Time to fertilize blueberries

How to get the right rate and timing with fertilizer applications

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 considerably 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 sand 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.

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 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 may be just as effective. N should not be applied to the soil after June because this may promote late flushes of growth that may 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% indicates rates should be increased and levels higher than 2.3% 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 thee bags to a reputable laboratory. Leaf analysis is well worth the time and money.

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

 

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