What does this mean: "Given a sizable tract in which to determine the optimum soil resistivity, some intuition is in order."

What this means is that you have an area of soil that is large enough to be mixed thoroughly, so that it can be considered uniform. In addition, the soil's resistivity means how much electrical resistance the soil loads in relation to a geometric constraint. 
Where it implies "intuition is in order," what it is looking for is some intelligence on your part. You must conjure a way in your mind to determine this resistivity. This may mean you need to devise an experiment to determine resistivity, or if the question is a real life application rather than theoretical, ask a proper authority for data on the soil.
If you must run an experiment, you can build a rig pretty easily at home for imprecise measurements by making a small box with electric contacts at both ends, measuring the base resistance across with a multimeter, and then filling the box with soil to take another measurement. More measurements can be made to build a data set. Finally, just measure the cross sectional area and box length to calculate the resistivity from the resistance and dimensions.