Thesis (MScAgric)--University of Stellenbosch, 2000 / ENGLISH ABSTRACT: Approximately 7000 ha of potatoes are cultivated under irrigation. The greatest part of
the planting is entered under the South African Certification Scheme as seed potatoes,
representing between 35 and 40 % of seed production in South Africa. The production of
high-quality tubers is, therefore, in the interest of the entire country. Some production
factors in the area, however, have a very negative influence on quality with a resultant
increase in input costs. These factors include mainly low clay content of the soil «5 %),
low organic matter content (% C = 0,2 to 0,4 %), a low pH value as well as a high level
of chlorides (400 mg.kg") in the irrigation water. Nitrogen conversion as well as uptake
can be influenced by these factors. There may also be negative influences on the
environment if leaching of nitrates to subterranean water sources is involved, which can
pose a threat to sensitive protected areas. Leaching of various nutritional elements occurs
as a result of the low clay content of the soils. Over-irrigation or intense thundershowers
could lead to the leaching of nearly all of the fertilisers from these soils. Various
management aids are therefore needed to facilitate the decision-making process for
corrective steps. Soil, leaf or total-plant analyses are possible, but it is a time-consuming
process and the results could become available too late to be of any use. Consequently,
the development of methods to determine the nutritional element status of plants on the
land and the establishment of norms are both essential. A method which has been tried on
a wide scale with varying results is the use of the nitrate content of the petiole, which is
determined by means of a reflectoquant and nitrate strips. To evaluate the sensitivity of
the method two sites in the Sandveld have been selected. A trial planting has been
established at Wadrift and information was also obtained from a leaching trial at
Sandberg. The cultivars Up-to-Date and Hertha were established in four nitrogen
treatments (100, 200, 300 and 400 kg N.ha-1
), and repeated over two seasons. At
Sandberg irrigation was applied at three application levels and three fertigation divisions
for one season. Drip irrigation was applied in both cases to enable the accurate distribution of water and fertilisers during the planting season. Petiole samples were
taken weekly or every second week in all the trials. The first mature leaf was sampled
(about 20 to 30 leaves per plot). During the initial planting at Wadrift nitrate N in petiole
sap (measured by means of nitrate strips and a reflectoquant) was compared to nitrate N
of dried petioles measured with an auto-analyser. In all test cases a good correlation was
found. The readings from the auto-analyser were about 14.5 times higher than the
readings from the petiole sap as a result of moisture loss during the drying process. A
better correlation was obtained during the second season when the petiole sap was used
for strips as well as the auto-analyser. The petiole nitrate content was increased by high N
application rates and the 400 kg N.ha-1 treatment showed significant differences from the
other N treatments. A substantially higher yield was obtained during the last season.
Varying results were obtained regarding specific gravity for cultivars and nitrogen
treatments. At Sandberg (1998) over-irrigation lowered the nitrate content of the petiole
sap significantly. It was found that the optimum irrigation level and frequency, is two
fertigations a week and kept at field water capacity. From the trial it was clear that the
use of nitrogen strips in combination with the reflectoquant gave a good indication of the
nitrate-N content of the petiole sap. It is, however, difficult to lay down a norm or critical
range, as conditions during the growth period on sandy soil as well as the growth stage
have a significant influence on the nitrate-No If yield is the only objective the 400 kg
N.ha-1 curve can probably be used as an optimum nitrate-N norm for the particular
cultivars. It is also not clear whether the nitrate N content of the petiole sap gives a good
reflection of growth and yield potential. / AFRIKAANSE OPSOMMING: In die Sandveld word bykans 7000ha aartappels onder besproeiing verbou. Die grootste
deel van die aanplanting word as saadaartappels onder die Suid-Afrikaanse
sertifiseringskema ingeskryf en verteenwoordig tussen 35 en 40% van Suid Afrika se
saadproduksie. Die produksie van hoë kwaliteit knolle is dus van landsbelang.
Somminge produksiefaktore van die area het egter 'n uiters negatiewe invloed op
kwaliteit en het 'n meegaande verhoging van insetkostes tot gevolg. Faktore wat veral
hieronder tel is die grond se lae klei inhoud «5%), lae organiese materiaal inhoud (%C =
0.2 tot 0.4%), lae pH asook hoë chloriede (400mg.kg-1
) in die besproeiingswater. Hierdie
faktore kan 'n invloed op die omsetting van stikstof en die opname daarvan hê. Daarmee
saam kan dit 'n negatiewe effek op die omgewing hê indien loging van nitrate na
ondergrondse waterbronne plaasvind wat sensitiewe bewaarareas kan bedreig. Weens die
lae klei-inhoud van die gronde vind loging van verskeie voedingselemente plaas.
Oorbesproeiing of hewige reënbuie kan tot gevolg hê dat bykans alle bemestingstowwe
uit hierdie grond geloog word. Bestuurshulpmiddels is dus nodig om die
besluitnemingsproses vir korrektiewe stappe te vergemaklik. Grond-, blaar- of hele
plantontledings is moontlik maar is tydrowend en resultate daarvan kan soms te laat
beskikbaar wees. Die ontwikkeling van metodes om die voedingselement inhoud van
plante op die land te bepaal en die daarstelling van normes is dus noodsaaklik. 'n Metode
wat reeds wyd deur ander navorsers met wisselende resultate beproef is, is die gebruik
van nitraat inhoud van die petiool wat met 'n reflektoquant en nitraat strokies bepaal is.
Om die sensitiwiteit van die metode te evalueer is twee proefpersele in die Sandveld
gebruik. 'n Proef is by Wadrift geplant terwyl data ook van 'n logingsproef te Sandberg
verkry is. By Wadrift is die kultivars Up-to-Date en Hertha by 4 N-behandelings (100,
200,300 en 400 kg N.ha-1) en vier herhalings vir twee seisoene aangeplant. By Sandberg
is besproeiings teen drie toedingspeile met 3 sproeibemestingsverdelings vir een seisoen
gedoen. In alle gevalle is van drupbesproeiing gebruik gemaak om akkurate plasing van
water en bemestingstowwe oor die plantseisoen moontlik te maak. Petiool monsters is
weekliks of tweeweekliks van alle behandelings geneem. Die eerste volwasse blaar is
gemonster en daar is tussen 20 en 30 blare per perseel geneem. Tydens die eerste planting te Wadrift is nitraat-N in petioolsap, gemeet met nitraatstrokies en reflektoquant,
vergelyk met nitraat-N van gedroogde petiole, gemeet met In outo-analiseerder. Hierdie
waardes was goed gekorreleerd vir alle gevalle wat getoets is. Die outo-analiseerder se
lesing was ±14.5 keer hoër as die petioolsap lesings weens die vog wat tydens droging
verwyder is. Tydens die tweede seisoen is In beter korrelasie verkry waar petioolsap vir
strokies asook vir die outo-analiseerder gebruik is. Petiool nitraat is deur hoë Ntoedieningspeile
verhoog en die 400 kg N.ha-I behandeling het betekenisvol van die
ander N-behandelings verskil en die laaste seisoen ook In betekenisvol hoër opbrengs
gelewer. Wisselende resultate is t.o.v. soortlike gewig vir kultivars en
stikstofbehandelings verkry. By Sandberg (1998) is gevind dat oorbesproeiing In
betekenisvolle verlagende effek op die nitraat-N inhoud in die petioolsap het. Daar is
getoon dat die optimum besproeiingspeil en frekwensie, veldwaterkapasiteit met twee
sproeibemestings per week is. Uit die ondersoek is dit duidelik dat die nitraatstrokies in
kombinasie met die reflektoquant In uiters bruikbare aanduiding van die nitraat-N in die
petioolsap gee. Dit is egter moeilik om In norm of kritiese nitraat-N grens aan te dui,
aangesien toestande tydens die groeiperiode op sandgrond en die groeistadium In
beduidende effek op die nitraat-N het. Indien opbrengs as enigste doelwit gestel word,
mag die 400 kg N.ha-I kurwe moontlik as aanduiding van 'n optimum nitraat-N norm vir
die betrokke kultivars dien. Daar bestaan ook onduidelikheid of nitraat-N in die
petioolsap wel In goeie refleksie van groei en opbrengspotensiaal is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/51728 |
| Date | 04 1900 |
| Creators | Brink, Pieter Paul |
| Contributors | Combrink, N. J. J., Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | af_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 61 p. : ill. |
| Rights | Stellenbosch University |
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