Objetivando-se comparar efeitos de fontes de nitrogênio no desenvolvimento e rendimento agrícola da soja, cv.IAC-8, conduziram-se experimentos em condições de campo, em solo classificado como terra roxa estruturada em Piracicaba, SP. As áreas experimentais diferiram quanto ao histórico de cultivo, sendo uma cultivada com cana-de-açúcar por dois anos e outra com soja durante três anos consecutivos. Foram utilizados sulfato de amônio e ureia como fontes minerais e inoculantes turfosos, oleosos e em emulsão como fontes biológicas. O tratamento controle não recebeu qualquer fonte de n. Os resultados obtidos revelaram que entre as fontes de nitrogênio utilizadas, os inoculantes turfosos foram os que permitiram maior nodulação de plantas de soja e rendimento agrícola em área sem histórico de inoculação. Em área com histórico, a resposta em termos de rendimento foi semelhante para todas as fontes. Essa área anteriormente colonizada com os rizobios por outros cultivos com soja apresentou plantas com maior porte e maior acumulo de matéria seca e rendimento em grãos pelo melhor aproveitamento da simbiose entre bactéria e hospedeiro

Experiments were carried out in order to compare the effects of different nitrogen sources on the development and yield of soybean (Glycine max (L.) Merrill), cultivar IAC-8, under field conditions. The sowings were made in 1990, during the first two weeks of December. The experiments were located on the experimental field of the Department of Agriculture, Faculty of Agriculture "Luiz de Queiroz", University of São Paulo, in the city of Piracicaba, State of São Paulo, Brazil. Plants were grown on a fertile tropudalf soil classified as 'Terra Roxa Estruturada'. The two fields were the experiments were established differed on their history of cultivation. One was previously cropped with sugarcane during two years (no inoculation history) and the other received soybeans for three consecutive years. The nitrogen sources consisted of mineral sources (ammonium sulfate and urea) and biological sources, by the use of the following carriers for the inoculum Bradyrhizobium japonicum: two peat carriers of different origins and, among the liquid ones, a mineral oil, a plant oil and a water emulsion carrier. A control check treatment with no nitrogen or inoculant addition was also tested. The soybean plant traits were evaluated, so that the potential effects of the different nitrogen sources on plant of development and on grain yield could be detected. A randomized complete block experimental design was utilized, consisting of eight treatments, with five replications in each experiment. The results from the evaluations performed over the crop life cycle led to the following conclusions: Among the nitrogen sources used, the peat carriers were those that resulted in highest plant nodulation and yield, in the experiment with no inoculation history. In the area with inoculation history, the response in terms of yield was similar among the different treatments. This area was previously colonized by Bradyrhizobium japonicum and this promoted and increased growth and dry-matter accumulation for the soybean plants, as well as an increased grain yield, due to the advantageous symbiosis between the bacteria and the host found in this field