A Natureza e as possibilidades do solo no Vale do Rio Pardo entre os municípios de Caconde, SP, e Poços de Caldas, MG

Resumo

The nature and agricultural possibilities of the soils of the upper Rio Pardo valley between the towns Caconde, SP, and Poços de Caldas, MG, ate described, based on analyses of 24 soils and evaluation of geology, topography, climate, and behavior of man as soil forming factor.

            The area is of very old amphibolitic or biotitic gneisses of wide range of metamorphism, from strongly lineated and schistose to massive with development of pyroxenes as katamorphic transformation of amphiboles. Both gneisses may occur side by side due to intense faulting and differential uplifting.

            The topography is mountainous, but recent alluviation of V shaped valleys endowed them with flood plains followed by deepening of streams due to further uplifting.

            Elevations range from 800 to 1300 m, latitudes from 21°35' to 21°43' S, and longitudes from 46"30%' to 46°40' w.

            The scattered population (about 5 inhab/sq km) is not accustomed to any soil conservation practice, manuring or irrigation. From the primitive tall and dense subtropical forest almost nothing was left More than 100 years of burning vegetation, either to "clean" the pastures or to prepare the soil for any cultivation, or just to show the right of property, resulted in decapitation of soil profile, leaching of bases, mineralization of phosphorus, and selection of bad weeds.

            Besides complete physical and chemical analyses of soils, detailed field description and location ate given for all samples which are typical for the chief features of topography, parent material and history of land utilization.

            Out of valley floors the soils ate reddish yellow latosols without any lateritic crust or concretionary layer because their formed high organic matter content (4-8%) was a powerful defense against laterization. Depletion of organic matter in the last decades seems to be responsible for the presence of small limonitized aggregates up to 2-3 mm in diameter, visible on top of B1 horizon underlying a previous A2. The clay minerals are caolinite and illite with a relatively small admixture af Al and Fe hydroxides.

            Clay content of alluviums ranges from 29 to 49%, that of the latosols from 8 to 25%. The latter present up to 20% pebbles (;,2 mm, mostly pieces of quartz veins). Moisture equivalent of alluviums ranges from 31 to 51%. that of the latosols from 10 to 28% The pH values are mostly from 5% to 6, humus of latosols from 1% to 2%, available phosphorus from O 12 to O 20 meq, exchangeable phosphors from o 6 to 0.9 mq. Relatively to the exchangeable, the available PO, is the better, the higher the pH, the humus and the average water contents without impermeability.

            The most common content of exch bases is 1 and a half-3 meq Ca, O 15-0 25 meq K. O 25-0 5 meq Mg, O 02-0 035 meq. Mn, O 010-0.02 meq. NH, O 2-0 5 meq Al, and 8-11 meq H;, 12-18 meq BEC (base exch capacity) with 22-40% base saturation.

            Comparing with the general soil conditions of the whole Crystalline Complex of the neighboring states of São Paulo and Minas Gerais, the soils of present report possess marked qualities: presence of high amount of rich minerals in decomposition, healthy subtropical climate with good isolation in the rainy mild summer, and ample possibilities of irrigation in the warm sunny winter because of abundance of streams and high hydroelectric potentiality

            But since in such a climate the decomposition of organic matter is very quick in limed and fertilized soils, and the needs of organic compost ate very high, bounding to daily transportation of great amounts of grasses and straws from the fields to the stables and corrals, and thence to the coffee plantations and other cultivated land, the topography becomes a strong limiting factor, so that only rich fawners, owners of big size property, can perform the program because they can possess a truck or tractor and maintain good roads.

            Yet the average size of property is small, and the most common land owner prefers to get an occupation in a few big size properties (500 to 1000 ha) instead of doing something on his own, besides burning vegetation and planting some corn. Having seen the ancient forest and the decay and erosion of the soil, this peasant knows well the extent of land exhaustion, but does not realize the new possibilities of the soil and climate brought about by use of organic compost, pulverized lin1estone, fertilizes, insecticides, irrigation, machines and electricity.

            Nevertheless, the agricultural research and many practical results already obtained in similar conditions of other areas (e g in campinas, SP, region) prove that high production of coffee is possible, besides excellent crops of rice, sugar cane, corn, cotton, sweet potato, manioc, tomato and a great variety of other cultures, since with irrigation the planting season covers the whole year irrigation is obligatory for high production of rice, coffee and all winter cultures. Mean frequency of frost is about once per 20 years only.

            Slopes steeper than 15% should be left for development of natural forest. Coffee, orchards and all cultivation should be made on slopes with less than 10% declivity, always according with level curves, including pastures. Between 10 and 15% only sugar cane and grass for cutting ate suitable on rich shallow soils, and reforestation with eucalyptus on deep poor ones.

            The complains that good temperature and high humidity, especially with irrigation in dry season, promote quick decomposition of organic matter are only reasonable for the poor helpless peasant who tries to get some corn without expenses. On the contrary, such quick biological activity of the soil is highly favorable to the industrious farmer, able to use green manuring, organic compost and modern amendments and implements, because conscious planned investments can be retributed by high crop responses with continuous amelioration of the soil.