Club of bologna proceedings

Fig. 
14
). This, however, is in large part a misleading 
evaluation of til lage effects, since it pertains to a 
transitory stage of soil modifications produced by 
deepening tillage (a real advantage for the N
utilization by plants can be seen in a more
complete exploration of the soil volume by the 
roots due to better soil aeration). Year after year 
the soil o.m. content and its related rate of
decomposition decline to a steady state level so 
that the physical effects of the structure
degradation become evident after a number of
decades. The contrary is expected when
introducing no
-
till manag ement: often higher N 
rates are req
u
ired in
i
tially and they can be
gradually reduced in later years. Soil tillage,
especially when deep, can initially promote
nutrient leaching through macropores and
preferential flow path. 
The interaction of tillage with phosphorus is
partially explained
b
y the above
-
mentioned 
aeration effect on mineralization of the P
-
organic 
fraction (
Fig. 15
). The depth of soil inverting
tillage (ploughing depth) is very important
because it distributes the P applied to top soil
layers at the beginning of the pr
e
vious gr owing 
season throughout the tilled profile; this implies 
accumulation of P in the top layers of no
-
tilled 
soils in contrast to rather uniform distribution
downwards, e.g. to 45 cm, with deep ploughing 
(see 
Fig. 13
). The surface concentration of P 
seems mor
e 
efficient for young seedlings,
especially for some crops (tomato, sugarbeet)
and early seeding (tomato), while P mixing down 
to a deeper profile is expected to be more useful 
for other crops (e.g. winter cereals), esp
e
cially in 
soils with initially low P
-
content. 
Many biological effects of different tillage
programmes have already been mentioned in the 
previous pages. The aeration mechanism strongly 
affects the soil microbial activity and the enzyme 
concentrations increase in parallel to this activity. 
Aerobic microbes normally show more intense 
activity; deli
v
ery of ammonia and subsequent
nitrate fo
r
mation are usually enhanced. When
large amounts of o.m. with high carbon to
nitrogen ratio (C/N) are available (say buried
wheat straw) the promotion of micro
b
ial growth 
more or less temporarily fixes the available N, 
which is in this case a limiting factor for microbial 
growth. When there is a lack of oxygen (residues 
buried in deeper layers) denitrification is
enhan-
ced. As regards propagation of soil
-
born 
diseases the effect of tillage is complex. Tillage 
might favour drainage and reduce root rotting 
or rhizomania. 
The inocula of foot and root rot diseases in
winter cereals are diluted and their attacks
reduced after deep ploughing
( 
Fig. 16
). 
No-
tillage and mu
l
ching usually show more
severe 
damage in years prone to average attacks
of t
hese diseases. 
A 60
-
year old experiment in Utah comparing
wheat stubble burning with its burying in the soil 
has shown that in the first 20 years burning gave 
a certain advantage (disease effect), while in the 
last 20 years the advantage was in burying the 
straw (organic matter
e 
ffect); the differences
were slight. 
The effect on the pedofauna is more unce
r
tain. 
Earthworms are normally very di
s
turbed by
tillage and the continuity of
the cylindrical pores 
they leave in the soil is i nterrupted. The hindrance 
to rodents, limax and some soil insects is usually 
an adva
n
tage. The effect on insects is still
controversial [2]. 

---

25
One of the most important effects of tillage is on 
weeds. Their control can be facilitated by: 
°
killing weeds existing at the time of the tillage, 
which is especially efficient when weeds have 
not yet produced seeds; a single tillage might 
favour weed seed germination, so that a
subsequent tillage can kill them, reducing
the 
seed bank (this is the principle of the Roman 
fallow) [17]; 
°
discouraging weed seed germination on a
rough soil surface left by primary ploughing;
°
leaving a uniform surface (after seco
n
dary
operations) on which herbicide can be
efficiently sprayed or incorporated (residue 
mulching reduces this applic ation). 
In general weed control is more difficult the
shallower is the tillage and with chiselling more 
than ploughing. Deep tillage usually reduces
certain species, especially perennial ones;
rhizomes and runner plants spread rapidly under 
roto-tillage. More frequent and shallow tillage
reduces the soil seed bank. Maximum seed bank 
concentration (4 to 5 times that of conventional 
tillage) and nu mber of species are found in the 
upper 10 cm of no
-
tilled 
s
oils; maximu
m differe-
ences in weed abundance on the field is
found, in 
I
taly, in late summer [32]. The weed problem is 
one of the main points of discussion in the choice 
of non
-
conventional tillage
programmes. 
Plant rooting can be affected by tillage. This
effect is however controversial for many crop
species and in many soils. Most neg
a
tive effects 
of no
-
tillage or minimum tillage due to mechanical 
impedance have been found in sugarbeet [12, 
14]. 
The effect on plant growth and yield are of
fundamental importance; these will be dealt with 
later.