| Resumo: |
From a study of the variation of nitrates in the soil known as terra roxa misturada (mixed purple soil), the following conclusions were drawn : a) The nitrate content of this soil reveals a periodic variation through the year. The same kind of variation is expected in the majority of S. Paulo soils, because the climate, with minor exceptions, is fairly uniform over the State. b) As nitrification proceeds at a higher rate in the hot wet (September-May) than in the cool dry season (May-September), nitrates accumulate in the soil during the first period and disappear in the second. However, as an exception to the above rule, there appeared to be a fall in nitrates during December. This may have been due, either to leaching by rain, or to absorption by weeds, then in their growing period in the experimental fields. Since inorganic nitrogenous fertilisers do not generally give good results in the State of S. Paulo, when applied at the beginning of the rainy season, it is logical to attribute the decrease in nitrogen content to leaching, although the investigation did not lead to a conclusive explanation. c) This study did not support the view of an upward migration of salts in our soils during the dry months, since a steady level of nitrate content was observed during this period. d) Although the author was able to establish an association between nitrate losses and rainfall, it was not possible to determine the relation quantitatively.
|
