This study was conducted to evaluate the accuracy of sap analysis using a portable nitrate ion meter for cauliflower (Brassica oleracea L. Botrytis Group, cv. Candid Charm) petiole nitrate determination. The relationship between NO3-N concentration in fresh petiole sap and in dried petiole tissue was studied for cauliflower grown in southern Arizona during the 1993–94 and 1994–95 growing seasons. Experiments were factorial combinations of three water rates and four N rates, both ranging from deficient to excessive. Petioles were collected throughout each season and were split for analysis of sap NO3-N and dried petiole NO3-N. Linear correlations between the two methods were similar in both seasons, with no consistent effect due to water application rate or crop maturity. Therefore, a single regression equation was derived: petiole sap NO3-N (mg·liter–1) = 0.047 × dry petiole NO3-N (mg·kg–1) + 218 (r2 = 0.772). This equation can be used to relate sap test measurements to existing guidelines for NO3-N concentrations in cauliflower petioles. These results suggest that the quick sap test, using the portable nitrate ion meter, is a valuable technique for monitoring N status of cauliflower.

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A no-tillage soybean-wheat cropping system has been practiced for many years on the "Terra Rossa" soils of eastern Paraguay. Soil compactness and soil erosion have recently been identified as potential problems. This study examines the effect of replacing unprofitable wheat by green-manure turnip (Raphanus sativus L. var. oleiferus Metzg.) on soil properties and soybean production. Gaseous phase, porosity, bulk density, water saturation, cone index, pH, exchangeable-cations, available-phosphorus (P) and aggregate size distribution of the soil were measured. Contrary to initial expectations, turnip did not reduce soil compactness. Instead, turnip stabilized the aggregate structure of the surface soil. Positive effects of turnip on subsequent soybean growth and yield were detected in a rather dry year but not in an exceptionally wet year. In a second part of this study, nutrient return from turnip and wheat residues were compared. Turnip produced 10.7 t ha -1 of shoot dry matter, and absorbed 294, 27, 302, 175, and 33 kg ha -1 of N, P, K, Ca, and Mg, respectively. Wheat absorbed 98, 11, 67, 11, and 7 kg ha -1 of N, P, K, Ca, and Mg, respectively. About 75% of the N absorbed by wheat was removed from the field at harvest whereas most nutrients in the turnip residue were returned to the soil before planting of soybeans with positive effects on soil fertility. Additional benefits of green-manure turnip would include a reduced chance for erosion through improvements in aggregate structure and through a more complete soil cover. Key words: soil compactness, aggregate structure, crop residue, nutrient return ADUBAÇÃO VERDE COM NABO PARA SOJA EM SISTEMA DE PLANTIO DIRETO NO LESTE PARAGUAIO RESUMO: Um sistema de plantio direto de rotação soja-trigo foi praticado por muitos anos em uma "Terra Roxa" no leste do Paraguai. A compactação do solo e a erosão foram reconhecidas recentemente como problemas potenciais. Este estudo examina o efeito da substituição do trigo antieconômico pela adubação verde com nabo (Raphanus sativus L. var. oleiferus Metzg.) sobre as propriedades do solo e a produção de soja. A fase gasosa, porosidade, densidade do solo, saturação em água, índice de cone, pH, cátions trocáveis, fóforo disponível (P) e distribuição de agregados foram medidos. Ao contrário das expectativas iniciais, o nabo não reduziu a compactação do solo. Ao contrário, ele estabilizou a estrutura dos agregados na superfície do solo. Efeitos positivos do nabo no crescimento e produtividade da soja plantada em seguida, foram detectados em um ano muito seco, mas não em um ano excepcionalmente úmido. Em uma segunda parte desse estudo foram comparados os retornos de nutrientes do nabo e do trigo. O nabo produziu 10.7 t ha -1 de matéria seca da parte aérea e absorveu 294, 27, 302, 175, e 33 kg ha -1 de N, P, K, Ca, e Mg, respectivamente, enquanto o trigo 98, 11, 67, 11, e 7 kg ha -1 de N, P,K,Ca, e Mg, respectivamente. Aproximadamente 75% do N absorvido pelo trigo foi removido do solo por ocasião da colheita, enquanto que a maio...

SUMMARYNitrogen removal in soybean grains at harvest may exceed biological N 2 fixation, particularly if grain yields are as high as typically achieved on "Terra Rossa" soils of Eastern Paraguay. Applying N fertilizer or coating seeds with rhizobial inoculants that enhance nodulation may represent a way of balancing the N budget. However, the effects of such treatments appear to be highly sitespecific. The objective of this study was to examine the effects of N application (N) and rhizobial inoculation (I) on nodulation, N accumulation and soybean yields in Eastern Paraguay. Field experiments were conducted in two consecutive soybean seasons. Dry conditions in the first year delayed sowing and reduced plant number m -2 and pod number plant -1 . Grain yields were generally below 2 t ha -1 but the +N+I treatment increased yields by about 75%. In the second year favorable conditions resulted in yields of around 4 t ha -1 and the treatments had no effect. Nitrogen accumulation was higher in the first year and could therefore not explain the observed yield differences between years and treatment combinations. The positive effect of the +N+I treatment in year one was associated with a more rapid root growth which could have reduced susceptibility to intermittent drought stress. Nodule biomass decreased between flowering and pod setting stages in the +I treatment whereas further increases in nodule biomass in the -I treatment may have led to competition for assimilates between nodules and developing pods. Based on these preliminary results we conclude that N application and seed inoculation can offer short-term benefits in unfavorable years without negative effects on yield in favorable years.Index terms: Glycine max, inoculum, nitrogen, nodulation, rhizobium. RESUMO: EFEITO DA APLICAÇÃO DE FERTILIZANTE-NITROGENADO E DA INOCULAÇÃO DE SEMENTES COM RIZÓBIOS NO RENDIMENTO DA SOJA NO PARAGUAI ORIENTALA remoção do N em grãos de soja na colheita pode exceder a fixação biológica do N 2 , particularmente se os rendimentos dos grãos forem tão elevados quanto os obtidos tipicamente nos solos "Terra Roxa" da região oriental do Paraguai. A aplicação do fertilizante nitrogenado ou a inoculação das sementes com rizóbios que estimulem a nodulação podem representar uma maneira de equilibrar o balanço de N, porém o efeito de tais tratamentos parece ser altamente específico do local. O objetivo deste estudo foi examinar os efeitos da aplicação do nitrogênio (N) e da inoculação com rizóbios (I) na nodulação, na acumulação de N e nos rendimentos de soja sob condições da região oriental do Paraguai. A pesquisa em campo foi conduzida durante duas safras consecutivas de soja. As condições de seca ou estiagem no primeiro ano atrasaram a semeadura e reduziram o número de plantas por m 2 e o número de vagens/planta. Os rendimentos de grãos foram geralmente inferiores a 2 t ha -1 ; no entanto, o tratamento + N + I aumentou o rendimento em aproximadamente 75 %. Nas condições favoráveis do segundo ano, os rendimentos foram em torno de 4 t ha -1 e...

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