It’s not so difficult to do seeding rate trials with today’s planting equipment and yield monitors, and it’s even possible to do several of these within a field in order to get an idea of how much responses vary within the field. The difficulty is that responses within parts of a field are not consistent across years: they are often more dependent on weather conditions than on soil zone or soil type.

Corn and soybean take up relatively large amounts of phosphorus (P) and potassium (K), and much of this P and K ends up in the grain that is taken off the field during harvest. In order to keep soil nutrient levels from dropping over time, the amounts removed need to be replaced by applying fertilizer or manure. In order to know how much nutrient a crop removes, we need to know how much there is in a bushel of harvested grain.