This field experiment was undertaken to assess the effects of two rates of Ca(OH)₂(0, 250 ㎏/l0a) and five rates of N-P₂O_5-K₂O (0-0-0, 0-10-10, 6-15-15, 12-20-20, 24-25-20 ㎏/l0a) on the grassland establishment, forage yield and quality, botanical composition, and persistence of different forage species in a mixed grass/clover sward established by oversowing method on new reclaimed steep mountain. This fourth part was concerned with the mutual balance of mineral nutrients and grass tetany hazard. The results obtained were summarized as follows; 1. Concentration of exchangeable Mg, relative proportions of Mg to CEC, K, and Ca in soil were considerably below the critical level for good forage growth and likelihood of grass tetany. These properties were handicapped by liming and increasing the NPK rate. 2. Comparing with the critical level for likelihood of tetany (Mg$lt;0.2%, K$gt;2.5%, and K/(Ca+Mg)$gt;2.2 in forages), mean concentration of Mg ranged from 0.09 to 0.15%, that of K exceeded 2.5% from 15㎏ K₂O/ l0a rate. The K/(Ca+Mg) ratios of mixed forages showed above 2.2 at no lime and below at liming, when averaged over NPK rates. The increase of Ca concentration and botanical composition of legume in mixed sward by liming resulted in the reduce of K/(Ca+Mg) ratio in mixed forages. 3. The ratios of K/(Ca+Mg) in mixed forages were increased by increasing the NPK rate and that exceeded 2.2 from 10㎏ K20/l0a rate at no lime and showed around 2.2 from 20㎏/l0a rate at liming. 4. The value of % (N x 6.25) x % K in mixed forages was increased and Mg concentration was reduced by increasing the NPK rate. As a result, it was expected for decrease of blood serum-Mg concentration in cattle, and for increase of tetany hazard. 5. To prevent grass tetany resulting from low Mg concentration and unbalanced ratios of mineral nutrients, Mg fertilization on grassland and/or extra Mg supplement to cattle were to be necessary. 6. The Ca/P ratios ranged from 1:1 to 3.2:1, and they varied with the levels of lime and NPK applications. At liming, higher amount of P fertilizer should be applied to maintain the optimum Ca/P ratio in mixed forages.