I think this your idea is a good and innovative ways to reduce stress to farmers in purchasing fertilizers. Many small scale farmers with can not afford that 50kg bags, even in the place they can afford can not use it all and left the remaining to spoil in their hut or room. At least it is a good idea.

I would like to appreciate the efforts you are exerting to help our farmers. In my opinion, the difficulty of assessing willingness to pay (WTP) for a product/service is all about how well you read the demand of your market. However, in your your case it is clear that the farmers are aware of the current market price of 50kg bags and thus you can convert the price for 50kg to 10 or 25kg bags and directly ask them if they are ready to pay for it. (NB: adjustments need to be made during price conversion considering costs of packing materials etc...e.g. price of 2litre bottled water is not twice that of 1litre bottled water). The other option is you can alternatively infer for their preference for 10 or 25kg bags as the price can be understood from the available 50kg bags or from the predetermined price. Thus, in this case it will no longer be WTP questions rather whether they prefer small packages over the 50kg bags or not.

I think 25 kg bag.