Re: Why do plants NEED phosphorus and Nitrates

Plants use phosphorus and nitrogen for many of the same purposes as animals. 
Phosphorus and nitrogen are in nucleic acids, (DNA, RNA) which are essential for 
all organisms. Phosphorus is in ATP, another key molecule for all living 
organisms. Plant cells have many of the same organelles as animal cells such as 
mitochondria, which function in cellular respiration. Hopefully your textbook 
will tell you that many enzymes (proteins) are required for cellular 
respiration. In plants, a great deal of nitrogen also occurs in chlorophyll and 
photosynthetic enzymes.

Reference

Stern, K.L. 1991. Introductory Plant Biology. Dubuque, IA: Wm. C. Brown.