Should you use any fall fertilizer?
By Dr. Carolyn DeMoranville [Excerpted from the Sept. 2009 issue of the UMass Cranberry Experiment Station’s cranberry newsletter. Used with permission.]
Each fall, growers ask me – should I apply fall fertilizer? The answer for many growers may be a cautious yes if the plants are looking weak or the crop is heavy. If this is your situation – how should you decide if this is needed on your bog and what should you use? First look at your vines, crop, and fertilizer use so far this year. Follow the decision tree below to decide if fall fertilizer is right for you:
|Are your vines lush or long?||Yes||No|
|Did you have a big crop this year?||No||No|
|Have you already applied more than
30 lb. N (50 lb for hybrids)?
|If you have deep organic soil with little need for
early fertilizer normally
But if your fertilizer use was modest, the vines are not pumped up, and you had a great crop, then consider using fall fertilizer. Choose a material and apply it between early and mid-November when the soil has dried from harvest but well ahead of the winter flood.
So what to use and how much? My best recommendation is to apply 5 lb/acre N, little or no P and as much K as you can find among the available fertilizer choices. The N will build up the vines, P is not needed in the fall (natural release from the soil is occurring), and added K may enhance hardiness. Also, N and K are the two elements that are removed in the greatest quantity in harvested fruit; P is at much lower concentrations in the berries.
Most growers try to apply in 100 pound increments for ease and uniformity of application, so let’s discuss fall fertilizer choices on that basis.
Look at your choices of fertilizer and determine how much N, P, and K (in pounds) would be applied in 100 lb of fertilizer material. Remember that you are looking for about 5 lb/acre N.
Many common materials (like 12-24-12, 15-15-15, and 18-8-18) are just too high in N. Some of the choices in the table would give around the 5 lb/acre N rate with 100 lb/acre of fertilizer but would add much more P than is desired, creating an environmental risk. Those include the 8-32-16 and the 6-24-24. While both give about the right N and fairly high K, seasonal P will exceed 20 lb/acre even if you used 18-8-18 during the season with these choices. If you choose the 8-32-16 at 75 lb/acre (to get the N down closer to the recommended 5 lb/acre), you still add more than 10 lb P/acre and now only about 10 lb K/acre.
The best off-the-shelf choices are 5-15-30 (good N, lots of K, moderate P) or 3-13-26 if you want less than 5 lb/acre N. I do not recommend using 3-13-26 at higher than 100 lb/acre rates to boost N since then you are paying for more materials to apply and again increasing P. The 5-10-10 is a good choice but a bit lower in K. This should be fine if your vines aren’t stressed or crunchy. If they are, I would prefer the 5-15-30 to get the additional K.
The best material for low P in the table is the 5-5-20. This gives very low P, target N and substantial K.
What about fish fertilizer? An application of 5-10 gal/acre can replace granular fall fertilizer. If you have pop-up sprinklers and can apply this post-harvest, that is an additional option. Remember, this fertilizer is taken up through the roots, so water it in enough to wash it to the soil.
Finally, what about zero P fertilizers? These are predominantly formulated for turf and most have 25-30 lb N in a 100 lb/acre application (first number is 25 to 30) leading to issues with applying the low N desired in the fall. If you can find one with N in the desired range, by all means use that.
The bottom line? — if you use fall fertilizer, choose one with the lowest possible P.
Carolyn DeMoranville – UMass Cranberry Experiment Station / 1 State Bog Road / P.O. Box 569 / East Wareham, MA 02538 http://www.umass.edu/cranberry
Authored by Carolyn DeMoranville, UMass Cranberry Station. Prepared for the UMaine Extension cranberry web site by Charles Armstrong, Cranberry Professional, UMaine Extension. Nov. 2009.