If you're in a fix try 20% Hoaglands and change it frequently

you can use ammonium nitrate or urea to decrease pH in rootzone

Please go through this paper its available online, Reclamation of highly calcareous saline-sodic soil using low quality water and phosphogypsum by M. A. Gharaibeh • M. J. Rusan • N. I. Eltaif • O. F. Shunnar, Appl Water Sci (2014) 4:223–230

DOI 10.1007/s13201-014-0189-3

One One hass to T

One hs to bring down the pH to between 5.6-77 by adding sulphur dust. One can do this by taking a sample & and titating it with sulphur dust. Once you brought it down to the good neutral region, you can keep it down by using Ammonium sulphate and or Urea as the N source. This is the cheapest method.

In such sandy calcareous soils , mango is comapratively tolerant to calcareousness , i would be good idea to keep additional 25% phosphorous for improvised rooting , preferably as single superphosphate and micronutrients like Fe and Zn , considered three most crunch nutrients . Irrigation is another factor ( otherwise calcareousness with pH 8.4 will create osmotic pressure , detrimental to growth of such young mango seedling) to keep partial pressure of CO2 well under limit for an optimised plant growth. We very often also add manures 9 farm yard mansucre or vermiocmpost , preferably 250 g with 25 g sulphur to keep calcareousness well under control through acids from these two divergent sources. You cant get any text book like prescription , its more of a your practical instinct than anything else. ...

In high calcareous soil for 1 year mango tree better to fertilize with FYM and Vermicompost for initial dose