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Mechanical weeding effects on soil structure
under organic vegetables
Bruce C. Ball
Abstract - Mechanical weeding of vegetables involves
the repeated use of cultivators and tractors. This
repeated tillage and compaction changes soil structure. We studied these changes in fields of organic
beans and carrots. Structure was described using
visual assessment and by measuring soil strength.
Under beans, the soil strength below the growing root
zone was highly variable and in some areas was
strong enough to restrict root growth. The carrots
were grown in beds containing zones of crop and
bare, weeded soil. The structure of the weeded soil in
the top 10 cm layer eventually became disrupted and
compacted enough to deter root growth. In addition
the topsoil and subsoil in the wheel-tracks between
the beds became very compact with little distinguishable structure. This compaction extended to the subsoil and persisted into the next cropping season. 1
INTRODUCTION
Assessments of mechanical weeding to date have
concentrated on the effect on the weeds and/or crop
productivity. Studies on soil appear to have been
limited to soil compaction and nitrogen dynamics.
Little work appears to have been done on soil structure and other soil physical attributes. Soil structure
can be defined as the spatial arrangement or heterogeneity of soil particles, aggregates and voids or
pores (Carter, 2004). The characteristics of the
pores determine the rate of movement, storage,
uptake and loss of nutrients as gases and liquids in
the soil. The soil particles and aggregates possess a
resistance to breakdown and penetration called soil
strength. The maintenance of adequate pore characteristics and soil strength is important to optimise
uptake of nutrients, penetration by roots and resistance to erosion.
Since chemicals cannot be used to control weeds,
organic vegetables rely heavily on mechanical weeding. This involves frequent passes by machinery and
wheels leading to excessive tillage and compaction
which can lead to deterioration of soil structure. This
can reduce soil quality and crop quality and yield.
Here we explored the influence of tillage and traffic
associated with mechanical weeding on soil structure
and strength for beans and carrots. We chose beans
because of the observed variability in crop height
during early growth and we chose carrots because of
the numerous tractor operations required during
their production.
Bruce C. Ball is with the Scottish Agricultural College, Crop and Soil
Research Group, West Mains Road, Edinburgh, EH9 3JG, Scotland
([email protected]).
MATERIALS AND METHODS
Sites
Both sites were in Scotland on land fully converted
to organic status. The site with broad beans (Vicia
faba L.) was located in East Lothian. We made our
measurements in April 2003, 3 days after the first
weeding by a light spring-tine harrow. The site with
carrots (Daucus carota L.) was located in Perthshire.
We made our measurements in June 2004 after
weeding by flame, brush and blade and in July 2004
after a further 2 or 3 passes of the blade weeder.
Soil textures were sandy loam or loam.
Soil structure and strength measurements
Soil structure was assessed using a visual and tactile
technique designed for use in Scottish soils (Ball and
Douglas, 2003). The size, porosity and strength of
the aggregates present on a spadeful of soil were
assessed subjectively and used to allocate a score
between 1 and 10. Score 1 is for a massive structure
of very low porosity where roots and water cannot
flow through. Score 10 is for loose aggregates such
as those visible under grass grown in ideal conditions. Soil strength was measured either as resistance to shear using a vane tester or as resistance to penetration by a cone using a penetrometer. Soil compaction status was estimated by
measuring dry bulk density.
RESULTS
Broad beans
Crop emergence and subsequent height and the
population density varied across the direction of the
tramlines (6m spacing) used by the tractor pulling
the weeder. Visual inspection of the soil revealed
that most of the bean roots were in the top 5 cm of
soil, even though the bean plants were 15 cm tall.
Visual inspection also revealed that soil aggregates
were bigger and stronger below 10 cm depth than
above. The vane shear strength at 12 cm depth, the
strongest layer in the topsoil, was greater and varied
considerably across the direction of travel of the
farm equipment used to establish and to weed the
crop. Some of the vane shear strengths were close
to or exceeded 65 kPa, the limit beyond which root
growth is likely to be impaired (Ball & O’Sullivan,
1982).
In the early season (June), soil physical conditions (Table 1) were favourable in the sandy loam.
Structure was loose in the weeded areas. The only
areas where any restriction to rooting was likely
were in the tramlines and in parts of crop bed adjacent to these tramlines.
ured cone resistance in the next season when the
field was sown to peas and barley. Cone resistances
were high at 30-45 cm depth (Fig. 1) indicating that
subsoil compaction from the carrots had persisted.
The structure in both topsoil and subsoil was also
poor, with low macroporosity. This, combined with
presence of the straw used to bed the carrots, made
the soil anaerobic, resulting in patchy, uneven
growth of the crop.
Table 1. Bulk density and soil strength (centred at 5 cm
depth) and topsoil structure on two occasions in 2004 in
mechanically weeded areas adjacent to rows of carrots,
sandy loam soil.
Soil property
11th June
23rd July
1.09
1.2
6.8
23
3
Bulk density (Mg/m )
Vane shear strength (kPa)
Structure score (out of 10)
8.4
6.6
1-35
1-15
Loose-friable
Friable
Aggregate size (mm)
Firmness
Later on, in July, bulk density (Table 1) and vane
shear strength had increased and soil structure had
deteriorated in the weeded areas. In the loam soil
in July, soil structure had also deteriorated to score
4.5. The soil was compact, breaking into large aggregates and clods of diameter 5 – 100 mm. The soil
in this weeded area was also strong as shown by the
high cone resistances at 6 cm depth in comparison
to the cropped area (Fig. 1).
Cone resistance (kPa)
3000
Bed, cropped
Bed, weeded
2500
2000
1500
1000
500
0
0
10
20
30
40
50
60
ACKNOWLEDGEMENT
I acknowledge the support of the Scottish Executive
Environmental and Rural Affairs Department. John
Taylor of Sidlaw Organics and Angus MacDowall of
Mungo’s Wells provided access to their fields and
helpful discussions.
3000
2500
Cone resistance (kPa)
DISCUSSION AND CONCLUSIONS
Repeated mechanical weeding gave some structural
deterioration in the zone of loosening. However,
compaction by wheels either as part of the mechanical weeding mechanism or, more importantly, by the
tractor pulling the weeder, was the main cause of
either soil damage or of compaction sufficient to
restrict root penetration. The tracks between the
beds of carrots tended to widen with use causing
some deformation of the carrots growing at the edge
of the beds. This also resulted in up to 25% of the
field receiving compaction which extended into the
subsoil. Subsoil compaction affects drainage, root
penetration and environmental protection on many
soil types (Jones et al., 2003). Natural loosening
does not penetrate to subsoil depth and loosening by
subsoiling is often only partially effective. Since the
compaction and associated structural damage is
hard to remove, this represents a threat to soil quality. Possible methods to avoid compaction and structural damage are to use wider beds (controlled traffic), to control the positioning of the tractor more
exactly between the beds (wire guidance techniques
are possible) and to use the weeder less frequently.
Also small tractors with even weight distribution
over both axles should be used. The weeder should
be used only in relatively dry conditions when the
soil is less compactible. The use of weeders which do
not include rollers might also be considered.
2000
1500
REFERENCES
Ball, B.C. and Douglas, J.T. (2003). Soil Use and
Management 19: 53-56.
1000
500
Ball, B.C. and O’Sullivan, M.F. (1982). Journal of Soil
Science 33: 609-622.
0
0
20
40
60
80
Soil depth (cm)
Figure 1. Cone resistance in the loam under carrots in July
2004 (upper) and, one year later at the same location in
May 2005 (lower). The vertical bars represent 2SE.
This probably resulted from the repeated tillage
during weeding. Cone resistance was very high in
the middle of the tramline as expected. Here the soil
structure was massive and very compact. We meas-
Carter, M.R. (2004). Soil & Tillage Research 79: 1-6.
Jones, R.J.A. Spoor, G. and Thomasson,
(2003). Soil & Tillage Research 73: 131-143.
A.J.