Time to fertilize blueberries
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EmailUse the right fertilizer at the right rate and time to achieve optimal growth and yields in blueberries.
Most Michigan blueberry fields need annual applications of nitrogen (N). Too little N reduces growth and yields, but too much can have similar effects, as well as wasting money and possibly impacting water quality. To be effective, use the right fertilizer at the right rate and time.
Fertilizers
Use fertilizers supplying ammonium-N, such as ammonium sulfate or urea. Ammonium sulfate is more acidifying than urea, and is the best choice if you want to reduce pH slightly. If pH is sufficiently low – below 5.0 – urea may be best since it has less effect on pH. The cost per pound of N is higher for ammonium sulfate than urea. Fertilizer blends work fine if most of the N is ammonium, but calculate the price you are paying per pound of N, not per bag of fertilizer.
Rates
General rates in pounds per acre are given in Table 1. These may be low for plants on very sandy soil with low organic matter, since these soils supply relatively little N from organic matter breakdown. High organic soils and mucks may require lower rates than those in Table 1 because these soils naturally supply high amounts of available N. The best way to judge whether you are using proper rates for your fields is to submit leaf samples for nutrient analysis in the middle of the summer. This will not help this year, but will give you guidance for next season.
Table 1. Nitrogen recommendations for Michigan blueberries (lb/acre).
| Age (years) | N | Urea | Ammonium sulfate |
| 2 | 15 | 35 | 75 |
| 4 | 30 | 70 | 150 |
| 6 | 45 | 100 | 215 |
| 8 | 65 | 150 | 300 |
Timing
Blueberries absorb little N until after budbreak. Active uptake begins during bloom or petal fall, and rapid uptake and strong demand continues from this time until harvest. Apply N between bud break and bloom. This will allow N to move down into the root system by petal fall. If the soil is sandy, a split application is usually beneficial. Apply half between bud break and bloom and half in early to mid-June. This will help maintain available N until harvest. On heavier soils or muck soils where N does not leach as readily, a single application just prior to bloom may be just as effective. Do not apply N to the soil after June because this may promote late flushes of growth that does not harden off adequately in the fall.
Monitoring
Collect leaf samples in the middle of the summer and have these analyzed for nutrient content. Leaf N levels will tell you whether rates for your specific fields need to be adjusted up or down. Leaf N below 1.7 percent indicates rates should be increased, and levels higher than 2.3 percent mean you are applying too much. Sample at least 50 leaves from different bushes in late July to early August. Select healthy leaves from the middle of this year’s shoots. If the leaves are dusty, rinse them briefly in tap water, spread them on a table top until they are dry to the touch, package them in paper bags, and send the bags to a reputable laboratory. Leaf analysis is well worth the time and money.
Dr. Hanson's work is funded in part by MSU's AgBioResearch.
