed te kwantifiseer.
Beide die konsentrasie van die voedingsoplossing en die verhouding tussen elemente het ‘n effek
gehad op die opname van water en nutriënte. Die totale nutriënt opname per wortel droë massa het
toegeneem. Terwyl water opname afgeneem het met ‘n toename in die elektriese geleding (EG) van
die voedingsoplossing vanaf 0.8 tot 4.0 mS cm-1 het die nitraat (NO3
-), fosfaat (H2PO4
-), kalium (K+) en
sulfaat (SO4
2-) opname verhoog. Behalwe vir Ca2+ opname was daar geen korrelasie tussen water en
nutriënt opname nie. Nutriënt opname kan dus nie bepaal word gebaseer op wateropname nie.
Alternatiewelik is die gebruik van ʼn meganistiese hoë-affiniteit Michaelis-Menten-gebaseerde model
voorgestel om die opname van makro-nutriente (Un, mg m-2 hr-1) te bepaal.
Water- en voedingstofopname is beinvloed deur die ligintensiteit vlakke. Voedingsopname word
bepaal deur die groei van die plant, daarom is dit verwag dat ligintensiteit vlakke die opname van
voedingstowwe sal beïnvloed. Die opname van al die ione het toegeneem met 'n toename in die
ligintensiteit vlakke en die tempo van NO3
-, K+ en H2PO4
- opname was hoër by 'n hoër
voedingsoplossing konsentrasie. Die Michaelis-Menten gebaseerde model is aangepas om die effek
van ligintensiteit vlakke op nutriënt opname te inkorporeer. Opname van water (Wu, L m-2 dag-1) is
gesimuleer as 'n funksie van transpirasie en blaaroppervlakte met behulp van 'n lineêre
regressiemodel en aangesien die blaaroppervlak ontwikkeling ook deur ligintensiteit vlakke beïnvloed
word, is dit opgeneem in die skatting van die blaaroppervlakte-indeks (LAI).
Die samestelling van die voedingsoplossing het die biomassa verspreiding beïnvloed. Dit kan nutriënt
gebruik en vrug opbrengs beïnvloed. Die voedingsoplossing samestelling het vrug opbrengs en -
kwaliteit beinvloed met kleiner vrugte, maar 'n toename in droëmateriaal %, totale oplosbare
vastestowwe (TOVS), titreerbare suur (TA) en likopeen inhoud by ʼn hoër EG.
Die resultate in hierdie tesis lewer 'n waardevolle bydrae tot ons begrip van die effek van nutriënt
beskikbaarheid (konsentrasie en verhoudings) en voedingstof behoefte vir groei (ligintensiteit vlakke)
op voedingsopname. Deur die inligting te inkorporeer in voedingsopname modelle het gelei tot die
ontwikkeling van 'n handige instrument om die veranderinge in die samestelling van hersirkulerende
voedingsoplossings te simuleer. Dit lei gevolglik tot die verbetering van die water en voedingstof
gebruik doeltreffendheid van grondlose stelsels.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/97988 |
| Date | 12 1900 |
| Creators | Kempen, Estelle |
| Contributors | Agenbag, G. A., Deckers, Ir. S., Stellenbosch University. Faculty of Agrisciences. Dept. of Agronomy. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 185 pages : illustrations |
| Rights | Stellenbosch University |
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