Club of bologna proceedings

particular biological interest are nitrogen
compounds (like ammonium and nitric ions), as 
are the metabolism of phosphorus and su
l
phur. 
No-tillage leaves crop residues to decompose
slowly on the soil surface, and th
e
ir intermediate 
products penetrate the upper soil layers (depth 
mostly in the order of 10
-1
5 cm) where they
accumulate (humus and adsorbed substances,
such as P compounds). Ploug
h
ing redistributes 
the materials left at the end of previous season in 
the ti lled layer; too deep tillage dilutes these
substances excessively (e.g. o.m., P); too shallow 
reduces the recapturing of percolated materials 
(e.g. herbicides, P, K; see 
)L
g. 13
). Chiselling 
has a moderate mixing effect. In general the
deeper the tillage, the greater the soil volume 
affected by the aeration; the latter, however, is 
hindered the deeper the layer. In clay soils with a 
shallow water table this depth effect on aeration 
can be strong enough to hinder o.m.
decomposition (especially when buried in
large 
amounts) and the redox potential falls to values 
that prevent a normal deco
m
position process
(maize stubble is easily found carbonized the
following year) and phytotoxic low carbon chain 
acids may be produced [30; 31]. These
conditions are unfavourable to structure stability.
As a whole, for each soil and watertable r
e
gime, 
there is an optimum depth for the rate of o.m. 
decomposition; at zero depth (no
-
tillage and top 
mulch) the rate of decomp osition is slowed down 

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24
by dry conditions (e
s
pecially in a
d
ry climate), 
while at low depth it is hindered and altered in 
quality by a noxia; these aspects are exaggerated 
in a wet climate and even more so by excess 
moisture in clay soils just above the watertable. 
The increase of o.m., especially when
concentrated in surface layers, leads to pH
lowering (soil acidification). 
In normal soil conditions the enhanced o.m.
decomposition brought about by tillage through 
aeration implies faster delivery of nutrients like N, 
P, S, etc.. This contributes to the usual
favourable effects of tillage (e specially the deeper 
one) on yields and can lead to reducing the
fertilizer application rate (mainly nitrogen;