Answer: Compost would not make the soil too heavy, in fact it helps to hold both air and water and improves soil structure. I would take a look at several factors in trying to decipher the problem here. First of all I would have the soil tested for both pH and fertility both in problem spots and spots that are apparently okay -- it is possible that the pH has somehow gotten out of whack (wood ashes for instance can have an effect on pH in addition to the lime you have been using) or that the nitrogen level is too low to sustain intensive plant growth. Next you might investigate if there are tree roots in the garden soil that are robbing it of nutrients and water, and make sure that whatever watering system you are using is functioning correctly. After that I would look for any patterns to the problem. I am a bit mystified because you have problems with say some of the cucurbits but not cucumbers, and your warm, rich soil lovers such as tomatoes are fine but the melons are not. For instance, is it possible that there is in fact a disease at work but it is showing up only as plants are rotated through a particular area of the garden? Are you experiencing an infestation of say squash bugs or other insect acting as a vector? Finally, I would suggest you take a sample or two to your county extension and see if they can identify the problem based on a visual inspection of the plants. I'm sorry I can't be more helpful.
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