No change in nitrogen is needed if the vines are making this amount of growth and produce three pounds of cane pruning per vine. Musadine canes are more numerous and less vigorous, and the desired growth is 30 to 36 inches. Phosphate occasionally limits grape production. Potash is recommended if the need is shown by foliar tests or if deficiency symptoms appear. Later in the season potash deficiency appears as black leaf, generally as interveinal blue-violet flecks or blotches on the upper surface of the exposed leaves.
Potassium deficiencies are more common on very sandy soils or where root systems are restricted.
Grapes require proper pruning on a regular, yearly basis to balance vegetative growth, fruit yield, and fertilization. Fertilization cannot correct improper pruning. Soil analysis for grapes is useful primarily for vineyard establishment and cover crop fertilization. Observations of leaf and cane growth and tissue analysis are of much greater value than soil test on an established vineyard. Rejuvenating vines that have been neglected several years usually requires very heavy pruning according to the type of grape and support.