Take about 20 samples with a hay-probing tool in as many bales; move the sample around the hay lot to get wider representation. The tool will have to be sharp to break the forage for a good sample. The sample can be sent to any number of labs for RFV (relative feed value) so you can decide how to balance concentrate fed against hay fed. While sampling, it's just as important to determine physical quality: fresh mown smell, green color, absence of mustiness or mold, leaves vs. stems and foreign matter. Absent mold, there is still a tremendous variation in hay quality due mostly to stage of maturity at cutting. You may have to buy twice as much poor quality hay as good quality to get equivalent feed value.
Using a spade or auger, probe into your soil to a depth of six inches. Get 20 sub-samples per field which are then mixed into one composite for the lab. Larger fields up to 100 acres should have 40 samples. Soil moisture at the time of sampling is best if not too wet or dry since probing is easier and the sub-samples easily crumble into one composite sample that should weigh about a pound.
Soils vary widely within a field. The pH can range from 4.0 to 6.0 in a field needing lime, and plant nutrients can vary as much. To get a representative sample, walk in a random zigzag pattern but avoid fence lines, watering and shade spots, especially in pastures because of manure distribution. Visualize the area as a whole for what is typical for the field and how it will be limed and fertilized. The first and best soil fertility action is lime followed by fertilizer nutrients.