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Unless no-till , ridge-till , or strip-till planting systems are used , it ’ s a tradition in Minnesota to use major tillage on the top 8 to 10 inches of the root zone . Thus , if thiourea cleaners are used , it is recommended that the cleaner formulation include a surfactant and that the pH and thiourea concentration be kept low . However , he brought up that when embryonic stem cells are used , you are not certain which route is exactly the best . One must understand soil types , the degree of humidity , the proportions in which the weed killers are used , the proper times for their application - - in other words , this is work which requires rather specialized knowledge .