VITICULTURE NOTE 1: SOIL TESTS

Often, the first thing a prospective grower is encouraged to do is to get a soil test. Prospective grower then digs a few holes, and spends money to have the soil tested. The grower then is told to add soil amendments based on the tests and only later finds that that he may have wasted money, or worse caused damage to the quality potential of his vineyard. How can this be?

Grape vines are deep-rooted perennials completely unlike shallow-rooted annuals like corn and soybeans for which soil tests make more sense. Often young vineyards will display a deficiency only to grow out of it as the vines develop deeper and more invasive root systems. Also cultivars and rootstocks differ widely in their requirements for and uptake of various macro and micro nutrients. It is very difficult to predict how your plants will be responding in a mature state four years after planting. I have reviewed tissue test results of vinifera and interspecific hybrids growing side by side which show dramatically different potassium/magnesium ratios. If you make the mistake of adding potassium to a soil with rootstock-cultivar combinations with high potassium uptake, you could well trigger magnesium deficiency and will almost certainly increase wine potassium levels. High wine potassium levels are the bane of winemakers worldwide.

So should you ignore soil characteristics and go ahead and plant? Clearly the answer is no. Soil characteristics are arguably the most important element to consider when planting grapes. It's no secret that wine grapes require soils with excellent internal drainage and depth for root penetration. Also pH is important, but not as critical as some make it out to be. As long as pH is above 5.5 everywhere in the rooting zone down for at least three feet you should be OK. Acid soils with a pH of 5 or below will profoundly affect the growth and viability of your plants. Limeing is not a long-term solution to the problem. Even if it is subsoiled in, within a few years the soil pH will return to previous levels and it will be virtually impossible to incorporate further lime additions at depth without damaging vines.

Here are my recommendations:

1. Get comprehensive soil maps and descriptions for your site from your extension agent. Carefully review them. If there is any question about internal drainage or pH, re-consider planting. Talk with neighbors who grow crops on the same soils and see if there are any glaring and consistent nutrient deficiency problems. If there are then again you should consider another site. It is possible that the soil survey could be wrong. In this case it is best to hire a soil scientist to sample at depth and independently map and interpret data for your site.

2. Layout and plant your vineyard with perhaps a handful of low nitrogen fertilizer incorporated just below the soil surface of each plant. Remember weed control is all important at this stage.

3. Observe the growth of your plants and if something looks seriously wrong, consult a viticulturalist or an expert grower. Often low levels of herbicide damage or disease mimic nutrient deficiency. If serious doubt exists consider tissue analysis where petioles (leaf stems) are collected and sent to a lab.

4. At four years after planting, consider either post-bloom or veraison petiole samples. By this time the vineyard should be well established and in full production. When analyzing the results remember that moderate to low nitrogen and potassium levels are not necessarily bad. Unless your vines are showing poor vigor or obvious signs of potassium deficiency, don't fertilize. Micronutrient problems can be solved with foliar sprays tank mixed with fungicide sprays.

5. Choose your soils carefully, manage your vineyard well, and you should seldom if ever need to fertilize. In ten years of growing grapes in Oregon, and 5+ years of growing grapes in the Bluegrass, I have never fertilized except at planting. Excessive vigor and high potassium levels remain my biggest obstacles to improving quality from the vineyard. My only significant nutrient deficiency problem is magnesium. None of the above could be predicted from soil tests.