Potassium (K), particularly in high concentration, has a significant impact in the Calcium (Ca) and Magnesium (Mg) absorption by plants due to the physiologic competition between K and Ca, and the use of common metabolic substances by K and Mg. The objective was to study the relationship of K, Ca and Mg in maize (Zea mays L.) hybrids at initial development stage, grown in nutritive solution. The study was done in a greenhouse in the University of São Paulo/Pirassununga/Brazil. The maize was grown in a greenhouse using a hydroponic system (Hoagland and Arnon, 1950). The treatments were K doses: D1=0.3(5%), D2=1.2(20%), D3=6.0(100%) e D4=12.0(200%)mMol.L^-1, and maize hybrids: DKB390ProR2(H1), Pioneer30F35(H2) e SyngentaStatus(H3), with four replications (Tukey 5%). At stage V4, the dry mass of shoot (DMS), concentration of K, Ca and Mg and the interaction of (Ca+Mg):K was calculated for the shoot. Visual symptoms of K deficiency were observed in all hybrids at D1 and D2. The interaction of K and hybrids was significant (p<0.01) for all parameters studied except Ca concentration. Quadratic models were adjusted (p<0.01) to DMS and K concentration along K doses for all hybrids, being the maximum estimated by the regression of 59.5 g Kg^-1 at the dose 8.4 mMol L^-1. The Mg concentration for H1 and H3 and Ca decreased along the increase of K doses, adjusting to a negative, linear model for Mg and a quadratic model for Ca. The ratio (Ca+Mg):K decreased from 0.7 (H1), 0.5(H2) and 0.8(H3) to 0.1 in D3 and D4 (p<0.05). The increase of K in solution relates to increase of this element in the shoot and to a decrease in Ca and Mg absorption, due to the completion with K, causing a decrease in the (Ca+Mg):K ration in maize hybrids at the V4 stage for three hybrids.