Take your total amount for 200 kg of soil which is 2,000 mg of PAH mix your 2 grams of PAH into one kg of dry uniform sieved soil. Take the 1 kg of mixed soil and dilute with 9 kg of dry sieved noncomtaminated and finally add 190 dry sieved and uncontaminated soil in your mixture. This should give a uniform test concentration at the rate your are seeking. Take appropriate safety and good laboratory practice not to expose yourself unwisely. For the final mixture of 200 kg you will probably need a small cement mixer rotated slowly for a good amount of time. 

It is hard to give an answer without information on your aims. Do you plan to grow plants on this soil or do you need the soil for analytical purposes? 

However, I suggest to measure the water content of your soil first e.g. by gravimetric method. From this you get an idea how much phenanthrene is needed to achive a concentration of 10 mg/kg dry soil. For the mixing your soil should be air dried and sieved (e.g. 2 mm)  to ensure a good mixing.  By using small portions and an electric mixer you can make sure to achive more or less homogeneous concentrations. 

Take your total amount for 200 kg of soil which is 2,000 mg of PAH mix your 2 grams of PAH into one kg of dry uniform sieved soil. Take the 1 kg of mixed soil and dilute with 9 kg of dry sieved noncomtaminated and finally add 190 dry sieved and uncontaminated soil in your mixture. This should give a uniform test concentration at the rate your are seeking. Take appropriate safety and good laboratory practice not to expose yourself unwisely. For the final mixture of 200 kg you will probably need a small cement mixer rotated slowly for a good amount of time. 

I think it is an appropiate reply by Dr. Hepperly. Since the amount of PAH(2g) to be mixed in 200kg is small,the mixing has to be done in three stages,first in 1 kg soil,then in 9(9+1) kg soil and then 190(190+10) kg soil.Also moisture content may be measured  in soil as suggested by Dr.Wiche and  PAH material may be mixed   on dry weight basis,taking appropiate amount of soil.

After spiking with Phenanthrene i have to grow plants for bioremediation purpose.