Comparing the effect of manure-extracted humic acid and commercial humic acid on the growth of maize and inorganic P forms in a calcareous soil

Due to the lack of suitable water resources and poor soil quality (high salinity, low organic matter, and plant nutrient deficiency), the increase of arable lands in arid and semi-arid regions has faced severe challenges. The use of organic amendments is recognized as a critical strategy for plant production in such soils. Nowadays, humic acid (HA) is known as one of the eco-friendly fertilizers that not only preserves soil quality but also plays a significant role in increasing the production of high-quality agricultural yields. The present study was conducted to extract humic acid from two sources of cattle and sheep manures and compare their effect on maize growth characteristics and inorganic phosphorus (Pi) forms in a calcareous soil compared to the commercial HA.

25 kg of sheep and cattle manure were collected from the farm of Shahid Bahonar University in Kerman and composted separately under controlled conditions for three months. Humic acid was extracted from composted manure using 0.5 N NaOH (1:10 material: extractant) in a dark environment under no oxygen conditions. The extracted HA's chemical and structural features, such as pH, EC, elemental composition, and functional groups, were determined. To investigate the effect of extracted humic acid on the growth of maize and soil inorganic P forms, a completely randomized design with seven treatments, including 0.1 and 0.2% of cattle manure extracted humic acid (CM-HA), 0.1 and 0.2% of sheep manure extracted humic acid (SM-HA), and 0.1% and 0.2% levels of commercial humic acid (HA), and a control treatment was designed in four replicates. The seedlings of maize were planted in treated pots and kept for 70 days under greenhouse-controlled conditions. At harvest time, the fresh and dry weights of the shoot and roots were measured. Soil Pi fractions, including H2O-extractable P, NaHCO3-extractable P, NaOH-extractable P, and HCl-extractable P, were also determined after maize harvesting.

The results showed that commercial humic acid had the highest pH and the lowest EC compared to the extracted acids. In contrast, humic acid extracted from cattle and sheep manure was more acidic than the commercial HA. The results of FTIR indicated the presence of phenolic hydroxyl, carboxyl, and aromatic rings in the extracted and commercial humic acids. The results showed that only 0.1% of commercial humic acid had no significant effect on maize's shoot fresh and dry weight. In contrast, other extracted acids, especially SMHA, significantly increased both growth characteristics of maize. In all treatments, the overall distribution of Pi forms was HCl-P > NaHCO3-P > NaOH-P> H2O-P. The results showed that HA application at the rates of 0.1% and 0.2%, mainly extracted HA from sheep manure, increased the contribution of more labile forms of inorganic phosphorus, including H2O-P and NaHCO3-P in the soil.

Since extracted HA from sheep manure and cattle manure have better quality than commercial HA and more effectively affect the growth of maize and labile forms of Pi, these resources can be used as an alternative for HA extraction and HA-based fertilizer production. In general, 0.2% of extracted humic acid from sheep manure had a more influential role in increasing the labile P forms in the soil and the growth of maize plants compared to other treatments.