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Now that spring has arrived, it is time once again to break out the trusty GDD and precipitation charts to monitor the progress of the growing season. So far, temperature and rain are following close to the long term average. GDD accumulation has been tracking right along the long term average line. Precipitation started out below average but the recent rains have brought us back closer to normal and the forecasted rain should peg us right on the long term average.
2003 Shoot and Cluster Observations
The GDD and precipitation information from 2002 shows that we had above average temperatures and plenty of soil moisture during the month following bloom. This suggests that cluster development should be normal to above normal for 2003. The late season dry weather in 2002 and long cold winter may have decreased wood maturity (pruning weights or ripe nodes of periderm) and bud survival; however, the buds that made it through the winter look like they are in good shape. At the Fredonia Lab, shoots originating from sun-exposed canes appear to have three good clusters. It is still early to be "counting berries before they are pollinated" because we still have to get over the threat of frost and go through fruit set. However, the tool of looking at the weather conditions the month after bloom in year one to get a rough estimate of bud development for year two appears to be holding up. Frost and fruit set are events independent of the previous year's weather conditions.
More on Nitrogen Fertilization
I received some more questions on nitrogen fertilization after last week's crop update. . . so here is my bare bones approach to nitrogen fertilization.
Nitrogen sources: There are basically three nitrogen sources for grapevine growth: mineralization of nitrogen from organic matter, remobilization of stored nitrogen from perennial tissues, and inorganic nitrogen fertilizers. By far, the largest pool of N for grapevine growth comes from the mineralization of organic matter. Although there are many biological, chemical, and environmental processes at work, the basic rule of thumb is that for every 1% of organic matter in your vineyard soil about 15-20 pounds of N is released for plant uptake. Stored N is probably the most easily used by the vines in the spring because it is already in the vine and just needs to be converted and remobilized. Inorganic nitrogen fertilizers are basically just used to supplement the other two sources during periods of peak N demand.
Nitrogen Demand and Fertilizer Efficiency: Concord vines demand and will take up large amounts of nitrogen during periods of rapid growth and with sufficient transpiration rates. More specifically, bloom to veraison. Relatively little nitrogen (around 5%) is taken up prior to bloom when compared to what is taken up after bloom. However, this 5% may be important if weak vines have inadequate nitrogen stores. The 5% is less important in healthy vineyards where nitrogen reserves directly provide nitrogen for growth - including new root growth that absorbs additional soil nitrogen. Research out of Michigan by Stan Howell indicates that mature healthy Concord vines need approximately 70 pounds of nitrogen per year. Further research (again out of Michigan) by Tom Zabadal and Eric Hansen shows that bud break applied N fertilizer is only 10% efficient and bloom applied N fertilizer is a bit more efficient at 15-20%. Meaning, for every 100 pounds on nitrogen applied at bud break, only about 10 pounds is making it to the vines.
Lets do some math: Say I have a healthy mature Concord vineyard with 3% organic matter. I figure that the vines need 70 pounds of N per year and that my soil is releasing approximately 60 pounds from mineralization. I need to make up 10 pounds of nitrogen through inorganic fertilizers. If I apply fertilizer only at bud break and get only 10% uptake efficiency I need to put on 100 pounds of fertilizer nitrogen just to make up the needed 10 pounds. If I rely on reserves and organic matter in the pre-bloom period and apply nitrogen around bloom and increase fertilizer efficiency to 20% then I only need to apply 50 pounds of fertilizer nitrogen to make up the needed 10 pounds.
Does our math work in the field? Long term research from the West Tier shows that ownrooted Concord vines on soil with 2-3% organic matter achieve maximum productivity with 50 pounds of N fertilizer. Going up to 100 pounds does not improve productivity and is a waste of money. At the nitrogen trial at the Betts' vineyard on a heavy soil type, the organic matter is a bit higher at 4-5%. In this case, we have recorded no differences in vine growth or productivity with 0, 50, or 100 pounds of nitrogen. It is both possible and probable that the mineralization of organic matter is providing the total vine nitrogen needs in that particular vineyard block.
What happens to the rest of the fertilizer nitrogen? Some of the nitrogen is used by the other macro and micro organisms in the soil such as weeds, worms, and bacteria. This nitrogen enters the living portion of organic matter and can eventually be used by the vines in the future. A very small amount of nitrogen is absorbed onto soil particles by cation and anion exchange. The rest of the nitrogen can be lost to the system through leaching, erosion, and denitrification. Nitrogen loss, especially leaching, is getting more attention these days because of its potential impact on the environment. It is in all of our best interests - financially and environmentally - to use efficient fertilizer practices.