ABSTRACT: Mechanical sugarcane harvesting increases soil compaction due to the intense traffic of agricultural machinery, reducing longevity of sugarcane crops. In order to mitigate the harmful effects caused by agricultural traffic on the soil structure in sugarcane fields, this study evaluated impacts of mechanical sugarcane harvesting on traffic lane under two soil tillage systems based on load bearing capacity models. The experiment was carried out in the region of Piracicaba, state of São Paulo, Brazil, on a Rhodic Nitisol, under conventional tillage (CT) and deep strip-tillage (DST). For CT soil tillage was applied to the entire area with a heavy disk harrow, at operating depths from 0.20 to 0.30 m followed by a leveling harrow at a depth of 0.15 m. For DST, soil tillage was performed in part of the area at a depth of 0.80 m, forming strip beds for sugarcane planting, while the traffic lanes were not disturbed. Undisturbed soil samples from traffic lanes were used in the uniaxial compression test to quantify preconsolidation pressure and to model the soil load bearing capacity. The surface layer (0.00-0.10 m) was most susceptible to compaction, regardless of the tillage system (CT or DST) used. In the DST, the traffic lane maintained the previous soil stress history and presented higher load bearing capacity (LBC) than the traffic lane in the CT. As in CT the soil was tilled, the stress history was discontinued. This larger LBC in DTS minimized the impacts of the sugarcane harvest. Under CT, additional soil compaction due to mechanical sugarcane harvesting in the traffic lane was observed after the second sugarcane harvest. There was a reduction in load bearing capacity from 165 kPa to 68 kPa under CT and from 230 kPa to 108 kPa under DST, from the first to the second harvest at surface layer. Water content at mechanical harvesting was the most relevant factor to maximize impacts on the soil structure in traffic lanes, for both tillage systems.

RESUMO Entre as matérias-primas renováveis destinadas à produção de etanol, distinção vem sendo dada ao sorgo sacarino, destacando-se o fato de ser uma cultura totalmente mecanizável. Entretanto, o uso intensivo de máquinas agrícolas provoca degradação estrutural do solo, principalmente quando realizada em condições inadequadas de umidade. Objetivou-se avaliar a qualidade física de um Latossolo Vermelho Distroférrico sob compactação e componentes do rendimento de sorgo sacarino para épocas de semeadura em safrinha no Cerrado brasileiro. O experimento foi instalado no delineamento de blocos ao acaso, em esquema de parcelas subdivididas, com quatro repetições. Nas parcelas, foram avaliados cinco níveis de compactação obtidos pelo tráfego de um trator agrícola nas seguintes intensidades de tráfego: 0 (ausência de compactação adicional), 1, 2, 7 e 15 passadas no mesmo lugar. As subparcelas foram constituídas por três épocas de semeadura do sorgo sacarino na safrinha de 2013 (20/01, 17/02 e 16/03). Foi avaliada a qualidade física do solo, por meio do intervalo hídrico ótimo (IHO), a limitação hídrica, além das variáveis produtivas e tecnológicas da cultura. O monitoramento do conteúdo de água no solo indicou redução da frequência de conteúdos de água no solo dentro dos limites do IHO (Fdentro) com o aumento da intensidade de tráfego agrícola (T0 = T1 = T2>T7>T15), tendo a produtividade associação direta com a disponibilidade hídrica do solo. Por sua vez, a semeadura feita em janeiro apresentou maior qualidade industrial, porém com redução da produtividade de colmos a partir da densidade do solo de 1,26 Mg m-3, com produtividade máxima de 50 Mg ha-1 nessa época de cultivo. O cultivo do sorgo sacarino na safrinha é uma alternativa promissora; no entanto, necessita de cautela quanto ao cultivo em condições de riscos climáticos acentuados por causa da baixa produtividade de colmos.

ABSTRACT Increasing attention has recently been given to sweet sorghum as a renewable raw material for ethanol production, mainly because its cultivation can be fully mechanized. However, the intensive use of agricultural machinery causes soil structural degradation, especially when performed under inadequate conditions of soil moisture. The aims of this study were to evaluate the physical quality of aLatossolo Vermelho Distroférrico (Oxisol) under compaction and its components on sweet sorghum yield forsecond cropsowing in the Brazilian Cerrado (Brazilian tropical savanna). The experiment was conducted in a randomized block design, in a split plot arrangement, with four replications. Five levels of soil compaction were tested from the passing of a tractor at the following traffic intensities: 0 (absence of additional compaction), 1, 2, 7, and 15 passes over the same spot. The subplots consisted of three different sowing times of sweet sorghum during the off-season of 2013 (20/01, 17/02, and 16/03). Soil physical quality was measured through the least limiting water range (LLWR) and soil water limitation; crop yield and technological parameters were also measured. Monitoring of soil water contents indicated a reduction in the frequency of water content in the soil within the limits of the LLWR (Fwithin) as agricultural traffic increased (T0 = T1 = T2>T7>T15), and crop yield is directly associated with soil water content. The crop sown in January had higher industrial quality; however, there was stalk yield reduction when bulk density was greater than 1.26 Mg m-3, with a maximum yield of 50 Mg ha-1 in this sowing time. Cultivation of sweet sorghum as a second crop is a promising alternative, but care should be taken in cultivation under conditions of pronounced climatic risks, due to low stalk yield.