Nitrogen is a key component of annual fertilizer programs for Florida blueberry production. Blueberries in Florida are typically planted either in pine bark beds or in sandy soil heavily amended with pine bark. Either scenario can result in inefficient fertilizer uptake and potential leaching or runoff from the application site. In addition to these soil conditions, shallow rooting depth of blueberry, frequent heavy rain during the summer growing season, the need for frequent irrigation in the absence of rain, and the extended growing season in Florida can add to the challenge of developing an efficient nitrogen fertilizer program. Our research was designed to determine if nitrogen fertilizer uptake is related to growth stages in young, recently planted, blueberry plants.
Summertime Growth Spurts
The study used young ‘Emerald’ blueberry liners grown in 57-L containers for two growing seasons using a mixture of sandy field soil and pine bark at a ratio of 1:1 (volume/volume). We used ammonium sulfate as the nitrogen fertilizer source with 10% 15N so we could quantify uptake and allocation to different tissues in the plant. The study was conducted in Alachua County in north-central Florida. The plants were dormant during winter, and samples were not collected from November through February.
During the first growing season, total nitrogen uptake by the young liners was very low until summer when it increased substantially along with the onset of rapid growth and plant dry weight gain. During the second season, nitrogen fertilizer uptake was again relatively low during spring (bloom and early fruit development). However, like the first growing season, nitrogen uptake increased during the summer and was greatest in August during a vegetative growth flush.
The majority of absorbed nitrogen was allocated to leaf tissue at all sampling dates except during early bloom, when most of the nitrogen was allocated to the developing flowers. In both years, nitrogen fertilizer uptake was low early in the growing season (March and April).
The low uptake during year one was likely due to the small plant size and limited root system. However, during year two, early season growth probably relied primarily on stored nitrogen reserves from roots and stems as has been reported in previous studies for both rabbiteye and northern highbush blueberry. Generally, the greatest nitrogen uptake occurred during late summer and fall when plant biomass gain was also greatest — October 14 in 2015 and August 5 in 2016.
Light Doses or Controlled Release
Young, recently planted blueberry liners appear to be inefficient at nitrogen fertilizer uptake. Until root system growth and establishment occur after planting, only a small portion of the fertilizer is likely to come in direct contact with the roots and be absorbed. However, blueberry root systems are highly susceptible to fertilizer burn, so fertilizer should not be concentrated near the base of plants. Frequent, light applications of fertilizer via fertigation, or use of controlled-release fertilizer, can potentially increase fertilizer use efficiency and reduce the chances of fertilizer burn.
Irrigation practices and fertilizer use efficiency are closely linked. Excessive or poorly timed irrigations can leach fertilizer below the blueberry root zone resulting in the need for additional fertilizer applications that otherwise would not be necessary.
Growers should monitor soil moisture carefully. When irrigation is needed, multiple light irrigations are preferable to a single large irrigation event, which can move water and fertilizer below the shallow blueberry root zone.
Most blueberry plants in Florida are grown on pine bark-amended soils or in pine bark beds. Pine bark can be a very good soil amendment or substrate for blueberries, but fresh bark can be difficult to wet and keep moist. When using fresh bark, apply it to the field ahead of planting along with some nitrogen to aid in its initial conditioning. The amount of nitrogen added to fresh bark varies depending on the quality of the bark and the wood content in the bark. Apply fresh bark to the field from one to three months before planting, again depending on the quality of the bark and the amount of nitrogen needed for conditioning.
More information on the use of pine bark in blueberry production can be found at https://secure.caes.uga.edu/extension/publications/files/pdf/B%201291_3.PDF.