The influence of conservation tillage practices on soil microbial communities and
soilborne diseases of potato
1
Carlson, René (1), Van Zyl, Jacques (2)
( ) Potatoes South Africa, Private Bag X135, Pretoria, 0001, South Africa
2
( ) Department of Agriculture: Western Cape, Private Bag X1, Elsenburg, 7607, South Africa
Abstract
Soilborne diseases of potato including common scab, powdery scab and black scurf and plant-parasitic nematodes pose a severe threat
to potato production as chemical control is either ineffective or non-existent. For this reason, potato production is increasingly reliant
on soil fumigation. The use of synthetic fumigants not only increases production costs, but also has a treadmill effect in that it severely
disrupts the soil ecosystem and repeated application in increased dosage is needed to achieve desired pest control. Such practices are
highly non-sustainable and the resulting ecological disturbances to the production system have devastating effects in the long run.
Through fumigation the soil is sterilised and beneficial micro-organisms are in so doing, destroyed. Pathogenic organisms that are
introduced by planting material proliferate under these sterile conditions where no competition is offered by beneficial microorganisms. This is especially problematic for the potato industry as potatoes are clonally propagated by planting whole seed tubers,
which promotes the spreading of pathogens from one area to another. A more sustainable approach needs to be investigated.
To address these and other concerns, a long-term conservation farming project was initiated in February 2013 in the Sandveld, the
second-largest potato producing region in South Africa. Treatments consist of three tillage regimes namely mouldboard plough
(maximum disturbance), rip plough (intermediate disturbance) and para plough (minimum disturbance). The aim is to evaluate the
effect on potato yield, the physical, biological and chemical status of the soil, and water use efficiency in the potato production system.
The biological part of this study investigates the influence of conservation tillage on indigenous soil microbial communities and the
incidence of soilborne diseases of potato and if a correlation exists between these. Conservation tillage is known to suppress soilborne
diseases by increasing organic carbon and indirectly influencing indigenous microbial communities. These changes in the microbial
community may lead to suppression of pathogen and plant-parasitic nematode populations. Culture-dependent and -independent
methods were used to assess the changes in microbial populations. Microbial activity was measured by measuring enzyme activities,
microbial diversity by measuring soil carbon substrate utilisation using the BIOLOG® system and microbial density by soil dilution
plating.
Results from the first season indicate a positive correlation between conservation tillage and active carbon, soil respiration, potato
yield, free-living nematodes as well as microbial diversity and a negative correlation with plant parasitic nematodes and common scab
incidence.
Keywords: conservation tillage, potato production, organic carbon, microbial diversity, common scab
1. INTRODUCTION
Conservation tillage embraces crop production systems involving the management of surface residues.
These practices and cover crop management have the potential to avoid the degradation of soil properties in
order to maintain crop yields as well as ecosystem stability. Conservation tillage provides the best opportunity
for halting degradation and restoring and improving soil productivity. The presence of crop residues may,
however, cause constraints which could reduce potato yield due to mechanical interference with the planting
operation, allelopathic or residue derived toxins, the promotion of soil-borne diseases and pests and the
reduction in the efficacy of fertilisers and pesticides. Alternative tillage methods are favoured due to the
negative effects of excessive tillage in conventional tillage practices in some of the potato producing areas in
the world. The frequent cultivation of fields removes plant residues and organic matter, causing increased rates
of soil mineralisation and results in losses of soil organic carbon, impacting negatively on soil quality. Effective
and active soil microorganism populations support and improve the sustainability of crop production systems
but require sufficiently high soil organic carbon content to sustain their populations in the soil.
Tillage plays a major role in modification of soil structure as it influences the distribution of energy rich
organic matter within the soil profile and thus impacts on the energy flow and the dynamics of soil geo-
chemical functions. Thus the modification of soil structure by tillage has implications for the organisation of
soil ecosystems. Tillage is a system component that is easily subject to manipulation leading to either positive
or negative consequences for agricultural sustainability. The pivotal role of soil tillage, in the context of
agricultural sustainability, is due to its ability to manipulate soil properties and processes and change soil
quality. Tillage, in relation to soil quality, can influence the capacity of the soil to store and cycle water,
nutrients and energy.
2. MATERIALS AND METHODS
2.1 Tillage treatments
The project consisted of three tillage treatments as follows:
2.1.1 Conventional tillage
Irrigate up to FWC before tilling
Mouldboard plough up to a depth of 350 mm
Level with a ghrop and broadcast fertiliser
Shearplough 600 mm deep between planting rows
Plant potatoes
Shearplough 600 mm deep after plant in tyre tracks
2.1.2 Conservation tillage: Rip
Irrigate up to FWC before tilling
Broadcast fertiliser + scarify
Shearplough 600 mm deep between planting rows
Plant potatoes
Shearplough 600 mm deep after plant in tyre tracks
2.1.3 Conservation tillage: Paraplough
Irrigate up to FWC before tilling
Broadcast fertiliser + scarify
Plant potatoes
Paraplough 600 mm deep between planting rows
2.2 Crop
The potato cultivar Mondial was planted on the 19th February 2013, very good emergence was achieved
and it was harvested on the 18th July 2013.
2.3 Nematode counts
During the season sampling was done twice, one before plant and again 10 weeks after emergence.
Nematode counts were done by using the method described by Jenkins (1964).
2.4 Soil penetrometer readings
Soil penetrometer readings were done after the tillage treatments were applied and again 10 weeks after
emergence. It was recorded with a Geotron P5 penetrometer.
2.5 Microbial activity parameters
2.5.1 Soil respiration
Soil respiration (CO2) samples were taken in February 2013 before commencement of treatments and again
10 weeks after emergence. Samples were weighed (20-20.010g) into glass vials of which the volume was
known. The vial was closed and left for 30min to create headspace. After 30min the gas was drawn through
septa in the lid using a gas tight syringe. The agilent GC was used in this procedure and fitted with a thermal
conductivity detector to measure CO2 and O2. The machine was calibrated using a standard containing 7.9%
CO2 and 11.9% O2.
2.5.2 Active carbon
Soil samples were taken 10 weeks after emergence and the active carbon content was determined by using
the method described by Weil et al. (2003).
2.6 Microbial diversity parameters
2.6.1 Microbial species variation
Soil samples were taken before commencement of treatments and again 10 weeks after emergence. The
soil samples were diluted in sterile distilled water (1:3,000) and inoculated into Biolog EcoPlatesTM (Biolog®
Inc., Hayward, USA) containing 31 carbon sources and a control well, in triplicate. The plates were incubated at
28 °C. Respiration of carbon sources by microbial populations reduce the tetrazolium dye, causing a colour
change which was measured twice daily over a period of 5-10 days at 590 nm to determine average well colour
development. The functional diversity of the soil microbial populations was determined using the amount and
equitability of carbon substrates metabolized as indicators of richness and evenness, respectively (Garland &
Mills, 1991; Winding & Hendriksen, 1997).
2.6.2 Eveness index
Soil samples were taken before commencement of treatments and again 10 weeks after emergence. The
Evenness index is used as an indication of how abundant species are within a soil microbial community, i.e.,
how close in “numbers” each microbial species are in a soil microbial community. If abundances / quantities of
different species in a community are measured, it will invariably be found that some species are rare, whereas
others are more abundant / dominant. Substrate evenness assumes a value between 0 and 1, with 1
representing a situation in which all species are equally abundant within a microbial population present in the
samples. This means less variation in microbial populations between species, thus, less dominance, and higher
diversity (Arias et al., 2005).
2.7 Yield parameters
The number of plants per square meter was calculated at mid-season. Potatoes were harvested and the
yield, size distribution and specific gravity (Nissen, 1967) were determined. Potatoes were sorted into five size
distribution classes namely large (above 250g), large medium (150-250g), medium (90-170g), small (50-100g)
and extra small (below 50g).
2.8 Quality parameters
Harvested tubers were evaluated for internal disorders and blemish diseases. Twenty tubers per replicate or
1440 tubers per treatment were evaluated. The percentage shown in the results is thus an indication of the
number of tubers showing symptoms of disease or internal disorders.
3. RESULTS AND DISCUSSION
The results obtained in the first season were very informative. Nematode sampling was done on each plot
before planting and it was established that a relative high number of free living nematodes were present. The
only plant parasitic nematodes that were extracted were the lesion and spiral nematodes in very low numbers.
The nematode samples taken at midseason of the potato growth period showed no significant differences
between the tillage treatments when free living nematodes were considered (Fig. 1). The number of plant
parasitic nematodes was significantly higher in the maximum tillage treatment compared to the other two
tillage treatments and it was especially the root-knot nematodes that were lower in rip and paraplough
treatments. Low numbers of spiral and lesion nematodes were found in the paraplough treatment.
Fig. 1. Number of free-living and plant parasitic nematodes (means followed by the same letter did not differ
significantly at the 5% level) under conventional tillage (maximum disturbance) in black, rip (intermediate
disturbance) in white and paraplough (minimum disturbance) in black and white.
The general compaction before plant was 3073 kPa at a depth of 30 cm, 3543kPa at 40 cm and above 5000
kPa in 90% of the sample points at a depth of 50 cm. After tillage treatments were performed and planting
were completed the paraplough treatment had a significant higher compaction up to a depth of 30 cm when
compared to the other two tillage treatments. The depths of 40 and 50 cm showed no significant differences
between all the tillage treatments and from a depth of 60 to 80 cm the paraplough treatment showed
significantly less compaction when compared to the other two tillage treatments (Fig. 2).
Fig. 2. Soil penetrometer readings (means followed by the same letter for each depth did not differ significantly
at the 5% level) under conventional tillage (maximum disturbance) in black, rip (intermediate disturbance) in
white and paraplough (minimum disturbance) in black and white.
By increasing the soils organic matter content, minimum tillage has a significant influence on soil microbial
communities. The activity, diversity and abundance of these microbial communities have been correlated with
disease suppression in potato and for this reason were evaluated in this trial (Peters et al., 2003).
Soil respiration (CO2) samples, as indication of biological activity, was taken in February 2013 before
commencement of treatments and was on average 5.36 mg/kg/h. The soil biological activity was significantly
higher in the paraplough treatment, ten weeks after emergence, when compared to the other two treatments
which at this preliminary stage probably indicates that the paraplough treatment enhances biological activity in
the soil due to less soil disturbance (Fig. 3). The soil respiration in the paraplough treatment rose from 5.360
mg/kg/h to 7.099 mg/kg/h in the season, respiration however dropped in the other treatments from 5.360
mg/kg/h to 5.271 mg/kg/h for the rip treatment and to 5.288 for the maximum tillage (mouldboard) treatment.
In addition to this, although not statistically significant, conservation tillage also positively affected active
carbon levels in the soil (Fig. 4). Since active carbon is an indicator of the fraction of soil organic matter that is
readily available as a carbon and energy source for the soil microbial community, this may have had a direct
influence on microbial diversity.
Fig. 3. Soil respiration (means followed by the same letter did not differ significantly at the 5% level) under
conventional tillage (maximum disturbance) in black, rip (intermediate disturbance) in white and paraplough
(minimum disturbance) in black and white.
Fig. 4. Active carbon under conventional tillage (maximum disturbance) in black, rip (intermediate disturbance)
in white and paraplough (minimum disturbance) in black and white.
Catabolic diversity of bacterial communities was determined by sole-carbon substrate utilisation, using the
Biolog® system. In the Biolog® system microbial communities give a characteristic reaction pattern, called a
metabolic fingerprint where from carbon source utilisation profiles (CSUP) are generated. By using the CSUP an
Evenness Index (EI) is generated and is as an indication of how abundant species are within a soil microbial
community. EI was influenced significantly by conservation tillage especially during emergence of the potato
plants (Fig. 5).
Fig. 5. Evenness index illustrating soil microbial species variation (means followed by the same letter did not
differ significantly at the 5% level) under conventional tillage (maximum disturbance) in black, rip (intermediate
disturbance) in white and paraplough (minimum disturbance) in black and white.
The yield and size distribution is shown in Figure 6. The paraplough treatment showed significantly the
highest yield with 61.2 t/ha. The intermediate (rip) treatment showed the lowest yield with 53.75 t/ha. The
paraplough treatment showed significantly higher yields of large-medium and medium sized tubers. The
specific gravity of the paraplough treatment differed significantly from the maximum tillage treatment and was
1.05621, it did not differ from the intermediate (rip) treatment (Fig. 7). The number of plants was significantly
higher in the paraplough treatment with 5.06 plants/m2, the other treatments did not differ significantly from
each other and were 4.48 and 4.40 plants/m2 for the intermediate and maximum tillage treatments
respectively (Fig. 8). Plant analyses of the petiole of the fourth leaf from the growing tip were done at
midseason to determine the elemental status of the plants in the different treatments. Significant differences
were only found in the case of phosphorus and iron where the paraplough treatment gave the highest values,
i.e. 0.649 % and 194.17 mg/kg respectively.
Fig. 6. Yield and size distribution (means followed by the same letter did not differ significantly at the 5% level)
under conventional tillage (maximum disturbance), rip (intermediate disturbance and paraplough (minimum
disturbance). The size distribution is arranged as Large, Large Medium, Medium, Small and Extra small, from
the bottom upwards or from dark grey to white.
Fig. 7. Specific gravity (means followed by the same letter did not differ significantly at the 5% level) under
conventional tillage (maximum disturbance) in black, rip (intermediate disturbance) in white and paraplough
(minimum disturbance) in black and white.
Fig. 8. Number of plants per square meter (means followed by the same letter did not differ significantly at the
5% level) under conventional tillage (maximum disturbance) in black, rip (intermediate disturbance) in white
and paraplough (minimum disturbance) in black and white.
Harvested tubers were evaluated for internal disorders and diseases. Although no internal disorders were
found, Fusarium stem-end rot (Fusarium oxysporum and F. solani) and common scab (Streptomyces spp.) did
occur abundantly on harvested tubers. Tillage treatments did not influence the incidence of Fusarium stem-end
rot (data not shown), but did the occurrence of common scab (Fig. 9). Lower levels of common scab were
observed on tubers harvested from the paraplough treatment.
Fig. 9. Common scab incidence (means followed by the same letter did not differ significantly at the 5% level)
under conventional tillage (maximum disturbance) in black, rip (intermediate disturbance) in white and
paraplough (minimum disturbance) in black and white.
4. CONCLUSIONS
Results from the first season indicate a positive correlation between conservation tillage and active carbon,
soil respiration, potato yield, free-living nematodes as well as microbial diversity and a negative correlation
with plant parasitic nematodes and common scab incidence. These results will be of great significance if the
trends are to be repeated in the following years as this could influence the tillage method implemented in the
future to achieve a more sustainable potato production system.
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