How many total atoms are
in the Fertilizer,
Ca(H2PO4)2?

15 total atoms are in the fertilizer I hope this helps out

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.