A compactação é uma das principais causas de decréscimo da produtividade dos solos agrícolas. A escarificação mecânica tem sido sugerida para reduzir a compactação do solo em áreas sob sistema de semeadura direta consolidada. Outra opção para aliviar a compactação é o uso de plantas de cobertura com sistema radicular pivotante e bem desenvolvido, como o nabo-forrageiro, com capacidade de crescer em camadas compactadas, formar bioporos estáveis e melhorar os atributos físicos do solo. O objetivo deste trabalho foi avaliar a eficiência do método mecânico (escarificador) e do método biológico (nabo-forrageiro) de descompactação do solo, ou de ambos associados, em promover a melhoria dos atributos físicos de um Latossolo de textura muito argilosa e o rendimento de grãos da cultura da soja manejada sob semeadura direta. Os tratamentos utilizados foram: aveia-preta em semeadura direta (SD-Av), consórcio de nabo-forrageiro e aveia-preta em semeadura direta (SD-Nb+Av), aveia-preta em solo escarificado (ESC-Av) e consórcio de nabo-forrageiro e aveia-preta em solo escarificado (ESCNb+Av).

SUMMARYVariable-rate nitrogen fertilization (VRF) based on optical spectrometry sensors of crops is a technological innovation capable of improving the nutrient use efficiency (NUE) and mitigate environmental impacts. However, studies addressing fertilization based on crop sensors are still scarce in Brazilian agriculture. This study aims to evaluate the efficiency of an optical crop sensor to assess the nutritional status of corn and compare VRF with the standard strategy of traditional single-rate N fertilization (TSF) used by farmers. With this purpose, three experiments were conducted at different locations in Southern Brazil, in the growing seasons 2008/09 and 2010/11. The following crop properties were evaluated: above-ground dry matter production, nitrogen (N) content, N uptake, relative chlorophyll content (SPAD) reading, and a vegetation index measured by the optical sensor N-Sensor ® ALS. The plants were evaluated in the stages V4, V6, V8, V10, V12 and at corn flowering. The experiments had a completely randomized design at three different sites that were analyzed separately. The vegetation index was directly related to above-ground dry matter production (R 2 = 0.91; p<0.0001), total N uptake (R 2 = 0.87; p<0.0001) and SPAD reading (R 2 = 0.63; p<0.0001) and inversely related to plant N content (R 2 = 0.53; p<0.0001). The efficiency of VRF for plant nutrition was influenced by the specific climatic conditions of each site. Therefore, the efficiency of the VRF strategy was similar to that of the standard farmer fertilizer strategy at sites 1 and 2. However, at site 3 where the climatic conditions

SUMMARYGenerally, in tropical and subtropical agroecosystems, the efficiency of nitrogen (N) fertilization is low, inducing a temporal variability of crop yield, economic losses, and environmental impacts. Variable-rate N fertilization (VRF), based on optical spectrometry crop sensors, could increase the N use efficiency (NUE). The objective of this study was to evaluate the corn grain yield and N fertilization efficiency under VRF determined by an optical sensor in comparison to the traditional single-application N fertilization (TSF). With this purpose, three experiments with no-tillage corn were carried out in the 2008/09 and 2010/11 growing seasons on a Hapludox in South Brazil, in a completely randomized design, at three different sites that were analyzed separately. The following crop properties were evaluated: aboveground dry matter production and quantity of N uptake at corn flowering, grain yield, and vegetation index determined by an N-Sensor ® ALS optical sensor. Across the sites, the corn N fertilizer had a positive effect on corn N uptake, resulting in increased corn dry matter and grain yield. However, N fertilization induced lower increases of corn grain yield at site 2, where there was a severe drought during the growing period. The VRF defined by the optical crop sensor increased the apparent N recovery (NRE) and agronomic efficiency of N (NAE) compared to the traditional fertilizer strategy. In the average of sites 1 and 3, which were not affected by (1)

The efficiency of nitrogen fertilizer in corn is usually low, negatively affecting plant nutrition, the economic return, and the environment. In this context, a variable rate of nitrogen, prescribed by crop sensors, has been proposed as an alternative to the uniform rate of nitrogen traditionally used by farmers. This study tested the hypothesis that variable rate of nitrogen, prescribed by optical sensor, increases the nitrogen use efficiency and grain yield as compared to uniform rate of nitrogen. The following treatments were evaluated: 0; 70; 140; and 210 kg ha-1 under uniform rate of nitrogen, and 140 kg ha -1 under variable rate of nitrogen. The nitrogen source was urea applied on the soil surface using a distributor equipped with the crop sensor. In this study, the grain yield ranged from 10.2 to 15.5 Mg ha-1, with linear response to nitrogen rates. The variable rate of nitrogen increased by 11.8 and 32.6% the nitrogen uptake and nitrogen use efficiency, respectively, compared to the uniform rate of nitrogen. However, no significant increase in grain yield was observed, indicating that the major benefit of the variable rate of nitrogen was reducing the risk of environmental impact of fertilizer.

Despite its relevance, nitrogen is poorly utilized by the plants when improperly applied. Thus, the objective of this study was to evaluate the yield and nitrogen use efficiency (NUE) in corn in response to doses and split application of nitrogen fertilization. The experimental design was a randomized block design, with three replications. Doses of nitrogen of 0, 30, 60 and 180 kg ha -1 were applied at sowing in order to create different nutritional status of corn plants and to obtain different values of Normalized Difference Vegetation Index (NDVI) measured with "Greenseeker ® " optical sensor. The subplots with nitrogen doses in topdressing of 0, 30, 60 and 90 kg ha -1 at V8 and a dose of 60 kg ha -1 at V12 were placed in experimental plots with doses of 0, 30, 60 and 180 kg ha -1 of nitrogen at sowing. Moreover, NUE was calculated in the experiment using agronomic indexes determined by applications of nitrogen in late topdressing (V8 and V12) and contrasted to the possible combinations at doses of 60, 90 and 120 kg ha -1 of total N applied. The results showed the occurrence of a linear relationship between nitrogen fertilizer dose and NDVI at V8 as well as at V12 stages. Late topdressing fertilizations (V12) did not cause a decrease in grain yield when combined with nitrogen fertilization at sowing, moreover resulted in higher NUE. Split the nitrogen dose showed better NUE than the combinations where nitrogen was not applied at sowing or in topdressing. The delay of nitrogen topdressing can be an alternative for the planning of the moment of the N fertilization according to the climate forecast in each region. Key words: Produtividade e eficiência nutricional do milho em resposta a doses e parcelamentos da adubação nitrogenadaApesar de sua relevância, o nitrogênio apresenta reduzido aproveitamento pela planta quando aplicado de forma inadequada. Assim, objetivou-se avaliar a produtividade e a eficiência do uso de nitrogênio (EUN) no milho em resposta a doses e parcelamentos da adubação nitrogenada. O delineamento experimental utilizado foi de blocos ao acaso, com três repetições. Foram aplicadas doses de 0, 30, 60 e 180 kg ha -1 de nitrogênio em semeadura visando criar diferentes condições nutricionais das plantas de milho e assim obter diferentes níveis de Índice de Vegetação por Diferença Normalizada (NDVI) medidos com sensor óptico "GreenSeeker ® ". As subparcelas com doses de nitrogênio em cobertura de 0, 30, 60 e 90 kg ha -1 em V8 e uma dose de 60 kg ha -1 em V12 foram alocadas experimentalmente nas parcelas com doses de 0, 30, 60 e 180 kg ha -1 de nitrogênio em semeadura. Além disso, foi calculado no experimento a EUN através de índices agronômicos determinados a partir das aplicações de N em cobertura tardia (V8 e V12) e contrastado às possíveis combinações nas doses de 60, 90 e 120 kg ha -1 de nitrogênio total aplicado. Os resultados mostraram que houve relação 352Telmo Jorge Carneiro Amado et al.