A few days ago I noticed the bottom leaves of my polytunnel tomatoes turning yellow. I was pretty sure this was a sign of a nutrient deficiency so I looked it up, and found I had the classic signs of magnesium deficiency in tomatoes – yellow leaves with green ribs. My tunnel is in my new garden, on the old veg patch, which has light soil which seems to have had a lot of ash added to it. Too much potash in the soil maybe (which according to the books can contribute to magnesium deficiency). I’ve not had this problem with my tomatoes before, maybe because of better soil and feeding with comfrey liquid in the past, but I’ve run out of that. Epsom salts are apparently the standard remedy, and are allowed in organic growing where there is a specific need for them. Having now given my tomatoes ‘a dose of salts’, I’ll let you know in a week or so how they look! Long term I hope to improve the soil so that this doesn’t happen again.