Pix4Dmapper does not have the max and min functions. However, there are ways through which you can mask the soil. In Pix4Dmapper what you can do is, keep changing the minimum value in color maps and prescription until you see soil has been removed. Even the Kent equation does not work perfectly when there are weeds on the soil, this will work perfectly where there is barren land. You have to guess a threshold that will remove the soil. Also, uncheck clamped. However, this can only be used for visualization.
I will show you another method in QGIS. Guess the threshold from Pix4Dmapper for soil. In QGIS use the raster calculator for creating a mask. A mask means a binary image where the values will only be 0 and 1.
This will give you the mask, where soil values are 0 and crop is 1. Again, this all depends on the threshold you have selected.
Now multiply this mask raster with your original NDVI map:
That will give you an NDVI map (masked) which has soil values as 0
You can then change it to pseudocolor from grayscale: