The environment surrounding plant roots is extremely complex and variable. For example, it is not uncommon for the surface one to two inches of soil to be very dry while very wet conditions exist at deeper areas within the root zone. Surface soil temperatures may differ by 20 °F or more from that observed at deeper layers within the rooting zone. A zone of compactions causes by wheel traffic may create very different conditions on one side of a crop row compared to the other side where no traffic occurs. The environmental variations occurring in the root zone tend to be much greater than those which occur above ground. To most effectively understand crop response to fertility practices, we should understand how crop root systems respond to the soil environmental conditions. This is particularly true with conservation tillage systems, because we tend to create more soil environmental variability with conservation tillage than we do with conventional tillage systems.