Answer: observation
Explanation: scientific method is the process which scientists use to provide an accurate and reliable explanation to a phenomenon.
It usually involves the observation and description of a occurrence,the formulation of hypothesis to explain the occurrence and then experiments to test the accuracy of the hypothesis.the experiments may prove the hypothesis right or prove it wrong,in which case the hypothesis may be modified or discarded.
In this case the farmer has noticed the yields in his crops and he might bring an explanation(hypothesis) that seems reasonable to him and will have to test the hypothesis to verify it.