It seems that light coloured straw, such as rye, lowers day-time soil temperature up to 2 °C more than darker substrates such as buckwheat due to reflection of light and heat.
Mulches and soil temp
The daytime soil cooling effect is increased with higher temperatures (i.e. in the afternoon as opposed to the morning) and the study was done in Poland at lower temperatures, so potentially there are much greater gains to be had in 40+°C .
It might be worth trying a more reflective mulch than brown leaves to deal with scorching summers that crack the clay.
The converse could also be useful for people trying to grow warmth-loving plants in cooler areas.
It seems like it would be easier to work with a thermally absorbent mulch than hauling water/stones/concrete in to provide thermal mass.
