This study aimed to evaluate the effect of different doses (0, 10, 20, 30, 40, and 50 t ha-1) of green coconut fiber in growth substrate on the early development and physiology of two soybean cultivars (Pampeana 40RR and Pampeana 60RR). The experiment was carried out in a greenhouse using a completely randomized experimental design in a 2x6 factorial arrangement (two genotypes and six doses) with five replicates. Each replicate was made up of one plant, totaling 60 experimental units. Biometric variables (height, number of leaves, stem diameter, leaf area, and dry matter) and physiological variables (photosynthesis, stomatal conductance, transpiration, internal carbon, water use efficiency, and photosynthetic pigments) were evaluated. After obtaining the data 30 d after sowing, the means were subjected to an analysis of variance and, when significant for the F test, they were subjected to regression analysis and comparison of means by the Tukey's test. Through the regression analysis, the ideal minimum dose for each variable could be calculated. We observed an increase in plant height, stem diameter, number of leaves, leaf area, leaf dry mass, stem dry mass, and root dry mass of around 51.10%, 31.60%, 52.83 %, 61.78%, 79.65%, 81.52%, and 6.06%, respectively, when we compared the values of the minimum doses with the maximum points found in each variable. Regarding the gas exchange, cultivar 60 RR was superior to cultivar 40 RR. In conclusion, the green coconut fiber compound had a positive influence on the growth and physiology of the cultivars, with the best response being obtained at the dose of 30 t ha-1.