Why do we need to put phosphate fertilizers along with rhizobium?
Phosphorus is indeed a macronutrient!!! If you're wondering why it is insufficient in natural soils then read on. Phosphorus is, in fact, the most limited of the three primary macronutrients in most natural soils (being nitrogen, phosphorus and potassium). This is because it is released very slowly from insoluble phosphates (on rock surfaces) and rapidly becomes “fixed” after being released, making it difficult for plants to obtain their phosphate ions (intake form of phosphorus). Few unfertilized soils release phosphates fast enough to support the high growth rates of many plant species. Also, plants tend to only obtain their phosphate ions from the little “polyprotic phosphoric acid” in the soil, they are often very deficient in phosphorus and for this reason phosphate ions are used in high quantities than nitrogen and potassium in NPK fertilisers (N:P:K = 1:2:1). 

P is found in natural compounds in soil (however this is very limited); it is more often found in NPK fertilisers in the forms of: Orthophosphates ( H2PO4- or HPO42- ) and Polyprotic phosphoric acid ( H3PO4 ).

The following reasons are why phosphorus is a primary macromolecule and why it is VERY essential to plant nutrition; 


To build ATP (the energy currency of plants) from respiration ; 
To build DNA and RNA (genetic information) ; 
To build the cell membrane ; 
To aid the growth of a plant in its early stages (seedlings and root development) ; 
To perform phosphorylation ( the modification of the activity of various enzymes – this is responsible for cell signalling (which is the ability of cells to perceive and correctly respond to their microenvironments – aka tissue homoeostasis))