Some Thoughts on Fertilizing 

 Blueberries Efficiently 



Eric Hanson 



Horticulture Department, Michigan State University 



Blueberries require annual applications of 

 nitrogen (N) containing fertilizers. Most blue- 

 berry soils are not able to supply naturally the 

 quantities of N required for optimum production 

 and growth. However, growers have a number 

 of choices to make in designing a fertilizer pro- 

 gram for their plantings. The most important of 

 these is deciding which N fertilizer to use and 

 how much to apply. 



Optimum N rates vary a great deal from 

 farm to farm. Plantings on organic soils may 

 require only 10 to 30 lbs N/acre because these 

 soils supply a great deal of N naturally. 

 Plantings on very sandy soils low in organic 

 matter may require 80 to 100 lbs N/acre because 

 these soils supply very little natural N. On 

 average, blueberry plantings in Michigan per- 

 form best with 60 to 70 lbs N/acre. 



This is a wide range in rates. How can you 

 be sure your rates are best for your plantings? 

 Careful observations of the growth, production 

 and appearance of the bushes is helpful. 

 Underfertilized bushes generally will not grow 

 or produce as much. Leaves tend to be small and 

 pale green in color. They tend to develop fall 

 colors and drop early. Annual shoot growth is 

 quite short and fruit production is low. Bushes 

 receiving too much N are dark green and lush 

 appearing. This may produce excessive growth, 

 and surprisingly, fruit production is also re- 

 duced. 



The best way to judge whether your N rates 

 are appropriate is to collect leaf sample and have 

 the N concentration analyzed. Nitrogen concen- 

 trations in blueberry leaves are an accurate 

 indicator of whether or not bushes are receiving 

 appropriate amounts of N. For optimum produc- 

 tion, leaves should contain between 1.6 and 

 2.2% N. Levels higher or lower than this will 



likely reduce yields. 



There are many ways to maintain optimum 

 leaf N levels in a blueberry planting. The 

 challenge is to do it at the lowest cost. Ideally, 

 fertilizer should be applied at rates and times 

 that match the seasonal demand of the bushes. 

 Bushes probably begin needing N at bud break. 

 The demand probably increases through June 

 as shoots are growing most rapidly, and begins 

 to decline in late July or August as growth 

 begins to slow. The most efficient fertilizer 

 program might be one that supplied small 

 amounts of N beginning at bud break, increased 

 the supply until perhaps late June, and then 

 began decreasing the supply thereafter. If this 

 were accomplished, it is likely that optimum leaf 

 N levels could be maintained by applying much 

 less N/acre than is typical today. 



The most obvious way to match N applica- 

 tions with demand is to apply N through a trickle 

 irrigation system. Injecting fertilizer in this 

 way provides a large amount of flexibility so that 

 N can be applied where and when it is needed. 

 Studies in orchards indicate that growers can 

 apply half as much N and achieve the same 

 yields by injecting N compared to standard 

 broadcast applications. Tom Zabadal and I have 

 started an extensive study at the Southwest 

 Michigan Research and Extension Center in 

 Benton Harbor to compare fertilizer injection to 

 broadcast applications on blueberries. 



What can growers who are not set up to inject 

 fertilizer do to improve their efficiently and 

 reduce their costs? Here are some suggestions. 



1. Apply fertilizer where most roots are lo- 

 cated. In most blueberry plantings, roots are 

 concentrated within the row. This is particu- 

 larly true in younger plantings. Fertilizer 



Fruit Notes, Fall, 1991 



