Dick Grimshaw asked me to respond to your enquries re gully erosion on Sodic soil. We too have a lot of gully erosion on sodic grazing land in Australia. The cause of gully erosion is concentrated flow of runoff water, therefore the principle of gully erosion control is to spread runoff water out as much as you can. I found planting vetiver hedges above gully head to spread runoff water is most effective, but for immediate effect you can build a diversion bank to take water off the site. Above each gully head, I recommend you plant 1 or 2 rows of vetiver on contour line. Plant the first row about 5m upslope and a second one only needed for very big gully) another 10m upstream. The row should be extended least 5m beyond the outer edge of the gully to ensure that water doesnot flow back to the gully. Normally I found that this treatment is enough but you can also plant a few short rows on the gully floor later to trap sediment. I hope this will help.