You can categrise your sample according the crop yield . Suppos there are 1000 acres of land cultivated and there are four crops, so you can select your sample in four categories. Otherwise your can select your sample according the directions of field. You can take your sample from four directions and one center point.
Dear, it depends on your target population, in your case, since u don't know the population size, you need to apply a non-probability based sampling method and the formula for non probability based sampling is given on google, u can search that which normally applied to more than 50,000 respondents.
You can categrise your sample according the crop yield . Suppos there are 1000 acres of land cultivated and there are four crops, so you can select your sample in four categories. Otherwise your can select your sample according the directions of field. You can take your sample from four directions and one center point.