Specifically for soybean yield, I have developed a spectral model based on EVI/MODIS data which is very useful for estimations in advance to crop harvest. Please take a look at Gusso et al., (2013) in the International Journal of Geosciences.
There exist many methods in doing this.such as the Crop yield estimator using spatial parameter broadcasting services where remote sensing is highly applicable
Most remote sensing estimates of yield are based on NIR/Visible information, which integrates leaf greenness, leaf biomass and, to a lesser extent, water content. Basically, they attempt to quantify what a good farmer's eye can see when predicting yield. In most cases, the indices perform better at critical stages, such as anthesis, although for some crops time-integration could be useful (e.g. for maize to determine green area duration). However, depending on your purpouse, accuracy is not very high, and the spectral features require site-specific calibration and, in earlier stages, when plant cover is limited, to consider soil properties.
Have a look at some of these papers:
Ferrio, J. P., Villegas, D., Zarco, J., Aparicio, N., Araus, J. L., & Royo, C. (2005). Assessment of durum wheat yield using visible and near-infrared reflectance spectra of canopies. Field Crops Research, 94(2), 126-148.
Cabrera‐Bosquet, L., Crossa, J., von Zitzewitz, J., Serret, M. D., & Luis Araus, J. (2012). High‐throughput Phenotyping and Genomic Selection: The Frontiers of Crop Breeding ConvergeF. Journal of integrative plant biology, 54(5), 312-320.
Weber, V. S., Araus, J. L., Cairns, J. E., Sanchez, C., Melchinger, A. E., & Orsini, E. (2012). Prediction of grain yield using reflectance spectra of canopy and leaves in maize plants grown under different water regimes. Field Crops Research, 128, 82-90.
Su, T., Feng, S. Y., & Cui, X. Y. (2013). Regional Yield Estimation for Spring Maize with Multi-Temporal Remotely Sensed Data in Junchuan, China. Advanced Materials Research, 610, 3601-3605.
Serrano, L., Filella, I. and Josep Penuelas, j. (2000)̃:
Remote Sensing of Biomass and Yield of Winter Wheat under Different Nitrogen Supplies. Crop Science 40, 723-731.
And these two may also be of interest:
Laigang Wang, Yongchao Tian, Xia Yao, Yan Zhu, Weixing Cao (2014): Predicting grain yield and protein content in wheat by fusing multi-sensor and multi-temporal remote-sensing images. Field Crops Research 164, 178-188.
GNYP, M.L., BARETH, G., LI, F., LENZ-WIEDEMANN, V., KOPPE, W., MIAO, Y., HENNIG, S.D., JIA L.L., LAUDIEN, R., CHEN, X.P. and ZHANG, F. (2014): Development and implementation of a multiscale biomass model using hyperspectral vegetation indices for winter wheat in the North China Plain. J. Appl. Earth Observ. Geoinf. 33, 232-242. DOI: 10.1016/j.jag.2014.05.006