Agriculture Reference
In-Depth Information
penetration resistance
cone cross-sectional area
cone index =
8
6
4
2
0.1
0.3
1.0
1.2
1.4
1.6
0.5
1.8
dry
soil
bulk
den
sity
(tons
per
m
)
Fig. 7.3
The effect of bulk density and water content on the cone index (From Sun et al.
2003
,
altered)
7.2.1.5
Resistance to Penetration
Soil resistance to penetration has for a long time been an object of investigations,
e.g.
in search of a
substitute
for bulk density. Traditionally, probes with a sensing
cone on the tip are pushed
vertically
into the soil. Yet this mode of operation hardly
is suitable for on-the-go sensing. Therefore, the interest has shifted to probes, which
move in a
horizontal
direction through the soil and can easily be drawn by a tractor
(Lüth
1993
). So continuous- instead of punctual signals are recorded. Adamchuk
et al. (
2008
) placed several horizontally oriented sensing cones that were staggered
along a vertical blade for simultaneous recording of the horizontal resistance at
several depths below the soil surface.
The signals obtained from penetrometers are strongly infl uenced by the
water
content
of the soil regardless of the sensing direction. The results shown in Fig.
7.3
are based on a cone index, which is penetration resistance divided by cone cross
sectional area. The effect of the water content of the soil on the cone index is several
times higher than the infl uence of the bulk density.
This explains why it is attempted to sense resistance to penetration and water
content simultaneously. The objective in doing this is to correct the cone index data
for the infl uence of the water content (Sun et al.
2003
; Hartung and Drücker
2009
).
Without such corrections, the resistance to penetration cannot provide reliable sig-
nals to control the cultivation depth, since the water content of a soil can vary
considerably on a temporal- as well as on a spatial basis.
Notwithstanding the infl uence of the water content, penetrometers are well suit-
able to detect
hardpans
in soils,
e.g.
those that might exist below the topsoil as a
