Is the browning from sunburn or perhaps a lack of nutrients in the soil?
They are also experiencing yellow and white tinges on the leaves.
No fertilisers have been used on the plants, as it was an experiment testing for the most efficient soil out of: organic garden soil, sandy loam soil and potting mix.
Normally, coriander are easy to grow on sandy loam soil and potting mix. Try to loosen the soil. It is dry and thirsty. Water it every three days until the leaves recover from dryness. Good luck.