Impact of humic acids extracted from organic sources on the nitrogen forms, available phosphorus and potassium in a calcareous soil

More than 60% of the country's soils have less than 1% organic matter. Due to the calcareous nature of most soils, nutrients are insoluble and therefor not absorbed by the plants. One of the effective solutions in this situation is the use of humic acid, because it is the active part of the organic resources and has a higher utilization efficiency than other organic fertilizers. This compound is extracted from various natural sources such as soil, compost, Leonardite (oxidized lignite), sediments, peat and different types of coal. In the present study, humic acid was first extracted from five organic sources (cow manure compost, urban waste compost, vermicompost, sheep manure and poultry manure). Then, added at 5 concentration levels (0, 0.25, 0.5 and 1 g of humic acid per kg of soil) to a calcareous soil under incubation condition (temperature of 25°C and field capacity moisture). After 70 days, forms of nitrogen, available phosphorus and potassium were determined. The highest and lowest amounts of nitrogen, phosphorus and potassium were observed in poultry manure and vermicompost, respectively. The effect of different types and concentrations of extracted humic acid on soil nitrate, ammonium, available phosphorus and potassium was significant at the probability level of 1% (P < 0.01). The highest value of nitrate was found at the level of 1 g kg-1 of humic acid of poultry manure (234.53 mg kg-1) which was about 10 times more than the control (23.97 mg kg-1). The highest value of ammonium at the level of 1 humic acid extracted from poultry manure (144.48 mg kg-1) was approximately 5 times more than the control (30.92 mg kg-1). The maximum value of available phosphorus in humic acid 1 g kg-1 of poultry manure (25.40 mg kg-1) was about 1.8 times more than the control (13.91 mg kg-1) and the highest value of available potassium at the level of 0.5 humic acid of sheep manure (472.47 mg kg-1) was approximately 1.2 times more than the control (394.69 mg kg-1). In general, results showed that with increasing the concentration levels of humic acids, the available forms of nutrients increased in the soil. The greatest effect on the availability of nutrients in the soil was observed in humic acid extracted from poultry manure at the level of 1 g kg-1 soil. In order to better analyzing and understanding the real effect of humic acid, it is better to measure the contents of nitrogen, phosphorus and potassium and other characteristics of humic acids in future studies.