its about temperature too but farmers also use many types of fertilizers. main fertilizer is urea which contain nitrogen. nitrogen helps to growth but it also effect the roots, stem and leaves of potatoes which invites diseases to the plants.
Here is lack of technology. most of farmers use their hands for spread fertilizers because of low cost. And another main reason is lack of land. Framers do not have a lot of land to grow potatoes that’s why they use their own hand instead of technologies.