Main Income in Field Track is not field specific. Instead it totals up your total sales and then uses the fields that are allocated to a "Sales Crop" type to determine how much income to give a field at any one time. Main income is a percentage of the total sales based on either the field's area, or the field's yield.
This method of income is limited but always functional, as a field will always have an area. The resulting main income, however, will not vary to the field's performance. "Area Based" is the default setting, however "Yield Based" would be the more realistic.
Allocating main income as a percentage of the total yield will mean the income varies to match the field's performance, however, this assumes the field has a "Sales Crop" and a yield.