Brazil is the sixth agricultural products exporter country in the world with a significant contribution of Rio Grande do Sul and Santa Catarina, states located at South Brazil ranked, respectively, as second and fifth national agriculture producer. 

These states have, at main agriculture regions, Cfa type of climate (Köeppen), classified as to sub-tropical without drought. The annual precipitation mean is around 1500 mm, regularly distributed throughout the year. The mean temperature of hotter month is near to 30 °C and the colder month is near to 8 °C. In South Brazil, the main soil orders used for agriculture production are Oxisol, Alfisol, Inceptisol and Entisol. The soils included in the Oxisol and Alfisol orders contain low activity clay, low bases saturation, poor soil fertility, presence of Al, Mn and acidity. The soils included in other two orders have higher soil fertility but they have limitations regarding to stone and slope, hardening the agriculture system mechanization. The main cash crops for these states are soybean, corn, tobacco, wheat, maniot, garlic and black beans. The most farm size, in both states, is smaller than 50 ha from where are estimated to produce 80% of agriculture grain. In Rio Grande do Sul state many small farms are mechanized, however, in Santa Catarina state most farms have steepland, restricting mechanization, using animal traction and micro-tractors. 

Land degradation by agriculture in south states has been related to climate, soil genesis and topographic configuration, but was intensified after 70’s decade due to anthropogenic, social and economical factors. Some operations, such as intensive plow and disking, winter fallowing, residue burning and downhill seeding were widely adopted during this decade. In addition to the latter, the most tropical soils with low organic matter, fragile structure, high and long slope and climate composed by intense rainfall, which has water drops with high kinetic energy paved the way to caused huge erosion rates. In this decade, research results for clayey soil indicated about 10 t ha^-1 of soil loss for each 1000 kg of harvested grain (Cassol, 1984). In sandy soils erosion was even a greater problem. Under natural rainfall, at Rio Grande do Sul, an experiment measured that the soil loss of a sandy bare soil for one year was similar to the lost of 1.13 cm layer, equivalent to what the nature would take about a hundred years to form (Debarba e Amado, 1997). In the same experiment was measured 7.12 t ha^-1 organic matter (O.M.) lost by water erosion, e.g., 10% of plow layer existing O.M. These results highlight the erosion potential in sub-tropical climate. 

The main consequences of erosion in South Brazil were: soil quality lost based on O.M. decline, low infiltration, rapid structure degradation, compaction, plant water available reduction, water pollution, decreasing productivity and rural emigration. Cassol (1984) estimated, for 80’s decade, that 2/3 of productive areas had some degradation scar. All factors discussed before indicated that the agricultural system was not sustainable. 

At beginning of 70’s decade started, in South Brazil, research of no-tillage as system to control erosion. During this decade up to half of next, the adoption by farmers was inconsistent, mainly due lack of technologies to weed control, lack of planters able to work with high amount of residues, lack of cover crops and crop rotation, a few research results and unsuccessful efforts of some pioneers farmers. Fortunately, as a result of farmers, extension service and researchers efforts the constraints for no-tillage systems were solved and the no-tillage adoption increased sharply from mid 80’s (Figure 1). The no-tillage area in Rio Grande do Sul reached 5 million ha for the last warm agricultural season, equivalent to 50% of seeded area. The main no-tillage system has black oat as a cool-season cover crop and soybean or corn, as cash crops, at warm season. In Santa Catarina state, the no-tillage area is lower, around 20% of seeded area, however, for some counties with lower soil slopes, the no-tillage areas are greater than 80% and includes corn, onion, tobacco, soybean and other cash crops.   

Figure 1. Evolution of no-tillage in Brazil

The no-tillage adoption, at beginning, started on loam and clay loamy soils, left but sandy or clayey soils. Besides, the pioneer areas had smooth slopes (mechanized), agriculture tradition, fertilized, limed, low weed pressure and medium to large size farms. After the positive result in these situation, the no-tillage were spread to large range of texture such soils with 10 to 70% clay. Early experience of conventional tillage in sandy soil showed rapid decrease in aggregation and consequent high erosion rates causing areas abandon and return to grass. These sand soils, based on old conventional tillage experience, were believed to be able to sustain grain production for only few years when associated with several years of fallow or natural coming grass. However, the no-tillage adoption induced rapid aggregation restoration reducing the main limitation - erosion susceptibility - converting this soil to be able to produce grain for long periods without fallow. 

On the other hand, the main limitation to no-tillage adoption in clayey soils was compaction susceptibility induced by mechanization. Even though it was observed increasing in bulk density in no-tillage areas it looks like hasn’t effect on crops yield. In reality, survey made by extension service, in Rio Grande do Sul state, for 300.000 ha clayey soil, was estimated yield increase of 40 and 80%, respectively, for soybeans and corn, as compared to regional average. (Polleto, 1998). 

Besides textural variation, the no-tillage has been successfully adopted in degraded, low fertility and steep soils (marginal to agriculture), small farms with animal traction and in never tilled soils with native grass without any early preparation. 

The no-tillage on never tilled soil (virgin soil) represents a new world scenario to expansion of the system. Among the advantages, Reinert & Eltz (1997) point out the chance to preserve the natural soil structure that took hundreds of years to be formed. In addition, this approach offers a way to begin no-tillage in soil with higher O.M. than those observed for cultivated soils. This strategy may save time necessary to reach gains in soil productivity. Monitoring no-tillage on never tilled soil has showed higher yields for first years than that yield gotten in no-tillage on early-cultivated soil. The latter has been estimated that is necessary between 3 to 5 years of no-tillage to obtain yields similar to those obtained in no-tillage on never tilled soil. These results are attributed to association of several factors which improve plant growth such as: maintenance of natural physical conditions; large and continuous pores, intense biological activity, vertical distribution of biological activities, high O.M., low weeds pressure and fertility construction due fertilizer application. One of the main limitation to expansion of no-tillage on never tilled soil was soil acidity, what suggested a need to incorporate lime by soil mobilization to mixture it into the soil. However, surface lime application using rates from ¼ to ½ of usual recommended to conventional tillage has been enough to obtain high yields (Reinert & Eltz, 1997). These results have been attributed to decrease of Al activity by organic complexion and the preferential root growth to points with low constraints. Despite recent, the results obtained by using no-tillage on never tilled soils are encouraging, once Brazil has wide area under native grass which, potentially, may be converted to successful grain production, especially when associated with animal production. 

No-Tillage Effects on Main Indicators of Soil Quality in South Brazil 

Chemical Attributes 

The organic carbon (O.C.) is the most important indicator of soil quality and sustainable agriculture due its impacts on physical, chemical and biological indicators (Larson e Pierce, 1991). The O.C. management for tropical and sub-tropical climate is complex, once oxidation rates are high. Thus, the soil mobilization by tillage increases gases exchange creating an oxidative environment having as a result fast decline in O.M. Results obtained by Bayer (1996) indicated that after nine years of a cropping system with high addition of residues (14 t ha^-1 ano^-1 of dry matter) associated to no-tillage, for 0 to 17.5 cm layer, a sequestration of 11 t C ha^-1 higher than the sequestration observed for cropping system with low addition of residues (6.5 t ha^-1 ano^-1 of dry matter) associated to conventional tillage (Figure 2). The latter shows retention of 2.1 t CO₂ ha^-1 ano^-1 for high residue addition and no-tillage system 
 

Figure 2 – Soil total carbon under tillage systems and residue management, Source: Bayer, 1996 

and liberation of 1.78 t CO₂ ha^-1 ano^-1 to atmosphere for low residue addition and conventional tillage. The soil carbon sequestration is an important strategy to improve soil quality and preserve the environment. 

The cropping systems for tropical and sub-tropical regions should be planned aiming to add high amount of residues at surface. Amado (1985) showed 68% of erosion control when small amount of residue ( 1t ha^-1) were evenly spread over soil surface. However, if the goal is to increase organic carbon and soil quality, amounts much higher than those are necessary. Looks like, higher than those traditional recommended, 4-5 t ha^-1 year^-1 of dry matter (Lal, 1993). 

Tropical and sub-tropical soils usually have clay with low charge density; thus, the soil carbon represents important role in increasing the cation exchangeable capacity (CEC). Bayer (1996), after five years of no-tillage, found that cropping system with high residue addition increased in 53% the effective CEC, for 0 to 2.5 cm layer, as compared to conventional tillage. This increment is important to fertility construction because reduce bases leaching, common process on tropical and sub-tropical climate. 

Soil analysis results from no-tillage farming, after a few years, indicate surface nutrient concentration, such as K, Ca, Mg and N with no apparent negative effects on plant nutrition. These results maybe were due to erosion control and increased CEC. An important point to stress is that although the conventional tillage was fertilized for years it was not observed soil fertility construction. 

The nitrogen (N) is the nutrient most affected by soil degradation, because the O.M. is the main source of N and it is sharply reduced by degradation (Aita, 1997). For tropical conditions, the soil N available is the main limiting factor to crop production; thus, increasing soil N is an important approach to improve cropping systems yield. Bayer (1996) and Amado (1997) observed after nine years, for 0 to 17.5 cm layer, that legume cover crop used associated with no-tillage caused an increment of 1000 kg ha^-1 in the total N as compared to grass cropping systems associated with conventional tillage. Moreover, Amado (1997) observed after 10 years of no-tillage that systems including legumes induced increments of 10.7 and 550 kg ha^-1, respectively, for N uptake and corn yield as compared to systems including only grasses. These results show the efficiency of long-term use of legume cover crop to promote increment in N supply and, in turn, increase soil productivity capacity. 

The no-tillage farming, also has promoted increase in soil phosphorus, specially, in surface layer. Reinert (1982) showed increment of 7.3 times of available P for no-tillage as compared to conventional tillage. The no-tillage looks like optimize the P cycle by means of increase organic P, lower mineralization of P from crop residues which in turn reduces its adsorption and lower retention of P from line fertilizers application (Sá, 1998). All these processes associated, help to explain the low answer to P fertilization (30 to 60 kg ha-1) in no-tillage farming. 

Physical Attributes 

The no-tillage system affects indirectly the physical attributes by increment in the O.M. content and biological activity and directly by tension history, mainly, related to mechanization. The effects are linked to soil structure modifications or structural stability and form modification. There is clear indication that O.M. level increase after some years of no-tillage cause increase in soil aggregation, independent of soil type. Campos et al. (1995) in Oxisol, after seven years of no-tillage, observed water stable aggregates increment of 2.7 times, for 0 to 5 cm depth, when the O.M. increased from 2.7% to 3.9%. The aggregation increase is the physical attribute which associated with soil cover explain the frequent observation by extension services and farms that with no-tillage the erosion reduces to values lower than soil loss tolerance (4.5 to 15 t ha^-1 ano^-1). 

The rate of aggregation associated with no-tillage is related to soil texture, management and cropping system. The degradation and the opposite process - restoration of structural stability - are at least twice faster in sandy soils than in clayey soils. The no-tillage use on sandy (73% sand) soil, degraded by conventional tillage, after 2 to 3 years, restored water aggregate stability to near 70% of original (Borges, et al., 1997), whereas, in clayey soil, to reach similar restoration values, were necessary 9 years of no-tillage (Da Rós, 1996). The soil management at start point of use a never tilled soil determines the following structural conditions. The Da Rós et al. (1997) data shows inverse relation between aggregate stability and soil mobilization at begins of no-tillage (Figure 3). The results are in ascending stability order as follows: 1 - conventional tillage every year (CT); 2 - no-tillage with chisel each three years (NT-Ch); 3 - no-tillage plowing in lime at first year (NT-Li); 4 - no-tillage with surface lime application and (NT-NV); 5 - never tilled soil with native vegetation (NV). 

The aggregation model in soil with native grass vegetation looks like is well accepted, however, the model including legume at short and long time is not well established. Some experimental data support the hypothesis that there are higher restoration rates at short time (6-12 months) when legumes are the cool-cover crops in no-tillage systems. Reinert (1993) found higher aggregation restoration when cover crops were grown on mechanically destroyed soil structure as compared to soil without cover crop. 
 

Figure 3 – Water aggregate stability under different soil management in Oxisol; Source: Da Rós, 1997 

The author reported aggregation increase around 2.6 and 1.6 times, respectively, for legumes and grass as compared to bare soil. Another similar experiment, but longer, it was observed at the end of third year, aggregation stability around 1.1 times greater for soil with grass than soil with legume, implying slower rates of restoration at short time, however, steadily increasing throughout the time (Borges, et al.,1996). The results discussed before lead to consider the no-tillage system to act toward improve structural stability, but its effects may have low or high rates of improvement, depending on global soil management and crop strategy adopted. 

There are general agreement that the indicators - water infiltration, macroporosity, mechanical resistance, bulk density, total porosity and microporosity - evaluates the state of soil structural form. These indicators are shown in a decreasing order of range values from base to top of Figure 4 as soil structure changes, implying a importance rank of soil physical properties as a way to evaluate soil management. The water infiltration is the indicator with larger range because integrates several effects such: pore size distribution and continuity, biological pores, cover crops etc. Biological pores are rounded, diameter greater than 2-3 mm mainly formed by mesophauna and root decomposition. Such pores has a great contribution to water infiltration, even though occupies small volume. The pores similar to those made by Dilobderus abderus, usual insect in no-tillage farming, were constructed to measure the impact of these artificial pores on infiltration rates. The results showed no decrease on water infiltration rates, for one hour when 250 mm h^-1 simulated rainfall was applied (Figure 5). In this experiment, the construction of less than 1% of large and continuous pores (11 cm of diameter and 23 cm long) was responsible by water infiltration under high intensity simulated rainfall. 

Figure 4 – Conceptual range of some soil physical properties used to soil structure evaluation. 
 

The soil compaction is a process where state of compaction is defined by tension history that some area has received, by mechanization or animal grazing. The no-tillage systems may, in many cases, increase soil compaction. This has not been a problem for sandy soils but has a crescent concern when clayey (>60%) Oxisols are considered. 

Compaction concerns are always present in agricultural systems including integration of animal and grain production under no-tillage. A loam soil which received continuous beef cattle grazing, during 100-150 days, with animal load of 1000 to 1500 kg ha^-1, did not show macroporosity and bulk density values which indicated critical values to plant growth. The root density values and grain and silage productivity when compared to mechanical compaction breaking did not show differences or restriction caused by cool season continuous animal grazing (Scapine et al., 1997; Silva et al., 1997). However, when high load for short periods were used it was observed bulk density increase and water infiltration reduction. In this experiment, soil compaction broken by soil disking induced significant increase of corn yield (Trein et al., 1991). 

Continuous no-tillage in clayey Oxisol has lead to soil compaction state close to suggest critical state of compaction, which would limit plant growth and productivity. Silva and Reinert (1998) reported values of bulk density, for 8 to 17 cm depth, near to 1.45 g cm^-3 and mechanical resistance higher than 2000 kPa for soil more than 10 years of no-tillage. For this soil the mentioned values should be around to those critical values, however, the soybean productivity for no-tillage was 3179 kg há^-1 and for conventional tillage was 3086 kg ha^-1. Important decision for no-tillage system planning for clayey soils should include operations which minimize compaction risks, such as: observance of soil water content, controlled traffic, controlled machinery weights and tire pressure. 

Another effect of no-tillage for tropical and sub-tropical areas is to reduce temperature range and maximum temperature. Amado et al., (1990) studying onion no-tillage, at Santa Catarina state founded, for no-tillage, temperature range (16 to 21⁰ C) closer to ideal and good soil moisture range (<0.5 bar) for onion production. The combination of these factors lead to 25% increase in onion production for no-tillage system as compared to conventional tillage (Amado, 1990). 

Biological Attributes 

The surface residues and the O.M. increase in soil under no-tillage, associated with reduction of soil temperature and water conservation has favored the biological activity, which in turn, has a important role in improving soil physical and chemical attributes. There are several indications of increasing in biological activity in no-tillage system. 

In the sub-tropical regions of South Brazil, the earthworms’ populations are low when compared to temperate climate. However, with no-tillage adoption has been observed a significant increase in earthworms’ populations inducing positive effects on biological porosity and aggregate stability. Krabe et al. (1994) measured earthworms population, by end of cool-season, which varied from 75 to 240 earthworms m^-2 in no-tillage system and 25 earthworms m^-2 in soil under conventional tillage. 

The microbiological activity, in sub-tropical climate, has been greater in soil under no-tillage than soil under conventional tillage. Campos (1993) measured CO₂ evolution twice as much for no-tillage as compared to conventional tillage. Ferreira et al. (1995) studied free N₂ fixation and found greater populations of Azotobacter, Azomonas, Derxia and Beijerinckia in no-tillage system which include legumes than in fallow or bare soil. Among legumes cover crops the lupine had higher population of nitrifiers. 

Some cover crops induce higher biological activities than others do with positive effects on soil properties. Scarton et al. (1994) comparing cover crops including grass and legumes found two times more fungi hype associated to lupine than to ryegrass, common vetch and black oat. This higher fungi activity induced significant increase in aggregate stability. 

As a sum up, the no tillage system for south Brazil conditions, after a few years, has improved soil biological activity, specially, mesofauna with direct effect on physical attributes. 

Small Farming No-Tillage 

The small farms have their own characteristics, which should be taken in consideration in order to have no-tillage success. Among this we may point out some: limited own capital to invest, limited access to governmental funding, labor availability, pressure for land use, diversity of production, can take low production risks and need to maximize farm inputs and minimize use of external inputs. Thus, the use of herbicides to burn down cover crops and to control weeds, common for mechanized agriculture, is restricted for small farms due lack of financial funds to buy then. Adding to it, the small farmers, usually, do not take care of individual protection. In this scenario, the use of cover crops to suppress weed and mechanical interruption of cover crop cycle with knife-roll are important strategy to reduce external input dependence. The N demand for cash crops, also can be partially supplied by legume cover crop (Amado et al., 1991). The main alternatives for cool-season cover crop are lupine, common vetch and forage pea and, for warm season cover crop are gray mucuna, pig beans, lab-lab, cowpea and dwarf guandú. For South Brazil, the cool-season legumes cover crops have from 70 to 130 kg ha^-1 of above ground N, whereas, the warm-season legume cover crops have produced from 100 to 190 kg ha^-1. The legumes, usually, are able to supply 2/3 of corn N demanding and it has N fertilizer equivalence of 60 to 100 kg ha^-1 (Reeves, 1994; Amado, 1997). 

The no-tillage adoption on small farms is recent (early 90’s). Based on rural extension service survey, the main difficulties to small farmers adoption of no-tillage were absence of planter specific for small farming, lack of technical support and lack of farmers training (Berton, 1998). The main difficulties were overcooked by extension service using strategies listed on Table 1. The efficiency of those strategies reflected on number of small farmers who got into no-tillage farming. In 1993 only 10% out of NorthWest (Rio Grande do Sul state) small farm total area used conservation tillage, whereas, in 1998 this proportion raised to 85%. 

Table 1 - Strategies used to no-tillage diffusion for small farmers in Rio Grnde do Sul from 1993 to 97 

Source: Berton (1998) 

Another survey among small farmers identified the main reasons to no-tillage adoption and the results are presented on Figure 6. Only to illustrate the importance of this change, it was computed that would have 65.6% in economy of labor time to make one ha of no-tillage corn than if were made by conventional tillage. This extra time would be used to other farm activities, such creation of poultry, swine, etc, diversifying production of small farms (Berton, 1998). 

Figure 6 – Small farmers main reasons to adopt no-tillage; Source: Berton (1998). 

Additional advantages of no-tillage in small farms can be listed: energy saving, use of low potential areas to agriculture, without degrade then sustain production at same area without fallow or shifting cultivation and yield stabilization. 

Future Perspectives of No-Tillage 

Brazil and some African countries have important world soil reserve with potential to agriculture, that due to limitation related to erosion susceptibility, low natural fertility or high risk of accelerated degradation are not used. These soils when well managed in no-tillage farming represent high potential to cause regional economical re-conversion. In these days, those soils are mainly under never tilled soil with natural grass, in fallow or in shifting cultivation. The last soil use has been used traditionally in sub and tropical areas where to incorporate new areas is needed deforestation or plow out virgin soils. This tradition goes on and new degrade areas are formed. The simple substitution of conventional burning and planting or plowing and disking for no-tillage farming, including cover crops and crop rotation represent as important approach to quit land degradation by shifting cultivation, deforestation and intensive soil mobilization. Instead the latter one would get improvement of physical, chemical and biological soil quality acting to become the agricultural systems sustainable. 

In order to reach main goal to have sustainable agriculture in tropical and sub-tropical look like any region should put efforts to make three main actions: technologies diffusion, farmers training and focus on no-tillage research in specific climate condition. 

References 

^{Dr., Professor of Soil Management, Soil Science Department, Federal University of Santa Maria, RS., 97105-900 Santa Maria, RS, Brazil. Fax:55-55-220-8695.} 

PhD, Professor of Soil Physics, Soil Science Department, Federal University of Santa Maria, RS., 97105-900 Santa Maria, RS, Brazil. Fax:55-55-220-8695