What would be the most suitable experimental design to select a most suitable type of bio fertilizer level for five paddy varieties? Taking into account the explanatory text of the question, my first suggestion would be a 5x4 factorial experiment in a randomized complete block design.
I agree with Aristidis. Aksana's point about uniformity is very important and before doing in a large field experiment I suggest at least testing the soil, or preferably the standing plants fr uniformity at the proposed plot scale. The in there is a lack of uniformity it can be managed by adjusting basal treatments and by a suitable design. Even with a 5x4 factorial you may need 4 replicates