My cotton has reddening of leaves and shedding of squares, is this potassium or magnesium?

Reddish-bronze older leaves with scorched margins in cotton, especially during boll filling, most likely points to potassium deficiency, which cotton is very prone to because boll development pulls heavy potassium. The classic cotton picture is older leaves turning yellow then reddish-bronze with the margins and between veins scorching, leaf shedding, and small poorly filled bolls. Magnesium deficiency also reddens older leaves but it shows more as clear interveinal yellowing with green veins before any reddening, and less of the burnt margin. The presence of scorched margins plus heavy boll load leans towards potassium. Please confirm with your Soil Health Card or a soil test, and rule out a sudden water stress spell or a wilt disease, which can also redden cotton leaves. If potassium is confirmed, muriate of potash or sulphate of potash to soil per your test, and a foliar potash spray during boll development, are the usual corrections for this high-demand stage. Match the dose to your soil test and local KVK advice and do not over-apply, since excess potassium can crowd out magnesium and calcium.