The soil test page references adding in non-foaming dishwasher detergent. Googling turns up very little on this. What have people been using?
I used this...
I basically assumed dishwasher detergent was non-foaming. contrast this to dish soap (Dawn) which foams right up.
The purpose is to act as a surfacant to allow better layering of the types.
Great question...you can certainly go with the brand that @Modawg2k has suggested. And, to piggyback on @Dub9 the larger majority of dishwasher detergent is non-foaming.
The below article helps clarify dishwasher detergent a little more.
Best of luck with your soil test!