Club of bologna proceedings

Fig. 4
). These changes affect not 
only macropores (pores with diameter 

> 2 

m), but also micropores ( 
Fig. 5 
), 
demonstrating an indirect chemical action. 
Soil structure is a complex soil trait of great 
agronomic interest and still uncertainly 
measurable. Its most commonly determined aspect 
(one of the many) is its stability. The Meyer wet 
seiving method (modified many times) 
combined with Hénin pretreatment criteria,
shows that in general the deeper the ploughing, 
the weaker the soil structure ( 
Fig. 6
); the most 
stable structure is usually found in the top layer 
under no-tillage. Structure stability changes 
during the year and responds to different 
implements, as shown in 
Fig. 7
; it is usually 
higher in non-saline clay soils and in soils rich in 
organic matter or under a vegetation cover. Some 
soils are particularly prone to losing their 
aggregate structure at the soil surface, under the 
action of rain and weather; they form a crust 
which reduces the water infiltration rate, promotes 
runoff or water-logging and severely damages 
seed emergence; 
Fig. 8 
shows the results of 
laboratory tests on a soil under different tillage 
managements. 
The tilled soil layer dries rapidly, reducing 
evaporation from underneath; this mulching effect 
gradually disappears, more rapidly in structurally 
unstable soils, so that later the no-tilled soils 
usually have wetter topsoil (this might have a 
favourable effect on yield in dry climates, but an 
unfavourable one in wet conditions). 
The effect on soil temperature is indirect, mainly 
depending on soil moisture. In general the mean 
soil temperature in the upper layer increases after 
tillage and this is mainly associated with higher 
maximum daily temperature, so that the daily 
excursion also increases [12]. The topsoil 
temperature is however much more affected by 
residue mulching combined into the tillage 
programme. Large amounts of residues can be left 
on the soil surface by some crops (e.g. maize, but 
not by others, such as sugarbeet or soybean) with 
no-tillage or tilling without soil inversion. The 
mulch affects temperature in many ways, some 
materials (straw) reflect a consistent amount of 
solar radiation. Within the mulch layer the water 
vapour gradient is reduced so that the 
corresponding flux (Flick's law) and evaporation 
are also reduced [20], so the soil surface is wetter; 
this increases the thermal inertia of the soil (the 
opposite effect to tilled not-mulched soils). These 
conditions improve seed germination and 
emergence wherever too high soil temperature is a 
problem (tropical environments), while they can 
dangerously delay seed germination and seedling 
development in colder temperate regions. Where 
the temperature drops beneath 0°C so il freezing 

---

22
affects the soil structure through a complex
mechanism which depends on the initial soil
moisture, thermal conductivity, soil hydra
u
lic 
conductivity curve and type of soil mine
r
als [21]. 
At optimal moisture content and soil freezing
conditions results in a uniform excellent crumbly 
structure (
Fig. 9
) which doesn't, however, last 
long under rainfall action, but can be very useful 
for spring seeding; in the worst cases the soil
becomes muddy at thawing.
Texture and soil structure are closely relat
e
d to 
pore size distribution and this implies a given
water retention curve and changes to this basic 
soil trait as a function of tillage and, even more 
so, of the soil management package [22]; 
Fig. 10
shows examples of such changes and their
variation ove
r
time. The hydraulic conductivity
curve is similarly affected; it changes through the 
soil profile and as a function of the tillage
programme in an inverse way compared to the 
bulk density
(
Fig. 11
). Soil tillage tends to
destroy ma cropore continuity (e.g . those left by 
earthworms or root decay). This has an
especially negative effect on the hydraulic
conductivity at least when the macroporosity
initially cr
e
ated by the tillage reduces after
subsequent soil compaction. 
Soil compaction occurs in part after
a
lternate 
wetting and drying processes; at the soil surface 
it is enhanced by raindrop impact so that the
resulting small soil particles are dragged down to 
the larger pores beneath and seal the latter,
contributing to compaction; this mechanism is not 
yet adequately understood (it appears relevant 
even in untilled soils). One of the most effective 
causes of compa ction is wheeled traffic; during a 
single trip it does not normally affect the whole 
surface, but often subsequent passes, unless
carefully progra mmed, can compact practically
the entire soil surface [10; 23]. Note that in
certain cases this overall compaction can be
useful for seed development, acting like rolling. 
A lot of research has been done on this in the 
last two decades, both experimentally
and
theoretically, and the effect of the weight of
different machines, different tyres and tec
h
niques 
have been examined. A criterium has been
proposed [24] to assess the influence of cultural 
systems on soil compaction and decompaction. 
Possibilities for
reducing traffic compaction rely 
on:
°
lower tractor and machine weight; 
°
fewer trips across the field; 
°
smaller area affected by the traffic e.g.
running all trips along the same tracks as in 
the permanent ridge technique [24]. 
With conventional ploughing or
chiselling this
possibility is limited to one cropping season due 
to track obliterations year after year. Some crops 
(e.g. soybean) are more sensitive to compaction 
than others (maize), for which shallower tillage is 
needed. 
The influence of soil compacti
o
n on plant growth, 
as well as on the mechanical strength opposed by 
the soil to tillage
i 
mplements, has long been
reported. Soil co
m
paction reduces the larger
pores that facilitate drainage, gas exchange and 
root penetration. Roots grow through compacted 
soil by di splacing particles and aggregates or by 
enlarging pores exerting a force greater than the 
mechanical strength of the soil. If the force
required is more than a quite small one, the rate 
of root extension is reduced and a shallow root 
system results unless a greater abundance of old 
root channels and cracks existed [25; 26]. One 
of the more practical ways to assess this soil trait 
is the use of a penetrometer (e.g. the standard 
Cone-Index; C.I.). This does not measure a
simple and clear
-
cut physical soil property, but is 
practical and has the great advantage of being
suitable for field measurements, giving much more 
reproducible results than those for other traits
(like bulk density). The C.I. value is strongly
affected by soil moisture, by bulk den
s
ity and by 
clay and organic matter content of the soil. Many 
equations have been proposed to link C.I. values 
to these variables; the type of surface response 
that seems to agree with those found over a wide 
domain is shown in