Club of bologna proceedings

Fig. 
19
). Deeper tillages are often adopted down 
to 40
50 cm, according to crop, and with
alternating depth in different years; 
°
minimum tillage is generally useful for
reducing tillage costs and soil compaction in 
conditions. When substantial amounts of crop 
residues ar e left on the soil su
r
face, this
should be considered as a co
n
servation 
system and can be reco mmended on sloping 
soils to control er
o
sion; this criterion is also 
valid for diffe rent mulch tillage systems (e.g. 
shallow tillage with no inversion of winter
cereal stubble). Herbicide requirements
increase. In a colder climate the surface
mulch can hinder or delay seed germin
a
tion, 
especially with earlier planting, due to
temperature effects; it can be useful in
tropical climates. In soils with no special
drainage problems, when winter is the dry
season, autumn tillage should not be deep
ploughing, to prevent leaving large clods on 
the surface to dry out and b
e
come hard over 
the winter. Minimum tillage is often applied in 
orchards; 
°
the adoption of ridge
-
furrow tillage is strictly 
limited to certain highly specia
l 
ized crop
rotations (mainly maiz
e-
soybean or
continuous maize); it has gained some
acceptance in the USA corn belt. It provides 
surface drainage and good top soil structure 
especially in the furrow bottom.
Different
versions exist, more or less stable over the 
years; 
°
paraploughing and strip tillage (cultiv
a
tion in 
the inter
-
row zone only) are somewhat more 
sophisticated systems and do not appear to 
be widely accepted; they are in any case only 
suitable for large interrow distance; 
°
no-tillage is attractive for its simplicity. It
consists of mulching and requires sp
e
cialized 
sowing implements; it relies on the highest use 
of herbicides and co
m
plementary fertilization 
(mainly nitr
o
gen). It is the most efficient
in 
controlling erosion on slopes. It requires a 
certain number of years to reach a steady
state in the soil system and is not suitable for 
soils with drainage problems nor with certain 
crops. It frequently gives good
r
esults for 
planting second crops (e.g . maize). A special 
case for no
-
tillage is orchards, where deep 
tillage is obviously excluded (except at
planting) and topsoil structure is of paramount 
importance.
I
nstead of a high use of
herbicides, oper ators favour grass cover,
which in a humid climate
and on wet soils is 
not only sui
t
able for soil trafficability and
structure stability, but also for drying excess 
water from the soil. In dry climates its use is 
limited by water competition between the
grass cover and the trees. 
Special problems might ar
i
se when selecting a 
tillage system. In the case of sod breaking, one 
has to consider the advisability of deep to very 
deep (1.2 m) ploughing once and for all to
favourably modify the soil texture by mixing, or to 

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29
favour leaching in saline soils, or to bre
a
k 
hardpans, etc.; the latter obje
c
tive can be
reached with the use of rippers. When no serious 
problems exist no initial deep ploughing seems 
suitable in soils with good o.m. content in the 
upper layers. When the soil has just been drained 
from a we
t
land, the deeper soil layers have a 
very low redoxy potential and deep ploughing
gives very bad results initially; it would be wiser 
to chisel the soil rather deeply plus a seco
n
dary 
very shallow inverting ploughing (in the following 
years the depth of this plo
u
ghing could be
gradually increased until good soil aeration
throughout the soil profile is reached).
In managing soil tillage, the basic criteria are:
°
to understand which is the best soil status for 
tilling (soil workability); 
°
what to do when the soil is 
n
ot at opt
i
mum 
workability. 
The first question is not the subject of this report; 
it is theoretically one of the most complex
problems, depending on soil moi
s
ture, texture, 
structure, content type and state of soil colloids, 
plasticity, cohesion and adhesi
o
n, working
implements and requir
e
ments for the final seed 
bed. 
Of great interest is the way of tackling inc
i
dental 
unfavourable workability conditions. Very often 
experimental requirements press for the adoption 
of a previously precisely
d 
esigned tillage
programme for each "trea
t
ment"; deviations from 
the scheduled proc
H
dure are however frequently 
imposed by weather conditions and one can face 
contingent problems, reducing the damage to a 
minimum, through reduction of mechanical impact 
(e.g. passing from
d 
eep ploughing to shallow
ploughing or chiselling or discing or sweeping or 
even planting on no
-
tilled soil), provided that an 
increase in
N
-fertilization and herbicide use is 
ensured (see
Fig. 14
). Higher rates of
N 
-
fertilizers and a "stronger" herbicide c
o
ntrol tend 
to suppress differences among tillage methods [8] 
(
Fig. 20
). 
In practice the modern operator feels more free. 
In general, when optimizing an equ
a
tion like n. 1, 
he prefers to choose a suff
i
ciently flexible system 
among those just discussed (this is the reason e.g. 
for the scarce diffusion of the ridge system
outside the USA corn belt). An important aspect 
of this flexibility is to permit adaptation to
different conditions (crop requirements, seasonal 
events, occasional concentration of labour an
d
machinery demand) without an excessive amount 
of costly equipment (like those ne
c
essary for a 
good direct seeding); the chosen system should 
allow to face highest number of conditions
occurring in a given ped
o-
climatic situation. In 
areas where conve ntional ploughing is usually the 
most advisable method, tools for mimimum tillage 
are easily found, since they are also used for 
secondary tillage; in the case of a very rainy
tillage season, shallow tillage without inversion is 
frequently applied, since the s oil upper layers dry 
more easily and quickly to reach wor
k
ability 
conditions. 
Apart from this deterministic evaluation of a
contingent problem, the operator has to face a 
more subtle uncertain aspect: how the weather 
will be in the immediate future, up unt
i
l seeding. 
In some climates there is a high probability of soil 
freezing at least for a few days and down to a 
sufficient depth. This gives the possibility of
delaying any secondary tillage until later, taking 
advantage of its effects; in a warmer climate
the 
fitting of tillage operations into a suitable period 
of the year, soon after or soon before the dry 
season, can be very important (autumn deep
ploughing before a dry winter season exposes to 
the danger of a very rough hard cloddy field, 
difficult to
p 
repare for seeding before next
spring). 
Finally, after flexibility, reliability is an a
t
tractive 
trait. Mean differences in yield among tillage
systems can be negligible, but the spread of
yields around their means can be wide or narrow. 
This implies that 
t
he risk of a relatively lower 
yield in some years can be very different within 
similar mean yields. Even in the case of a normal 
distribution of yields around their means, low

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3 0
yields have an asymmetrical economic weight: low 
yields have more important con sequences. For this 
reason the operator (especially one on a small family 
farm and/or in economically under-developed 
agriculture) is prone to choose the system which 
ensures the most stable yield over the years. This 
seems to be one of the reasons for the preference for 
the additional tillage programmes.