Ana and Racio  , both have responded very nicely to your very intriguing question .  . Mohamad , in perennial crops , best premise of taking leaf samples is the period where leaves are about to enter into the phase of source ( Supplying accumulated nutrients to developing fruits )  from initial period of sink ( Period for which leaves continue to accumulate nutrients ) , so it is the time of intersection of sink-to-source. And this period needs to be standardized according to your crop , soil , management practices and agroclimate . The time of soil sampling should then commensurate with the time of leaf sampling.

It is better if you go to agricultural experiment design. take leave samples before applying nitrogen fertilizer and then set your experiment design in order to have treatment plots and control plots.then do compare in between diffrenet plots and the control

Mohamad your question is very valid , and its a kind of enigma , everyone of us is confronted , especially with perennial crops . I really do not have much experience on oil palm . But in this crop , various well established systems of fertilizer application is followed viz., French system , Foster System , Porin open system and  INFERS system are frequently followed . Out of these systems , INFERS addresses the various interactions involving crop , soil , environment in a much better way .  Like any perennial crop , the best time of evaluating the response of fertilizer application is through leaf analysis , when leaves have attained two-third of  maximum size. I am enclosing excellent PDF exclusively on Leaf analysis -based fertilization in oil palm , with a hope , they will be of some use to you .

depends on what you want to find out: if its the initial N status, then take before applying; if its the effect of the fertilization on the N status of the tree, then chose younger fully developed leaves from this years branches and watch their change of colour after fertilization (you might use the MUNSELL colour chart for plants or  a SPAD meter for this as well). When you observe even the slightes change in colour intensity its time to sample.

If you want to know initial status before to take an assay, before applying. However, if you want to know general nutritional status of trees, sampling must be carried out in appropriate date, because in this date, foliar reference levels are made. In citrus, in November. In  Loquat, in August. In kaki, In June-July.

Do you have a control area? In order to evaluate the N uptake and translocation to the leaves, you can set a control area without N addition. Then, measure N content in control areas and the different plots (if you use different N amendment or quantities in order to compare them). the way of application as well as the N-product you use can change your results. Measure N in soil and in different aged leaves at the begining of tour experiment. Then, measure periodically in old and young leaves to determine changes and..in soil at the end of the experiment. Be careful with the Nmeasurement method and take into accoun other parameters as Chl content, (a and b), leave color, size, etc.

Thanks to all. I want to take a picture by satellite and UAV to compare between nitrogen content in aerial images and ground data that's why I want to know when is the best time for leaves sampling after applying nitrogen in soil. generally not exactly 

 Ana and Racio  , both have responded very nicely to your very intriguing question .  . Mohamad , in perennial crops , best premise of taking leaf samples is the period where leaves are about to enter into the phase of source ( Supplying accumulated nutrients to developing fruits )  from initial period of sink ( Period for which leaves continue to accumulate nutrients ) , so it is the time of intersection of sink-to-source. And this period needs to be standardized according to your crop , soil , management practices and agroclimate . The time of soil sampling should then commensurate with the time of leaf sampling.

It would be better to design the experiment with different amount of fertilizer(e.g. Control, acute, normal and excess application of fertilizers). then you can see the difference affect of fertilizer based on time and the amount using remote sensing data. 

The rate of nitrogen in plants are dependent on the soil conditions. Optimal conditions for nitrification - good aeration, soil moisture 60-70% capillary water capacity, the temperature of 25-32 ° C and рН near neutral. If soil moisture is sufficient, the nitrogen begins to act very quickly in the plant, so you can have a few days to observe. I also consider it important to do at least three observations to be able to follow the dynamics of nitrogen in the leaves.

Hello Mohamad

It is only possible to say when the N supply is sufficient if what" sufficient " means is already known. I also do not think the time can be given for the many reasons given by earlier responses.  In annual crops  the response to N supply would be determined by setting up an experiment with replicated plots of plants all grown with the same basal  treatment, with very little N (perhaps already some in the soil).  N in this basal crop would be measured - in leaves, stems and perhaps roots at the start of the N treatment.  Also the size of the crop and the component plants would be measured.   This would give the basal starting conditions (control).  Then N would be applied in different amounts to plots (and properly  replicated of course), giving a range of N, starting with the basal  up to a large excess of N which does not increase growth etc.  Then the crop and plants would be measured at regular intervals to provide a comparison with the control, and between the different N applications.   From the data it is possible to determine both the amount of N giving the best plant response (it might be yield, for example) and the time required for this to be achieved.  

Perennial crops, such as date,  are more difficult to study but the same ideas apply.  A plantation with sufficient established trees grown under the same conditions  would be needed for an experiment. The size, characteristics (e,g, N, chlorophyll, protein content of the last fully mature leaf and yield if applicable) would be measured together with any aerial measurements such as you want to make, before applying the N treatments. Then different amounts of N would be applied to groups of replicated trees, so that the response  could be measured. The same  measurements would be made over time until the response was large enough to see.  The amount of N giving the desired effect (largest yield perhaps) and the time taken to reach it after N application would then be known.  The aerial measurements would then be correlated with the measurements.  This would provide good evidence of the amount of N required, how quickly N caused the effect and allow aerial measurements to be used with reasonable confidence to guide N application in future.

It is a lot of work but I think there is no way around the problem if aerial measurements are to be used effectively. Hope this helps

Best wishes

David Lawlor 

Generally,  2 wk. after N-apllication.

Nitrogen usually plants response to N quickly. so after application of N to plant you can measure leaves sample within 15-30 days. But first you should study factors. plant age factor is most important.

I am not familiar with leaf sampling from perennial crop for Nitrogen uptake but i believe the same principle used for annual crop leaf sampling applies. Sample young growing leaves 25 - 30 days after fertilizer application and analyze or monitor their change of colour intensities after fertilization using SPAD. i hope that helps