Nutrient Management Research Database

Apple trees are often not responsive to the levels of N supplied to many orchards in the Pacific Northwest. This has been demonstrated in a series of experiments at PARC, over a number of years. One of the main reasons for this is that at planting the trees are replete with  N  from  the  nursery,  and  subsequently  store and internally cycle N for their growth each  year.  While  orchards  will  require some  N  fertilizer,  understanding  both  the processes  of  N  storage  and remobilization,  and  any  physiological  limitation  to  nitrate uptake  will  enable  precise  advice  about  the timing  of  applications  to  ensure  that  N applications  are  used  with  the  greatest  efficiency  and  avoid  costly  wastage  of  fertilizer due  to  applications  when  the  trees  are  unable  to  take  up  the  N.  The  results  from  this project have shown the futility of applying N fertilizers in the spring and highlighted the opportunities  for  manipulating  both  the  N  status  of  the  trees and  influencing  the development of buds by manipulating the N status of the tree in the fall.

This experiment investigated:

1. The effects of temperature on apple root nitrate uptake.
2. The impact of tree nitrogen status on tree nitrate uptake.

These objectives were accomplished by a study conducted in an apple orcahrd (cv. 'Fuji') established in May 2001.

In 2001, nitrate was applied as calcium nitrate via drip lines. Rates were 10, 50, or 90 ppm in a total volume of 62 gal of water per tree.

In 2002, nitrogen was applied as foliar urea after bud break to half of the trees receiving 50 ppm N the prior year.

Root N uptake was measured by sampling a portion of the root system from each tree, placing them in a calcium nitrate solution, and then measuring the depletion of calcium nitrate from the solution. This was performed at room temperature (73^oF) and at an elevated temperature (82^oF).

To determine how tree N status influenced tree nitrate uptake, a greenhouse experiment was performed. Half of the trees were allowed to drop their leaves normally, which allows for N contained within the leaves to be transported to other parts of the tree for storage. The other half of the trees had their leaves removed before this could occur. Movement of N within the tree was then monitored.

No effect of temperature on nitrate uptake was found in the field or greenhouse experiment, as temperatures rarely dropped below 50^oF, the level where nitrate uptake is normally impeded.

Tree nitrogen uptake was not slowed when trees were manipulated to have higher total N levels.