Preparing the Samples for Analysis - A "blank" and two "check" samples are placed in each set for quality control. The blank is space left in the group where there will be no soil but only the chemicals which are added for extraction. At the end of the process, when the plant nutrients are analyzed, all the nutrients in the blank "sample" should read zero. The two check samples (in this case checks "D" and "F") are soils with a known nutrient content. When they are analyzed, they should give nutrient levels within a certain numerical range. Notice the range of colors of soil from the same state. Also, many labs analyze 5 soil samples supplied quarterly through NAPT (North American Proficiency Testing Program). The labs send their results back to NAPT and receive a summary report from NAPT including results from all the participating labs. The labs can then compare their results with many other labs across the nation.