I have had nematodes in my soil since I started gardening in this spot over 35 years ago. I quickly went to RKN resistant hybrids and didn't have much trouble with them agian until I started seriously growing heirlooms again because I tired of the same old same old round, red, only moderately tasty tomato from the hybrids. The one exception was Big Beef which I found had good flavor and great resistance.
Once I was seriously growing heirlooms I had to figure out a way to slow down the destruction caused by the RKN. The absolutely most effective thing I found after reading an article that gave me the idea was to put in fresh horse manure and till it in immediately. I did this several times over a few months to a couple of my beds that were the most infected and the next spring season planted tomatoes in those beds. I had almost no nematode damage for several years in those beds but then had to repeat the process. It is a job mucking out stalls in the heat of summer and spreading the stinky fresh manure and tilling it in several times but it was well worth the effort.
None of the efforts I made to combat RKN would reverse the problems I was having with fusarium wilt which only grew worse when my beds ended up infected with all three races of fusarium wilt. I couldn't even count on Big Beef anymore.
After a few years of constantly replacing dying plants I gave up and started grafting and since I started grafting using a rootstock that is both resistant to all three races of fusarium wilt and RKN I have not had problems with either. I agree that you should definitely build up your organic matter as much as you can to lessen the sandiness of your soil which will help some but not enough to really cure the problem. If you have a cheap source of fresh horse manure and most stables will gladly let you muck out the stalls when the horses are out in the pastures then I think you should go ahead an try using it and see if it makes a big difference next year. However I think the best thing you can do is learn to graft your own plants and you can start practicing during the summer so you become proficient by winter when you will need to graft a good number of plants.
As for the French Marigolds I tried them for several years and they did make a slight difference but you need a lot of them for them to do any good at all. You basically need your tomatoes growing in a dense growth of them and they will take away a lot of nutrients from your tomatoes. I found them much more effective at slowing down nematodes in cucumbers. I also found that spider mites liked them and that caused more spider mites on the tomatoes above them.
I found solarizing a waste of time and money. Maybe if the nematodes weren't so prevalent as they are here it might be worth the effort. Go to a garden center and set out some Big Beef plants and see if they do okay during the rest of the summer and fall. They won't be as big or produce as large a tomato as earlier in the season but if you can keep the foliage diseases and the spider mites at bay you should be able to grow some decent tomatoes; but if you really want to grow heirlooms with the problems you have then try grafting tomatoes and building up your soil for the best results.
Bill
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