If you want to get the application rate of 20 Metric Tons per hectare in a 7 kg pot of soil.  This is how I would approach it. 

In the soil layer there is 2,000,000 kg of soil per Hectare. check this number for its a rule of thumb you can probably get or calculate something more exact. 

Divide the Soil per hectare rate for your amendment by the weight of your pot replication giving a factor of 280,000 approximately.

Take your 20 metric tonnes converted into kilograms which is 20,000 kg divide your 20,000 kg by the 280,000 then multiple by 1000 to get the quantity you would apply in grams per pot.

20 tons/hectare = 20000 kg/2,000,000 kg soil = 0.01

for 7 kg pot = 7 * 0.01 = 0.07 kg /pot = 70 gm fertilizer/ pot

Fertilizer: Urea（N, 46%）, Potassium sulphate（K2O,51%）,Superphosphate（P2O5,12%）

The average bulk density soil is 1.120 g/cm3, that is 1.120 t/m3.

Soil depth for topsoil (plowing layer) is 20 cm (0.2 m). Then the volume for topsoil per hectare (ha) is 10000 m2 × 0.2 m = 2000m3

Then the weight of topsoil (0.2 m) per ha is: 1.120 t/m3 × 2000 m3 = 2240 t = 2240000 kg

Generally, fertilizers is mainly used in the topsoil, so we can calculate the fertilizer weight in pots according to the fertilizer weifght in field.  Soil weight per pot in your experiment is 7 kg/pot, dry soil?

then you can use @sabry Abdallah MENTIONED METHOD