First you need to know the bulk density of exsisting field by which the recommendation of fertilizer rate is given and the rooting depth of your crop. From that, you can compute the total volume and total mass of soil in the given fertilizer recommendation rate in relation to the rooting depth of your crop. Since you are going to fill you pot using bulk density, you know the mass of soil needed per pot. Finally, convert the amount of fertilizer needed in mass to mass ratio from field to pot.