Have the Rains Washed Away All of the Nitrogen?
By: Lee Stivers, Penn State Extension Educator – Washington County
June 2015 is certainly shaping up to be one of the rainiest on record for many parts of Pennsylvania. During growing seasons like this, when it seems that every day brings another shower or thunderstorm, most Pennsylvania crop producers start to think about applying supplemental nitrogen to their field, forage and vegetable crops. But what about grapes—should wine grape growers be concerned about nitrogen losses in a rainy year?
Nitrogen is a critically important nutrient for any plant to grow and thrive. However, managing nitrogen in the vineyard can be tricky because vine growth and fruit quality are affected when nitrogen is available in excess amounts as well as when it is deficient. Too much nitrogen can stimulate excessive vegetative growth, throwing the vines out of balance. This is especially true under rainy, wet conditions. That extra vine growth may need to be pruned out later in the season in order to get sufficient air and light penetration into the canopy for proper fruit ripening. Yields can also suffer from excessive nitrogen uptake.
We don’t want to provide too much nitrogen to wine grapes, nor do we want to provide too little. Insufficient nitrogen can reduce crop yield through a reduction in clusters, berries, or berry set. Nitrogen deficiency in wine grapes is not easily recognized, but a typical symptom is a uniform light green color of leaves, compared to the dark green of healthy grape leaves. Typically, soil organic matter decomposition supplies much of the nitrogen needed by grape vines. Nitrogen depletion will occur most rapidly in soils with low organic matter. If soils are not supplying enough nitrogen, then a supplemental fertilizer application may be needed. Soils with adequate organic matter will not necessarily lose much nitrogen during a rainy growing season since organic matter is not soluble.
How do you know if your vines need supplemental nitrogen fertilizer? There is no simple answer to this question, but Tony Wolf provides a very useful set of observations and measurements for assessing the nitrogen status of a vineyard in the “Wine Grape Production Guide for Eastern North America” (page 159-160). By making observations over time of canopy fill, leaf size and color, shoot growth rates and fruit quality, and also measuring yields, cane pruning weights, and bloom-time petiole nitrogen concentrations, you can determine if your vines have a nitrogen status of deficient, adequate, or excessive. In addition to these factors, it is also important to test your soil periodically to monitor soil organic matter levels, as well as other plant nutrients.
How to collect and submit a petiole sample for plant tissue analysis. It’s easy to collect and submit a petiole (leaf stem) sample to Penn State’s Agricultural Analytical Services Laboratory, and you don’t even have to visit an Extension office to purchase a kit. Full instructions can be found here: Penn State Plant Tissue Analysis Submission Instructions and you can download the proper form here: Penn State Plant Tissue Analysis Submission Form.
For routine analyses, petiole samples must be taken at bloom, or at veraison, at least 70 days after full bloom, as these are the only times of year when nutrient levels in the plant are relatively stable. If samples taken at other times of the year are submitted, faulty interpretations could result due to incorrect sampling technique.
To collect the sample, remove 50 to 75 first fully-expanded leaves. Use the higher leaf number for varieties which have small petioles. Samples should be taken from fruiting shoots located halfway between the ground and highest trellis wire. After removing leaves from vines, separate petioles from leaf blades. Send only petioles (leaf stems) for analysis. Loosely wrap the petioles in a dry paper towel. Fill out the form completely, and attach a check, payable to Penn State University, and mail to Penn State Agricultural Analytical Services Laboratory, Tower Road, University Park, PA 16802.
How to take a soil sample and submit it for nutrient analysis and recommendations. Penn State’s Analytical Laboratory also offers soil testing services for commercial growers. This important soil management tool aids in monitoring soil fertility and determining optimum lime and fertilizer requirements for all types of crops, including wine grapes. Full instructions can be found here: Penn State Soil Analysis Instructions and you can download the proper form here: Penn State Soil Analysis Submission Form.
To collect the sample, you will need a trowel, shovel or auger, and a clean bucket. Obtain thin slices or borings from 13-15 randomly selected places in the vineyard. Sample to approximately 12 inches in depth. Collect the samples in the bucket, and mix thoroughly into one composite sample. Spread the soil on newspaper in a warm room to air dry overnight. Do not heat. Take 1 cup of the sample and place it in a sturdy bag or plastic container.
Fill out the form completely and attach a check, payable to Penn State University. Note that there is an additional fee for the test for organic matter. Mail the sample and the form to Penn State Agricultural Analytical Services Laboratory, Tower Road, University Park, PA 16802.