Control of aphids in greenhouse raised bed.
Is Neem mixed with a detergent effective in getting rid of aphids in the soil before they get onto plants?
Archuleta County Colorado
Unless you have confirmed that you have root aphids, applying neem oil with a detergent will have little effect. It is best to wait until you have an actual outbreak of aphids before applying any pesticide.
Make sure that you understand that applying neem oil with detergent will result in a process called saponification and will change the characteristics of the pesticide.
If you are using an insecticidal soap and neem, these pesticides have different modes of action that may not be fully compatable. For instance, neem is slow to act and changes the insect's physiology. Insecticidal soaps kill the insect through exoskelaton degradation. The extension publication below will give you a good idea on what is going on.