Others (I hope) will reply how to do this in sms.

In case you're open trying new things, you can do it easily in GeoPard.

Here is a live demo. The logic is following, you select yield layers and attributes (dry or wet yield usually), set weights to each layer (just averaging is often suboptimal). There are also tools to make optimum clustering of zones for you (natural breaks, equal areas, spatially localized (great for soil sampling)). A bit bigger picture - you can also integrate lidar topography (we provide), soil sampling data into zones. In case you need more control - you can do this with Equations.

In case you need to clean/calibrate data - here is the module; 
In case you don't have yield for some years - GeoPard can generate synthetic yield maps (based on your avg or total yield input).

You can try it free (no credit card, 250 acres, 1 month), then if you like there is a cheap usage-based plan.