Grapes are voracious growers that pull nutrients out of the soil to provide food for their extensive growth both above ground and below. According to research done in India by Jagdev Sharma, Thompson grapes receiving fertilizer through their irrigation instead of the normal ground application "at the rate of 40% produced the yield equivalent to 100%." This means when you mix your fertilizer in water, you can use less than half of the normally recommended amount.to do the job.
Water your Thompson grape plants in the morning, when they have plenty of time to dry out. They are the most susceptible of all the grapes to powdery mildew so you need to be extra careful to keep the actual leaves and stems dry as much as possible. Of course, rain is inevitable but providing space between the plants will allow good air movement. You should never fertilize a plant while it is dry so if you are planning on adding fertilizer to your water, make sure the plants are well hydrated first and then give them a second watering with the fertilizer.
Mix the dry granular fertilizer (10:10:10) at the rate of 1/8 cup to 1/2 gallon of water for each plant. Pour the 1/2 gallon of dissolved fertilizer around the base of the plant without getting it on the leaves as soon as you see new growth appearing after planting. Repeat this application again about a month later.
Feed the plants again the following years in the same way, except use 1/2 cup of the dry fertilizer to the 1/2 gallon of water. Apply around the base of the plant into the soil as soon as you see the buds swelling on the vines. You need to use this increased amount since the plant will have easily doubled in size by this time.