Humic acid (HA) improves the growth and nutrition of eucalyptus changing the plant metabolism and soil properties. HA effects on plants and soil are regulated by concentration and mode of application. The aim of this study was to evaluate the nutrient availability in soil and the eucalyptus nutrition and growth as related to concentrations of HA added to soil or used to soak eucalyptus seedlings roots in humic solutions. The HA concentrations added to whole soil ranged from 0 to 100 mg kg−1 C-HA, while 0 to 5 g L−1 C-HA were used in the seedlings root soaking. The shoot growth and P, K, Ca, B, Mn and Fe accumulated in shoot were increased in HA soaking roots, though HA did not alter soil nutrient availability. HA added to the whole soil improved the P, K, Mg, and Fe availability in soil, besides increased the accumulation of P, K, Ca, Mg, B, Fe, Mn, and Zn in the eucalyptus shoot, as well as shoot and root seedlings growth at optimum C-HA concentrations in the range of 37.4–81.8 mg kg−1. Root soaking with HA is an effective practice to increase the shoot growth and eucalyptus nutrition over the use of HA in whole soil in higher rates.