The critical value for K deficiency in maize is

Potassium deficiency symptoms in corn appear first on the lower leaves because potassium is easily moved within the plant from older to younger leaves. Leaf symptoms are yellowing to necrosis of the outer leaf margins.
The critical value for K deficiency in maize is 1.0%. Potassium deficiency symptoms were observed when the tissue content was 2 - 3% of K and exchangeable K was less than 5 meq K/100g.