You may try to do some experiment primarily with the Venturi to understand how much is it passing
Try to record the delivery of the outlet of your pump where the Venturi is connected (with Ventury shut off), this gives the time taken for 1000 lts
Fill a drum with 100 lts of water
Repeat the measure of delivery of the outlet of your pump with Venturi switched on.
Try to understand how much of water has been sucked by Venturi from the 100 lts drum to draw 1000 lts of output.
This gives the percentage of fertilizer passing thru your water outlet going to drip irrigation.
Measure the requirement of water for your entire crop may be use Water meter.
And you know that you need 4KG of UREA to be supplied with this
Your delivery is 1000/min and it has taken 8 lts from the fertilizer tank (0.8% suction)
and you need total of 40000 lts for all your plants (40 mins)
and you wish to send 4kg of Urea in this 40,000 lts water.
Just mix the 4Kg Uria into 320 lts of water and supply with Venturi for next 40 mins, this will equally send your 4 kg UREA to entire farm.