Dear Mr Thakur, phosphate can be determined using ammonium molibdate. There is formation of a blue complex called molibdenium blue. The method is based on reaction between phosphate with molibdate ions in acid medium follows of reduction of the compound produced using ascorbic acid. The color has an absorption at 710 nm. The reaction is:

7PO43-+ 12(NH4)3Mo7O24 + 51H+ → 7(NH4)3PO4·12MoO3 + 51NH4+ + 36H2O

See the paper:  Korn, M.; Primo, P. M.; de Sousa, C. S. Influence of ultrasonic waves on phosphate determination by the molybdenum blue method. Microchem. Journal 2002, 73, 273. 

Urea is better determined using a biosensor with immobilized enzyme (urease). See the article: Minni Singh, Neelam Verma, Arun Kumar Garg, Niha Redhu. Urea biosensors. Sensors and Actuators B: Chemical, 2008, 134 (1), 2008, 345–351. The construction of a urea biosensor is very simple and you can use a glass electrode as a transducer of the signal. On the other hand, a spectrophotometric method for urea is descibed in this paper: T. K. With, Tove Dreyer Petersen, and Birgit Petersen. A simple spectrophotometric method for the determination of urea in blood and urine. J Clin Pathol. 1961; 14(2): 202–204. The method was applied in biological samples, but is possible adapted it. Good lucky. I hope that these suggestions helps you.

 Hi Thakur,

You will find the prosecure for Phosphorus analyzes in the following publications.

Cordially...

Article Nitrogen Fertilization of Golden Shiner Ponds

Article A Reassessment of Nitrogen Fertilization for Sunfish Ponds

Article A Sodium-Nitrate-Based, Water-Soluble, Granular Fertilizer f...

Try this colorimetric  method for the urea.