Growth and Yield Response of Maize (Zea mays L.) on Acid Soil to Different Rates of Humic Acid and NPK Fertilizer Putri Wulandari 1 , Endang Sulistyaningsih 1 *, Suci Handayani 2 and Benito Heru Purwanto 2 1 Department of Agronomy, Faculty of Agriculture, Universitas Gadjah Mada Jln. Flora no. 1, Bulaksumur, Sleman, Yogyakarta 55281, Indonesia 2 Soil Science Department, Faculty of Agriculture, Universitas Gadjah Mada Jln. Flora no. 1, Bulaksumur, Sleman, Yogyakarta 55281, Indonesia *Corresponding author: endangsih@ugm.ac.id ABSTRACT The main constraint in the extensification of maize on the dry land of acid soil is the low availability of P in soil. This study aimed to determine the effect and the optimal dose of humic acid on the growth and yield of maize on acid soil. This research was conducted from February to May 2017 at Tri Dharma Field Laboratory, Faculty of Agriculture, Gadjah Mada University. Acid soil with low P availability used was from sub-district Cigudeg and Jasinga, West Java. The research was arranged in Randomized Complete Block Design (RCBD) with 2 factors. The first factor was maize cultivar, namely Bisi 2 and Pioneer 35. The second factor was the rate of humic acid and NPK fertilizer, consisting of 0 kg.ha -1 (without) NPK and 0% (without) humic acid, NPK + 0% (without) humic acid, NPK + 5% humic acid, NPK + 10% humic acid, and NPK + 15% humic acid. NPK fertilizer applied was NPK 16:16:16 at a dose of 350 kg.ha -1 . The results showed that humic acid application on acid soil increased C-humic content in the soil, soil P availability, total dry weight of the plant, and kernel dry weight at harvest (15 weeks after planting). The increase in soil P availability did not improve the plant growth but increased the accumulation of plant biomass. The application of humic acid at 15% (52.5 kg.ha -1 ) combined with NPK fertilizer on acid soil significantly increased total dry weight of plant and kernel dry weight up to 13.14% and 21.81%, respectively, thus, it is recommended for maize cultivation on acid soil. Received: 03 rd July 2018; Revised: 13 rd August 2018; Accepted: 11 st June 2019 INTRODUCTION Maize is an important crop with multiple uses, which include the uses for food, industrial material, and bioenergy. The demand on maize for industrial material continuously soared in 2011-2015 from 3.67 million tons in 2011 to 3.88 million tons in 2015 (Kementerian Pertanian, 2016). Hence, it is necessary to increase maize production through agricultural intensification and extensification. Agricultural intensification is the management for maintaining food production without forfeiting natural resources for future generations by keeping it in balance with environmental function. The use of superior seed and good field management are some ways to improve agricultural production. The hybrid maize cultivars are widely cultivated to respond the environmental stress by improving morphological, physiological and yield capabilities (Sajedi et al., 2010). Bisi 2 is one of the widely cultivated maize cultivars in Indonesia. This cultivar was released in 1995, and it has a potential yield up to 13 tons ha -1 with 8.9 tons ha -1 as the average yield. This cultivar also has a good capability of growing on lowland up to 1000 m above sea level (Badan Penelitian dan Pengembangan Tanaman Pertanian, 2012). In 2016, cultivar Pioneer 35 was released as a maize cultivar that has a potential yield up to 12.1 tons ha -1 with 9.2 tons ha -1 as the average yield. Maize production can be increased through corn field expansion. In Indonesia, 79% of maize field are located on dry land (Zubachtirodin et al., 2008). Indonesia has 148 million ha of dryland resources. However, the varying climates and high rainfall lead to high loss of base content in the soil, causing 102.8 million ha of dry land to turn into acid soil (Mulyani Keywords: Humic acid, acid soil, corn. Ilmu Pertanian (Agricultural Science) Vol. 4 No. 2 August, 2019: 76–84 Available online at http://journal.ugm.ac.id/jip DOI: doi.org/10.22146/ipas.36680 ISSN 0126-4214 (print) ISSN 2527-7162 (online)