Improper fertilization of grapevines is at best a wasted expense, but of greater concern is the potential for a detrimental effect on vine growth and crop maturity. The first step in accurately determining the nutritional needs of grapevines is to thoroughly analyze the soils in the vineyard. It is strongly suggested that both the topsoil (0"-8") and subsoil (8"-24") be sampled. Randomly select locations across the vineyard to collect slices or core samples. If there are obvious differences in soil types, its best to map and sample those areas separately. Collect soil and place in a non-reactive container (plastic or glass), remove stones and organic matter, and place samples in an open area to dry. Mix each sample well and submit about two cups of soil from each sample for analysis. The results will be a starting point for determining fertilizer inputs and are generally enough to work with up for a non-bearing vineyard.
Soil analysis can reveal what is potentially available to the vine, but does not give us a good indication of soil/plant interaction. Soil depth, texture, water-holding capacity along with varietal and rootstock differences along with various cultural practices all impact how a vine absorbs nutrients from the soil profile. Soil samples are essential however in understanding fertilization approaches when a need is detected. For perennial fruit crops, tissue analysis has shown to be the single most reliable way of determining nutritional needs and for grapevines the most predictive tissue is the petiole (leaf stem). Petioles can be sampled at two different times during the growing season. Bloom-time petioles are the best indicators of zinc and boron status, while sampling at veraison will provide more accurate readings of potassium and magnesium levels.
Bloom-time sampling should occur at approximately 50% cap-fall. To sample a given block, randomly select thirty vines that represent the average vine condition. Sample different varieties, problem areas and blocks of dissimilar age separately. From each of these vines, choose two primary bearing shoots that are well exposed to sunlight. From each of these, remove the subtending leaf of the basal cluster. Remove and discard the leaf blade and retain the petiole. These sixty petioles constitute a sample. Be sure to store the petioles in a breathable bag (not plastic) while you are in the field to keep the petioles from wilting.
Petiole sampling at veraison is similar in procedure, but rather than subtending leaves, choose the most recently matured leaf on the stem. The most recently matured leaf is the last leaf on the shoot to achieve full size at the time of sampling. Again, this timing is the best to gain insight into the potassium/magnesium balance. Although veraison is suggested at the time to sample "fall" petioles, this procedure can be accomplished later in the growing season as long as the canopy is still healthy and functional.
It is strongly suggested that petioles be washed before they are analyzed. Briefly (ten seconds) submerge and gently rub all of the petioles from a given sample in a mild dish soap/water solution (phosphorus-free is best). Then rinse them two or three times, using distilled water for at least the last rinse. This removes soil and fungicide residues from the surface of the petioles, each of which can result in spurious readings. After rinsing, place the petioles on a paper towel or in an open paper bag in a warm, well-ventilated area. Allow the petioles to dry completely (crisp) before sending them to the lab for analysis.
Some growers use bloom petiole nitrogen readings to adjust seasonal applications, but visual inspection of the vine is as, or more reliable. Canopy fill and basal leaf color are commonly used to gauge nitrogen status. A full trellis and a deep green color of basal leaves indicate adequate nitrogen. Excessive vegetative growth indicates that nitrogen levels may need to be reduced.
With both soil and petiole analysis, growers can accurately predict vine needs for the coming season. Remember however, other cultural practices can impact tissue analysis results. Insufficient irrigation or weed control will result in spurious results. Random and thorough vine selection will also improve the reliability of results. Soil and tissue analysis are available from both Texas A&M University and several private laboratories across the state. With analysis comes a recommendation for fertilizer amendment application. If there is any confusion or the need for further interpretation, don't hesitate to call.