Sustainable Soil Management Is More than What and How Crops Are Grown - Principles of Sustainable Soil Management in Agroecosystems - page 348

resulting in 192,200 ha that had lost at least 10 cm of topsoil (truncated) by rill and
gully erosion (Gianluppi et al. 1979). The loss of seeds, fertilizers, and agrochemi-
cal from croplands resulted in US$33 million of damage (Gianluppi et al. 1979).
Another environmental indicator of the intensity of soil erosion verified during this
period was at the Passo Real Dam, which had 1.6 kg of soil per 1000 m 3 of water,
resulting in a total of 6 M t of suspended soil sediments in the water.
The estimated soil erosion in South Brazil during the conventional tillage period
was approximately 25 t ha -1 year -1 . After 15 years, adoption of conventional tillage
practice resulted in two-thirds of the agricultural land in Southern Brazil showing
soil degradation, expressed by the depletion of SOM, reduced water infiltration
rate, structural degradation, soil compaction, and an increase in plant susceptibil-
ity to short duration droughts. The social consequences of high erosion and soil
degradation were as follows: sedimentation of rivers, smallholders forced to migrate
to cities increasing the unemployed population, sales of small farms, and inter-
regional migration of farmers (south to central and north) (Cassol 1984; Amado
and Reinert 1998; Pöttker 1977; Bolliger et al. 2006). During the time period that
conventional tillage was the prevalent soil management practice, increases in
crop yields were very modest regardless of the increase in inputs and germplasm
improvement.
In Paraguay, the semideciduous subtropical forest was replaced by agricultural
land use, which, along with conventional tillage practice, promoted soil degrada-
tion (Riezebos and Loerts 1998), similar to that verified in South Brazil. Prior to
deforestation, SOM ranged from 2.09% to 2.42% but decreased to 1.59% under con-
ventional tillage management. Mechanically tilled fields appear to have a more rapid
decline in organic matter than manually tilled fields (1.59% vs. 1.89%) suggesting
more severe soil degradation in mechanized agriculture. In South Brazil, a decline
in SOM in conventional tillage pulled by tractors also was noted, although the effect
of poor management in reducing soil carbon was more pronounced in soils with
lower content of clay and iron oxides and under high soil erosion rates (Fabrizzi et al.
2009). Séguy et al. (1996) reported that in degraded soils of Brazil, the SOM stocks
were depleted by as much as 30% to 50%.
Conventional tillage causes the physical destruction of crop residues, increases
the soil-residue contact, promotes higher aeration and higher soil temperature, and
increases soil N mineralization (Amado et al. 2006; Aita and Giacomini 2007).
These processes in combination cause a sharp increase in microbial biomass activity
that consume crop residues and labile SOM resulting in an exponential rate of decay
(Pes et al. 2011). Soil tillage causes the disruption of soil aggregates and exposes
particulate SOM to microbial biomass attack (Amado et al. 2006; Pes et al. 2011).
In summary, the main causes of soil degradation in South America were associ-
ated with the cumulative effects of the reduction of plant biomass being returned to
the soil, reduction of crop diversity, soil erosion, soil disturbance by tillage, main-
tenance of bare soil or limited soil cover in periods of high rainfall erosivity, deple-
tion of SOM, depletion of soil fertility by unbalanced input-output agroecosystems,
deterioration of soil structure, soil compaction, loss of microbial biomass diversity,
and decrease in soil quality.