What a gigantic effort you have put in there Plantmark! Could the yellowing be residues of horrible weedkillers in the hay/ straw bedding from the horses?
I wished to add an interesting observation from this season; I multi-sowed Santero onion in modules and had good germination. Upon planting, all but one odd module of plants went under fleece for the whole of April – there just wasn’t quite enough fleece for that last clump! The result is the interesting bit; the fleeced onion plants are approximately four times as large as the uncovered clump. Same soil, same amendments – possibly the influence of warmth provided by the fleece?