A multi-factor study of cabbage production in the Umlaas River valley.
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Date
1995
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Abstract
A research and extension programme monitored 59 cabbage crops and many factors
associated with cabbage production in the Umlaas River Valley of KwaZulu-Natal, from July
1991 - June 1993. A large data bank was developed, accumulating all information regarding
soils, weather patterns, farming practices, management, crop water stress, yields, plant
populations, weeds, pests, diseases, soil fertility, leaf nutrient concentrations and the presence
of hormone herbicides in the rain. Analysis of this large data bank of information enabled
the identification of limiting factors and optimized production practices.
Excessively high or low levels of soil nutrients and soil acidity problems, reduced cabbage
yields in the Umlaas River Valley. Identification of critical values and an ideal cation ratio
in soils, and recommendations to rectify limiting factors, could drastically improve yields.
Wide variation and excesses of soil nutrients were seen from soil analysis of 59 cabbage
crops in the Umlaas River Valley. Soil P > 155, K > 486 and Mg > 568 mg 1(-1),
exchangeable acidity> 0.11 cmol(c) 1(-1), acid saturation> 2 % and pH < 4.51 resulted in
lower yields. Top yields were found between a certain range of soil nutrient levels and
critical values lay within this range. Optimal yields resulted with an ideal ratio of Ca, Mg
and K (65 :25: 10) in plots monitored. A soil acidity complex affected cabbage yields in the
Umlaas River Valley. High exchangeable acidity, acid saturation and low pH increased Mn
and Zn availability in the soil. Both Mn and Zn were absorbed in large quantities by the
plant and maximum levels of 406 mg kg(-1) and 114 mg kg(-1) respectively were recorded in
most recently mature (MRM) cabbage leaves at headform. Mn concentrations > 100 mg
kg(-1) at headform and 50 mg kg(-1) at harvest reduced yields.
Nutrient analysis results of a MRM leaf at headform, and a 1/8 head slice of a mature head
at harvest, generated leaf nutrient norms and crop removal figures for cabbage. Adequate
nutrient levels of MRM leaf at head form were: N = 3.3 - 4.8 %, P = 0.32 - 0.55 %, K
= 2.1 - 4.2 %, Ca = 1.3 - 2.5 %, Mg = 0.25 - 0.65 %, Mn = 15 - 100 mg kg(-1), Zn =
18 - 60 mg kg(-1) and Cu = 4 - 100 mg kg(-1). Nutrient concentrations above or below adequate
ranges resulted in lower yields. Nutrient removal figures were: 1.9 kg N, 0.3 kg P, 2.03
kg K, 0.43 kg Ca, 0.19 kg Mg, 2.8 g Mn, 2.6 g Zn and 0.4 g Cu t(-1) fresh material. These
leaf nutrient norms were developed for reliable identification of soil fertility problems and
the crop removals for improved fertilizer recommendations.
A study of fertilization practices showed that most farmers supplied enough (+ 200kg) N to
cabbage crops but used incorrect pre- and postplant proportions (1/4 and 3/4) for top yields.
For maximum yields, 2/3 of the total N should be applied preplant and 1/3 at 4-6 weeks.
Topdressings of N should be applied once only and not split as is common practise in the
area. Application of P on cabbage crops was proportional to soil requirements but was
generally excessive. Sufficient quantities of K were applied but scant attention was given to
soil analysis results for K, Ca and Mg requirements. Input costs could be decreased and high
yields maintained if soils were fertilized up to critical values and no further nutrients added
when soil test values exceeded critical values. Fertilization was not generally based on
results of soil analysis and few farmers limed their soils in spite of soil acidity problems.
The more acidifying NH4 containing fertilisers were mostly used in preplant applications.
The study recommended using more N0(3) based fertilisers to slow down acidification or, at
least, corrective applications of lime.
Good pest and disease control is essential for maximum yields in the Umlaas River Valley.
This study identified optimum infection periods and maximum and minimum temperatures
for infection by blackrot, club root , ringspot, damping-off, sclerotinia, downy mildew and
alternaria leaf spot; and evaluated disease effects on yields. Clubroot, blackrot and damping-off
significantly reduced cabbage yields. Clubroot incidence was generally associated with
soil acidity problems, waterlogged lands or sandy soils subjected to slight over-irrigation.
Blackrot was reduced by adequate N and K fertilization, increased with higher concentrations
of Mn in most recently mature leaves (MRM) at head form (also associated with soil acidity)
and always occurred with the warm, wet conditions of summer.
Aphids and thrips occurred throughout the year; cutworm, bollworm, greater cabbage moth,
leafminer and diamond-back moth, occurred mostly during the warmer season; and webworm
over the cooler period in autumn. Aphids, thrips and webworm were the most important
cabbage pests, causing serious losses especially during the first six weeks after transplanting.
Factors such as amounts of fertilizer applied, soil acidity problems, slope of the land,
infiltration rate, soil nutrient status, water stress, leaf concentrations of each element, weeds
and disease were all associated with the incidence of various pests. This study highlighted
the importance of good management practices, which affect all these aspects of cabbage
production, and therefore pest incidence. An awareness of these factors and
recommendations made in this study, could improve cabbage management practices in the
Umlaas River Valley and therefore, reduce pest infestation.
The study also evaluated relationships between weather patterns, hormone herbicide-like
symptoms which occurred simultaneously on various garden trees and shrubs and vegetable
seedlings, and pest and disease outbreaks or hormone herbicide symptoms on field crops of
cabbage. Hot days with temperatures> 30°C, high VPD and high solar radiation followed
by a rapid drop in temperature, high relative humidity and low solar radiation (stress weather
cycles) were characteristic of weather conditions with an approaching frontal low in the
Umlaas River Valley. All the dates of hormone herbicide-like symptoms on some garden
shrubs and trees and vegetable seedlings in the nursery (indicator plants) were correlated with
stress weather cycles, however not all stress weather cycles were correlated to these
symptoms on indicator plants. It appeared that an unknown factor (Factor X) occurred with
these weather cycles, and together with the stress induced by the harsh weather conditions,
caused these symptoms on the indicator plants. A Canary creeper (Senecio tamoides DC.),
fig tree (Ficus natalensis Hochst.) and an indigenous Cape chestnut (Calodendrum capense
(L.F.) Thunb.) showed symptoms of leaf bubbling, twisting, burning, deformities and leaf
drop. Lettuce and cabbage seedlings were twisted, etiolated and suffered from downy
mildew outbreaks. All trees, shrubs and nursery crops experienced these symptoms 1 - 4
days after the stress weather cycles and Factor X. It is possible that low levels of hormone
herbicide < 25 ng 1(-1)) deposited during dynamic fumigations (associated with approaching
frontal lows), were Factor X. However, when examining the relationship between field
cabbages and all related factors, no clear correlation could be established between weather
patterns, Factor X, cabbage plant health, and pest and disease incidence.
An overall study examined the effects of a range of factors on cabbage yield including: soil
analysis data; management and crop water stress ratings; total nutrients supplied; weed, pest
and disease indices; tillage operations; soil characteristics; plant populations; percentage
marketable yield and headmass. Poor water management, high incidence of weeds, pest and
disease, excessive nitrogen and phosphorus applied as fertilizer, and too many tillage
operations, were associated with low yields. Conversely, good water management, good
liming and soil fertility practices, higher levels of soil potassium and better weed, pest and
disease control, resulted in higher yields. A greater percentage of marketable cabbage heads
was more important than head mass for higher cabbage yields.
The methodology used in this study provided an efficient, reliable method of identifying
factors which limited and maximized vegetable crop production and provided data for the
compilation of production guidelines for cabbage. The research and extension aspects of this
study also provided an opportunity for the rapid transfer of technology to the farmer and
excellent farmer participation.
Description
Thesis (Ph.D. Agric.)-University of Natal, Pietermaritzburg, 1995.
Keywords
Cabbage--KwaZulu-Natal., Cabbage--Diseases and pests--KwaZulu-Natal., Cabbage--Yields--KwaZulu-Natal., Cabbage--Nutrition--KwaZulu-Natal., Theses--Horticultural science.