Club of bologna proceedings

)L
g. 12
[27]. The C.I. was 

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23
used to evaluate the drag forc
e op
posed by
tillage implements as well as an 
i 
ndex for soil
resistance to root development (e.g. C.I. < 2 
MPa is often found acceptable). 
As a consequence of the above
-
mentioned 
effects, soil moisture and balance are strongly
affected by type of tillage. This comes from:
°
soil drying facilitated by soil loosening;
°
enhanced water intake, especially when
tillage is combined with residue mulching and 
improved soil structure stability;
°
reducing direct evaporation from deeper
untilled soil layers (mulching effect from both 
soil mulching and residue
-
mulching); 
°
controlling weeds that disperse water taken 
up from deep layers. 
The effects on runoff and soil erosion after a
rainfall event mainly depend on slope, soil
infiltration rate and
s
oil cover (vegetation or
residue mulch) and from any ditches and their 
direction on the slope. The infiltration rate can be 
high in rough soil left after til
l
age, provided the 
soil has a stable clod structure and crust does not 
form. More than runoff, soi
l
erosion is reduced 
by vegetation and residue mulch (e.g. down to 
about 1-10%); large amounts of residues (e.g. 
from maize) are more efficient than small ones 
(e.g. soybean) [2]. 
Chemical effects of tillage are not immed
i
ate; the 
related processes, howeve
r
, start to develop at 
once and evolve over time. They are interpreted 
in terms of soil aeration (modes and intensity) and 
mechanical displacement of materials through the 
soil pr
o
file, the latter effect being magnified by 
soil
inversion or mixing operatio
n
s (
Fig. 13
). The 
most spectacular effect comes from 
e 
nhanced 
organic matter (o.m.) decay through the stages of 
humus formation and final mi
n
eralization 
(according to a first approxim
a
tion law o.m.
decreases asymptotically) [28]. It is essentially
from the
aeration promoted by tillage that this 
decrease in soil o.m. content (often to about one 
half to one third that of virgin soil) occurs over a 
number of decades; this usually implies
degradation of soil structure to a certain degree 
and
i 
ncreased need for
m
ore accurate and
intensive agronomic practices to maintain the
yield at satisfactory levels (an old Italian farming 
adage is that "once you start ploughing your soil, 
you have to plough it for ever"). The final steady 
state level depends on general pedocl
i
matic 
conditions and agronomic pra
c
tices; and almost 
all arable, still fertile land in the world is subject 
to this rule. If these general conditions are
unfavourable, ho wever, as may happen in some 
semi-arid areas, drought can follow the ploughing 
[29]. 
A
ll nutrients are gradually released; of