There are some deposits of nitrates and phosphates in earth's crust. Nitrates are more soluble in water. Nitrates are difficult to reduce under the laboratory conditions but microbes do it easily. Ammonia forms large number of complexes with transition metal ions. Hybridisation easily explains the ease of sigma donation capability of NH$$_3$$ and PH$$_3$$. Phosphine is a flammable gas and is prepared from white phosphorus.

White phosphorus on reaction with NaOH gives PH$$_3$$ as one of the products. This is a: