Como o milho é freqüentemente cultivado em solo anteriormente ocupado pela cultura algodoeira, achou-se conveniente verificar se o BHC empregado nesta última cultura poderia permanecer no solo e prejudicar aquela planta. Com êste objetivo fêz-se um ensaio em vasos de Mitscherlich, no qual diferentes doses de BHC foram incorporadas à terra. As doses de BHC empregadas foram correspondentes àquelas recebidas pelo solo no fim de 1, 2, 3, 4 e 5 anos de cultura algodoeira, tratada com a quantidade oficialmente recomendada daquele inseticida (108,5 kg/ha/ano, de BHC contendo 2% de isômero-gama). Nos vasos, onde as condições eram as mais favoráveis para o manifestação dos efeitos tóxicas do BHC, a decomposição dêste inseticida na terra foi suficientemente rápida para que os efeitos tóxicos de uma dose três vezes mais forte do que a oficialmente recomendada desaparecessem completamente no fim de dois anos. Nas condições de cultura os efeitos tóxicos do inseticida devem ser ainda bem menos intensos. Não são de se esperar, pois, prejuízos à cultura do milho, causados pelo acúmulo de BHC no solo, quando aquela fôr feita em terreno anteriormente ocupado pela cultura algodoeira, desde que nesta a quantidade empregada do inseticida, não tenho sido muito diferente daquela oficialmente recomendada.

Corn is frequently cultivated on soil previously used for cotton. Cotton plantings usually receive heavy applications of BHC for insect control, what might lead to an accumulation of the insecticide in the soil. An attempt was made to verify whether varying amounts of BHC added to a sandy soil in Mitscherlich pots would injury corn plants. The amounts added were calculated on an area basis to correspond to those received by the land at the end of 1, 2, 3, 4 and 5 years of cotton cultivation when BHC is applied at the yearly rate of 108 kg per hectare. Other types of soil were not used in this experiment because the toxic effects of this insecticide seem to be more serious on sandy soils. Other techniques employed in this work were exactly the same described in a previous paper (1). The experiment was terminated one month after sowing and the weights of the plant tops and roots were determined. The soil from the pots was passed through a sieve after harvesting, returned to the pots and left undisturbed for one year. Corn was then planted a second time on the same soil. In the first planting, dose 1 had no injurious effect on plant growth, but the heavier doses depressed it severely. In the second planting only doses 4 and 5 showed injurious effects, but injury was less severe than that in the first planting. The results seem to indicate that there is practically no risk that residual BHC in the soil from a previous cotton planting that received standard applications of the insecticide will injury corn plants raised on the same land the following year.

A fim de verificar o efeito do acúmulo de inseticidas no solo, fêz-se um ensaio em vasos de Mitscherlich, empregando-se os inseticidas BHC, toxafeno, lindano e DDT, incorporados ao solo. Os três últimos foram experimentados também na forma de emulsão, enquanto que o BHC entrou no ensaio apenas na forma de pó. As quantidades de inseticidas empregadas foram correspondentes àquelas recebidas pelo solo no fim de 1, 3 e 7 anos de cultura algodoeira, tratada com as quantidades recomendadas para cada um dêles. Em terra arenosa, do tipo arenito Bauru, a dose de BHC correspondente à recebida pelo solo em um ano de cultura com o algodoeiro (108,5 kg/ha) já causou um decréscimo de produção de 20%; a dose correspondente a 3 anos produziu um decréscimo de 56% e a correspondente a 7 anos anulou a produção. Entretanto, após ter o solo permanecido sem cultura durante um ano agrícola, os efeitos tóxicos do BHC decresceram, a ponto de mesmo a dose correspondente a 7 anos de cultura ter permitido alguma produção. Dos outros inseticidas sòmente o toxafeno em emulsão e quando empregado em dose 7 vêzes mais forte do que a recomendada, que é de 18,6 l/ha de emulsão a 50%, causou algum prejuízo à produção, reduzindo-a de 30% na cultura feita logo após a sua aplicação no solo. Em conseqüência de um acidente ocorrido e mencionado no texto, não se obtiveram dados de uma segunda cultura com êste inseticida, após ter o solo "descansado" durante um ano agrícola. Em terra-roxa os efeitos tóxicos do BHC foram menos intensos. Neste tipo de terra a dose correspondente à recebida pelo solo no fim de um ano de cultura algodoeira não prejudicou em nada o algodoeiro. As doses correspondentes às recebidas pelo solo no fim de 3 e 5 anos de cultura algodoeira causaram sensíveis decréscimos de produção. Na segunda cultura, entretanto, feita nos mesmos solos após terem êstes permanecido sem cultura durante um ano, não houve nenhuma diferença entre as plantas tratadas e testemunhas, quer quanto ao desenvolvimento ou à produção, o que mostra que na terra-roxa os efeitos tóxicos do BHC, nas doses estudadas, desapareceram completamente do solo no fim de dois anos.

The effects of increasing amounts of insecticides added to the soil on the growth of cotton plants were studied. This was an attempt to verify whether or not a possible accumulation of insecticides in the soil might become detrimental to plant growth. The experiment was carried out in Mitscherlich pots with two types of soil: one sandy (arenito de Bauru) and the other clayey (terra-roxa). The amounts of insecticides added to the pots were calculated on an area basis to correspond to those applied to cotton plantings at the end of 1, 3, and 7 crops, and are called in this paper doses 1, 3, and 7, respectively. The amounts of insecticides as recommended for cotton in the state of São Paulo are as follows: BHC, 108 kg; Toxaphene, 14.5 k; Lindane, 0.4 kg and DDT, 3.6 kg. In the sandy soil, doses 1 and 3 of BHC caused 20% and 56% reduction in yield, respectively; dose 7 inhibited the production completely. Dose 7 of Toxaphene (72.24 kg/ha) when applied in emulsion form showed some toxicity. The same insecticide applied at the rate of 101.5 kg/ha in powdered form did not have any toxic effect nor did the emulsifier (15 ml of tritton 177 + xilol q.s.p. 100 ml) when applied alone. Lindane, emulsified Lindane, and DDT did not have any adverse effect on the cotton plant0 growth and yield. After the first crop the soil in each pot was passed through a sieve, returned to the pot and left undisturbed for one year. Cotton was then planted again. The development of the plants in this second planting was very irregular. This was due to aluminum toxicity, because the protective painting of the aluminum trays of the Mitscherlich pots came off in many spots and the metal wos attocked by the salts used as a fertilizer, forming aluminum salts that were detrimental to the plants. It wos possible, however, to see that toxic effects of the BHC added to the soil decreased fast enough after two years to allow even the plants in pots which received dose 7 of the insecticide to come to bearing. In the experiment with clayey soil only BHC was used since it was the only insecticide that showed a dangerous level of toxicity. The doses employed were 1, 3, and 5. Here too these numbers mean that the omounts of BHC added to the soil correspond to the omounts applied to cotton fields after 1, 3, and 5 years of cultivation, respectively. In the clayey soil, dose 1 of BHC did not decreose the yield nor caused any other toxicity symptom. Doses 3 and 5 decreased the yield in 14 % ond 27%, respectively, but the analysis of variance did nat show a significant difference between the two results. The growth and yield of the plants sown offer the soil had been left undisturbed far one year did not show any difference between the control ond treated plants, thus indicating that the toxic effects of the BHC had completely disappeared.