Club of bologna proceedings

7D
ble 1
refer 
to a given trait so that different soil traits can 
l
ead 
to different conclusions. In order to help the
decision-maker in max imizing an equation of the 
type n. 1, a suiable weighting of all traits involved 
in the evaluation of a technique is needed.
The above is an analysis of the logical pro
c
ess. 
Unfortunately we are far from having such
detailed knowledge on the behaviour of the
material system. The need for research is evident, 
while caution in using current knowledge should 
be recommended. 
3. Main basic processes in the system 
The increasing contributi on of research on tillage 
has clarified many important aspects of the role 
of this technique and its adapt
a
tion to the main 
environmental conditions. A number of very
valuable reviews on the subject have been given 
[6; 7; 8; 12, 13; 14]. 
It is not the cas
e 
to discuss the results of their 
analyses. We shall only mention some of the most 
important effects of tillage tec hniques, especially 
considering the conditions where they seem to fit 
better. 
The main objectives of soil tillage are:
°
suitable seed bed prepa
r
ation (this goal
varies for different crops; the related state 
of the soil for the seed bed was once 
known as "tilth", in German "Bodengärung");
°
weed control, often considered important; 
°
aeration of deeper soil layers; 
°
soil water traits manageme nt; 
°
burying materials such as crop residues, farm 
manure and fertilizers; 
°
reducing soil erosion. In some cases soil
inversion for texture improvement, harpan 
breaking, surface shaping etc. are occasional 
goals [15; 16; 17]. 
Most common drawbacks are: 
°
reduction of organic matter content of the
soil; 
°
formation of ploughpans or tillage pans;
°
compaction from wheeled machinery.
Unsuitable tillage can lead to undesirable soil
layer mixing, erosion, too deep burying of organic 
material. 
Unsuccessful tillage can s ometimes be corected 
by the application of fertilizers, irrgation, 
herbicides, or pesticides [11]. The success of a 
tillage programme may depend on these
applications and especially on the availability of 
special machinery (for direct seeding and in some
cases for minimum tillage). The direct and indirect 
costs must fr equently be pooled with those of the 
whole technique package; this aspect is not
always adequately considered. 
The direct actions of tillage are of a physical
nature; many important indirect
effects are
physical, chemical and biological, the latter
including yield which is usually one of the main 
goals of the culture.

---

2 1
Mean bulk density of the soil (

a
; apparent 
volumic mass) is instantly decreased by tillage. 
The clods resulting from traditional primary 
tillage may be grouped into two classes, one with 
unchanged porosity, and the other with reduced 
porosity due to tool compression; the pooled 
porosity is then essentially increased through
interaggregate voids [18; 19]. Rainfall at this stage 
can easily infiltrate the soil, eventually 
accumulating on plough pans (soils behave 
differently); runoff and erosion are strongly 
reduced. Roto-tilled soils, by contrast, are too 
loose, uniform and highly exposed to erosion on 
slopes; their porosity rapidly reduces under the 
first rainfall. This reduction occurs in any case 
after whatever tillage operation, mostly at the first 
rainfall events and gradually continues ( 
Fig. 3
); to 
some extent it can last for more than a year even 
in the deep layers. The plough-pan remains even 
longer and can hinder water flow and root growth; 
this process varies in different soils and conditions 
(the subject requires more research). When 
starting from virgin soils the bulk density 
obtained by plough or chisel tillage gradually
reduces in tropical climates (mainly in oxysols) 
over a number of years, and more slowly, over 
decades, in temperate climates, due to structure 
degradation and organic matter reduction; tropical 
soils also tend to harden when dry. The bulk 
density is affected mainly by the tillage of that 
year; some effect of the previous year may be 
also evident (