A field composite sample should consist of at least 20 selectively random soil cores. Selectively random sampling means that the field is sampled only in areas which represent most of the field area. Unusual landscape features such as eroded areas, saline or sodic zones and old building lots are not sampled. Also, avoid sampling in dead furrows or back furrows, under old manure or hay piles, sugarbeet tare piles, animal droppings, next to ditches, sloughs and roads, known banded fertilizer locations, and small depressions. The number of subsamples required for a composite soil sample for NO 3 -N with various levels of accuracy for an 80 H precision level.
Some samplers collect one composite sample per section or one per quarter-section.
Others separate the field into large landscape zones and treat each as a field. Some may divide a quarter into three to four equal sub-fields and sample each individually. Generally, the smaller the area, the more representative of the area the sample values will be. Figure 5 shows two examples of a suggested way to obtain representative samples from fields. Composite soil sampling plan for glacial landscapes.