Weathering is advanced in some areas and many soils have undergone a relatively intense leaching. In spite of a relatively uniform climate, the differences in the soils from the physical, chemical and biological point of view, as well as associated characters (mainly topography and drainage), determine that soils vary a lot in their suitability for agricultural use and in the modalities of handling that they require.
The undulating relief of the country is a product of the erosion processes; it is a landscape of peneplain that covers about 80 percent. This peneplain as a whole does not exceed 200 metres in height. The highest point is the Catedral hill, which reaches 514 metres. The higher parts of the orographic system, called “cuchillas”, are the principal dividers of the hydrographic basins. Lower forms of relief are located on the periphery of the peneplain, formed from sedimentary rocks, arranged horizontally with gentle slopes and difficult drainage, where marshlands are frequently found.
Soil, relief, vegetation and climatic changes such as temperature and particularly rainfall pattern determined the hydrographic network which is very dense and ramified. Rivers and streams are fed by rainwater, therefore their volumes have a very irregular regime, which goes from very pronounced low waters to great floods; this is favoured by the shallowness of the river beds.
Figure 3. Soil Groups (CIDE, 1963)
In Group I shallow soils associated in variable proportions with deep soils of different characteristics predominate, which are the ones that determine to a large extent the potential of the areas. In this group there are two zones: One with Lithosols and Pellic Vertisols, developed on basaltic rocks; this area occupies 21.2 percent of the country. The shallow soils have a low capacity to store water, high drought risk, areas with stones and high phosphorous fixation. The relief is level to gently undulating. In the other zones the predominant soils are Lithosols and Luvic Phaeozems, developed on igneous rock, metamorphic and lava [extrusive] rocks; they are 11.2 percent of the area. The relief is rolling to hilly and steeply dissected to mountainous (8 percent to more than 30 percent).
Deep, medium textured, poor drainage and low to medium fertility soils predominate in Group II. This group has a single zone with Eutric Planosols, medium textured, with Orthic Solonetz associated; it occupies 5 percent of the area of the country and is a large plain with a very low slope. They are soils with low organic matter and fertility and high humidity.
Group III includes deep, medium textured, with low permeability and medium to low fertility soils. In general the igneous acidic and sedimentary rocks of medium granulometry with low calcareous level, tend to develop under prevailing climatic conditions, with topography undulating and smooth undulating, medium textured soils with heavy subsoil and strongly structured, with periods of saturation of humidity and periods in which soils become very hard when they get dry. Three soil zones of soils with different aptitudes and different handling problems can be distinguished in this group.
In one of these zones, which occupies 8 percent of the total area of the country, medium textured Luvic Phaeozems with Eutric and Mollic Planosols predominate. The second zone occupies 15.5 percent, with fine textured Luvic Phaeozems, Mollic Gleysols and associated Pellic Vertisols and Lithosols as intrusions; the relief is level to undulating. In the third zone fine textured Luvic Phaeozems predominate with Pellic Vertisols and Mollic Planosols associated; the relief is level to undulating and the area is about 5 percent. The main problems of these soils are the high risk of drought, susceptibility to erosion and low to medium organic matter content and medium fertility.
The soils of Group IV are deep, light textured and of low fertility. Sedimentary materials of coarse texture with low calcareous level tend to develop in Uruguay, light textured soils with top soil generally deeper and with heavy subsoil with colours ranging from dark red to dark grey, going through yellows and light dark reds. They are the most leached soils of the country.
In this group three zones are also distinguished: One which occupies 3.4 percent of the area with medium textured Orthic Luvisols, and Luvic Phaeozems and Chromic Luvisols associated. In the second zone medium textured Luvic Phaeozems predominate, with Orthic Luvisols and Mollic Planosols associated; they occupy 4.5 percent of the area. In the third zone with an area of 7.1 percent, fine textured Pellic Vertisols with Luvic Phaeozems and Mollic Planosols predominate. The relief of all three zones is level to rolling. The main problems of these soils are that they are prone to hydraulic and aeolian erosion, of easy leaching, low organic matter content and fertility, and the presence of interchangeable Al+++, mainly in the first zone of this group.
Group V comprises the soils of the highest potential in the country. These are deep soils, heavy textured with slow to moderate permeability and high to medium fertility. In the prevailing climatic conditions, the igneous basic rocks and the ones of fine textured sedimentary, tend to form black soils, heavy textured, with slow to very slow permeability. The sediments are medium to fine textured, with a good carbonate level, and give in general, medium to heavy textured soils, with colours tending towards black, with heavier subsoils and moderately good drainage. In this group four zones can be separated: one with a predominance of fine textured Pellic Vertisols, with Luvic Phaeozems and Mollic Planosols, with an area of 5.5 percent . A second with fine textured Luvic Phaeozems, and associated Haplic Phaeozems, and 3.9 percent of the area. The third with fine textured Pellic Vertisols, with Luvic Phaeozems and Lithosols, with an area of 5.2 percent. The last cover 2.2 percent of the country, in which Luvic Phaeozems predominate, and Pellic Vertisols and Mollic Planosols are associated. In the four zones the slope varies between zero and 8 percent, so from level to rolling. The main problem of these zones is the loss of the great part of the top soil by the effect of tillage and erosion; in some zones a plough pan is frequent. (Soil classification and nomenclature from FAO-UNESCO, 1971).
Studies on the soils of Uruguay have continued in greater depth and
detail, which at present allows the separation within these groups of
73 soil units with their chemical, physical and handling characteristics
(Division de Suelos y Aguas, 2001).
Figure 4. Annual mean rainfall in Uruguay (Dirección
Nacional de Meteorología del Uruguay)
Although rainfall is distributed throughout the year, it is characterized by great variations between years. The highest precipitation occurs, in general, in summer and autumn; in the first season, precipitation is very irregular, having summers without precipitation and others with more than 600 mm of rain; in the second season, precipitation has minor variability. Although in winter precipitation has a somewhat smaller volume than that of other seasons there is no marked rainy season. It is possible to emphasize the great irregularity of rainfall, as much as in regularity as in intensity, which leads to droughts and floods that can happen in different seasons of the year. This irregularity is the main cause of problems in forage production. Rainfall distribution is shown in Figure 4.
Mean temperatures of the coldest month (July) are 10.8 °C and 13.0 °C, and the warmest month means (January) are 22.6 °C and 25.1 °C for the Southern and Northern regions, respectively. Except in winter, where temperature can rise for some days, the seasons are more or less are marked. In general great thermal amplitude is registered, especially in the North.
The average date of first and last ground frost is from mid-May to mid-September in the North and West, from the end of April to middle of October in the East, from beginning of June to beginning of September in the South and from beginning of May to the middle of October in the centre of the country, respectively. The average days with ground frosts is 20 in the North, 33 in the West, 37 in the East, 10 in the South and 25 in the centre.
Figure 5. Agro-ecological zones (Ferreira, 2001)
Three groups of Farm Decision Making Units (FD-MU) can be distinguished, according to their production systems and technological demands in a study developed in the Basaltic zone (Ferreira, 1997). The first group, with 56 percent of the FD-MU, are those which have natural resources with lower potential and use a defensive strategy when making decisions; this leads to very low levels of technology adoption since the available technology for shallow soils does not show production, economic increases and stability attractive enough to the risk aversion of these producers. The second group, represented by 18 percent of the FD-MU, have a greater response to technology adoption and a proactive behaviour in relation to the technical change. They are not only aware of new technologies, but also are continually experimenting and analysing the impact of adoption of technical changes on their production systems. The third group (26 percent) are the FD-MU with the largest farms, with willingness to copy, incorporating technologies that have been successfully applied by other producers. The cited author concludes that the supplies of technology must be different for each type of FD-MU group, “recommendation domains” identified in the study and what is even more important, that the processes of technological identification for each one also must be different.
Zone 2. East “Sierras”.
Zone 3. East Plains. Rice growing.
Zone 4. Granitic (Crystalline) Centre (4A) and East Lomadas (4B).
The eastern hillocks (4B) cover 1,276,000 hectares with 4,700 farms. The landscape is characterised by rolling hills with slopes of between 2 and 12 percent and rocky areas (patches) are infrequent. Natural pasture represents 80 percent of the area and cultivated, improved and annual pastures 14 percent. There are approximately 900,000; 870,000 and 34,000 beef cattle, sheep and horses respectively. This agro-ecological zone has a high potential for development and has adequate infrastructure and services.
Zone 5. Sandy soils (5A) and Northeast (5B).
Zone 5B is in the Northeast where 3,500 farms manage 1,500,000 hectares. This zone is characterised by the heterogeneity of soil properties, such as texture, fertility and depth. The landscape is mainly rolling hills. Forages are mainly natural grasses that represent 87 percent of the area and cultivated, improved and annual pastures 10 percent. There are about 1,069,000 cattle, 953,000 sheep and 45,000 horses. Soils of the Northeast zone have a high potential for increased productivity and are suitable for winter, summer crops and cultivated pastures.
Zone 6. Deep Soils, crops, intensive livestock and dairy production.
Sub-zones 6 B and 6 C have the most intensive livestock and crop production systems of the country and have a high proportion of cultivated pastures with the use of silage and hay to conserve forage. The main crops are wheat, barley, sunflower, sorghum, maize and soybean. Sub-zone B has 1,323,000 hectares and 2,861 farms. Cultivated, improved and annual pastures represent 24 percent of the area. Crops represent 8 percent and natural pastures 58 percent. In this zone 9 percent of the area is planted with trees such as Eucalyptus and Pinus. Beef cattle are 830,000 and sheep and horses 610,000 and 29,000 respectively. Sub-zone C has 2,758 farms that manage 711,800 hectares. This is the area with the highest percentage of sown, improved and annual pastures reaching 38 percent. Crops cover 21 percent of the area and natural pastures only 40 percent. In this zone beef and dairy cattle are 558,000 and sheep and horses 221,000 and 14,500 respectively. This is the zone where the number of sheep is lowest.
Zone 7. Deep Soils: