Effect of urea containing humic acid on maize growth and¹⁵N utilization

Objectives In this study, effects of urea containing humic acid (HAU) on maize growth and fertilizer N uptake were investigated for providing support for rational use of humic acid in the improvement of urea efficiency .

Methods Humic acid was added into urea in proportion of 1%, 5%, 10% and 20% (w/w) by melting method, and the four products were recoded as HAU1, HAU2, HAU3 and HAU4, respectively. A pot experiment was conducted using maize as test material and¹⁵N tracer technique. The maize growth, fertilizer N uptake and the fertilizer N distribution in soil profiles were investigated.

Results 1) Compared with the urea (U) treatment, the aboveground biomass and grain yields of the HAU treatments were significantly increased by 5.5%–13.8% and 6.3%–17.3%, respectively, and the increases were enhanced with the increase of HA adding proportions. 2) The HAU treatments increased the fertilizer N contents in grains, stems and cobs, but decreased those in maize bracts. The total fertilizer N uptakes in aboveground parts were increased significantly by 11.6% to 17.0%, and those in grains were increased significantly by 16.8% to 25.9%, however, those in leaves and bracts were decreased. The nitrogen harvest indices of HAU were higher than that of the U treatment by percentage points of 2.5 to 4.2. 3) The HAU treatments significantly increased the N use efficiencies and N residual rates by percentage points of 5.9–8.6 and 1.4–2.5, respectively, while the N loss rates were decreased by percentage points of 7.3 to 11.2. 4) The total fertilizer N residues of the HAU treatments were 5.2%–10.1% higher than that of the U treatment in 0–90 cm soil layer, and the fertilizer N was mainly remaining in the 0–50 cm soil layer.

Conclusions Compared with the conventional urea, the addition of humic acids increases the effect of urea in maize yield increase, and the effect will be enhanced with the increased proportion of humic acid (ranging from 1% to 20%). Humic acid containing urea can improve the nitrogen harvest index and promote the transport of fertilizer N from leaves and stems to grains, increase the fertilizer N residual rate in soil and decrease the leaching loss of fertilizer N.