Activating a detector at 30 cm depth was clearly not necessary for little beetroot plants. So we slowly cut back the water as can be seen on graph below (the cumulative irrigation line starts to rise more slowly). Although the water use was not excessive, the precipitous drop in nitrate through August was almost certainly due to leaching.
For much of the second part of the season we irrigated in such as way so as NOT to activate the detector. This would help bring the leaching under control, but we were now not sure if we were giving too little water.
This was going to be difficult soil to manage. Next we set up a much more detailed trial to answer these questions and more.