Typically, twenty to thirty soil cores make up a composite sample. Those cores are randomly located within similar areas of the field.
It is important to consider the sample use when deciding how to sample a field. Next, decide how the field is going to be split based on field characteristics. Within the field areas that are alike should be grouped together. For example, soil texture, elevation and cropping patterns make great boundaries for dividing uniform areas. Certainly, it takes some prior planning to get a good representative sample.
Often times, when composite soil sampling crop consultants split irrigated field into halves or quadrants. Consequently, the uniform patterns result in consistent soil sample tests over time. Consistency is important because it lets you see how the soil test levels change from year to year.