Why are there trenches in this tea plantation?

I've posted this picture and asked this question on my FB page yesterday. The exact answer is very difficult, but several readers came quite close.

Here's the answer: The first thing you notice is that the tea trees on this plantation in Mingjian are very small. They will take 2 to 3 years before they'll start producing a significant amount of tea leaves. That means that this plot of land would not produce any income for all this time! On the other hand, the trees are so small that there's a lot of space between them... That's why the farmer is going to grow something else between the rows of tea trees. In this case, the deep trench indicates that he has chosen to cultivate ginger this year!

The best answer I received was pineapple or cabbage, which are 2 popular plants for the same purpose. But they would not have necessitated such a deep hole. In a 2008 article, I also posted an article where flowers were grown in a field of Ruby/Hong Yu tea. Growing another plant is mostly done to compensate the lack of revenue at the start of a new plantation. However, growing a different plant also benefits the soil, because it's not a monoculture.