linkage-Math+agriculture
If you cant think of ideas for your project you aren't well read in enough in the field you want to spend several years working in. Generally, when you read papers in your field you will find holes or areas that need further attention, thus giving you ideas. It is one of the reasons the US has coursework, it forces you to get an understanding of the field so that you can accurately choose a project.
Let's assume that you are well read and continue reading about your chosen fields but having trouble structuring an idea big enough for a PhD. So I am going to give an example of how to maybe come up with an idea where the field is sustainable agriculture and the methods have to involve fractal maths.
Start with a problem like sustainable agriculture, where we need to make more food with less damage to the environment. Take a point in the previous statement and elaborate, eg, how do we measure damage to the environment? We do further research into our intermediate question, eg, how we damage the environment? And we find and read about one small topic like soil erosion, which we find out is hard to measure on small scale but we can do it on a large scale. We then think how can we improve measurement techniques. Then after more reading, we come up with a hypothesis can, can you measure the bulk properties of soil and use that to find the microscopic properties of soil using fractal equations? We then do more reading to find out is it possible and if anyone else has done it, if you can answer yes and no respectively, you have a possible PhD idea.
I hope that method is clear on how to possibly form an idea. Good ideas dont appear out of nowhere and take a lot of effort to form. ie READ MORE!
PS: your two topics are very far apart and so my honest opinion is it will take a lot of work to make a connection
