Snow peas are planted in boxes 3 metres by half metre by half metre high soil and is mainly sheep manure (second year). Top two thirds of foliage very yellow.
Snow Pea Plants Turned Yellow
Certified GKH Gardening Expert
Peas can turn yellow for a number of reasons. Some possibilities include too little water, nitrogen burn from the manure (though you would have seen that sooner, I think), root rot and high temps.
As long as the containers are draining well, I would try increasing water to the plants - this will help if it if it is too little water or high temps.