So, how do you use them? I put it in the soil. I get a reading, but where do I find out if it is a good number for my soil? chart?

Yes, you need to judge the nutrients of the soil through the corresponding chart data,Nitrogen , Phosphorus , Potassium , Fertility grading(Unit: (N:mg/kg)(P:mg/kg)(K:mg/kg))Level 1: 150, 40, 200 FertileLevel 2: 120~150, 20~40, 150~200 medium and upper fertilityLevel 3: 90~120, 10~20, 100~150 medium fertilityLevel 4: 60~90, 5~10, 50~100 lack of fatSoil NPK Sensor with high sensitivity probe, accurate measurement, stainless steel probe, the error range is reduced to 2%. The professional soil meter is used to test the NPK of the soil to help achieve the optimal level of crop growth.