Although we're not expecting actual ashfall like after Mt St Helens or Montserrat, there will be deposits of fine ash or dust from any rainfall over the coming days. Apparently the main risk to agriculture is to livestock consuming the deposits off grass/crops - as well as being put off their feed and requiring supplement. Ash itself contains many nutrients beneficial to new plant growth, albeit slightly inhibitive to older plants if too thickly deposited. This is not because of the nitrogen content, but rather other minerals that microorganisms can breakdown. There is also research to suggest that diffuse sunlight from high atmospheric ash content is actually better for photosynthesis.
Just to allay any fears people may have of this eruption 'choking' the pastures green (unless Katla goes off, as everyone keeps saying) :)
Ashfall mitigation strategies for agriculture from the USGS
Looks like the best advice for growers is to simply wash ash deposits into the soil off any foliage, especially off animal fodder crops.