I have gone through a few manuals on soil analysis. I have followed the hot calcium chloride method for extraction. I have doubts about the method of preparing standard solution (Stock and working both) of Boron for the spectrophotometric method. Please mention the steps to make standards with a proper description of the reagents to be added.
Secondly, which method should be most sensitive for the spectrophotometer and which will be for AAS?
You cannot detect Boron on a AAS! As I recall Boron is detected spectrophotometrically as Borate ion using dry Sodium Borate as your stock solution (100 ug/mL). The concentrations of working standards are about 0.10, 0.20, 0.30, 0.40, 0.50 ug/mL.
If you prepare the standards yourself, e.g., from boric acid, it would be good practice to purchase a small quantity of a standard solution of certified concentration to confirm that your preparation is accurate.
There are numerous colorimetric options, e.g., look up azomethine-H. There is also an option of measuring B as tetrafluoroborate using an ion-selective electrode. My experience is that ICP is much more effective than colorimetry.
https://uwlab.soils.wisc.edu/wp-content/uploads/sites/17/2015/09/available_B.pdf
http://researchjournal.co.in/ONLINE/RKE/RK%20Eng%20%20%207%20(2)/7_40-41_9999.pdf
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