For a practical example of that process - and also of how it can be reversed - look no further than Brazil's southern state of Santa Catarina. There, until recently, thousands of hectares of cropland were sliding towards the point-of-no-return between stages 2 and 3. "Modern farming arrived in Santa Catarina in the 1970s with incentives to use mineral fertilizers, toxic pesticides and heavy machinery, such as tractors, disc ploughs and harrows," says a new FAO case study (see box below). "While some farmers achieved maize yields of up to 7.8 tonnes per hectare, that technology was to contribute to subsequent degradation of their land."
European models. Over the previous century, Santa Catarina's settlers had cleared for farming more than two-thirds of the state's forest cover, and established land preparation procedures based on European models. For example, it was normal practice following the harvest to plough crop residues and other biomass into the soil - or simply to burn it - and leave the land fallow throughout the winter. Spring ploughing and harrowing practices aimed at leaving the soil bare of vegetation, and loose and well pulverized for seedbeds. Farmers also tended to grow the same crop - usually maize for the pig industry - from one year to the next.
Removal of surface vegetation during winter not only left the soil exposed to intense, damaging rainfall, but also removed one of the main sources of soil nutrient recycling. Burning of crop residues destroyed important meso- and macro-fauna, such as worms and insects. Monocropping also played a role in soil degradation, by depriving the soil of alternate root systems with different depths of penetration, which improve soil aeration and promote favourable microbial activity.
"Research showed natural physical factors such as slope, stoniness, soil depth and drainage may favour soil erosion, but they are not the dominant factors responsible," the case study says. "The manner in which farmers were managing the land was causing physical, chemical and biological soil deterioration."
Direct sowing. The solution in Santa Catarina has been a steady shift over the past two decades to conservation agriculture, a soil management approach that seeks to limit damage to soil composition, structure and natural biodiversity. Encouraged by the state's rural extension service, many farmers now keep their fields covered with living or dead biomass during the fallow and planting seasons, in order to protect the soil from the direct impact of rain drops, excessive sunshine and wind. Many of their "cover crops" are spontaneous species, such sweet grass and cushion grass, which are slashed or dried before each new planting.
The adoption of cover crops has been accompanied by the emergence of minimum tillage, which limits to a narrow strip of 10-50 cm the area of soil that is disturbed in land preparation. Direct sowing of seed is carried out using a variety of engine-powered and animal-drawn machinery and implements, many of them developed and manufactured locally. The advantage of these practices includes reduction in soil losses, labour requirements and production costs, and more flexibility in the timing of sowing. The FAO study also suggests that minimum tillage may contribute to a reduction in levels of the "greenhouse" gases responsible for climate change: soils in direct-sowing systems emit into the atmosphere up to eight times less carbon dioxide than ploughed soils.
Conservation agriculture techniques have been adopted on a large scale in Santa Catarina, the case study found: between 1994 and 1997, the area using conservationist tillage systems increased from 124,000 to 685,000 ha, or more than one-third of the state's total cropped area. "Nevertheless," the study notes, "neither the scientific community nor the farmers consider minimum tillage to be sufficient to permanently resolve the problems of soil degradation. Direct sowing must be considered as a system, and not merely as a method of land preparation. For the system to be successful, it is necessary to introduce crop rotation, i.e. the use in time and space of a sequence of species. Crop rotation is the basis for the sustainability of direct-sowing systems."
Published May 2001