Humic acid as a biostimulant and chelating agent for improving rice productivity in the Onattukara sandy soils

Abstract

The study entitled ‘Humic acid as a biostimulant and chelating agent for improving rice productivity in the Onattukara sandy soils’ was carried out at the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani during 2021- 2024. The major objective of the study was to assess the effect of humic acid (HA) as a biostimulant, biosurfactant and as an iron chelating agent for improving nutrient use efficiency and rice productivity in the Onattukara sandy soil (AEU 3). The research work comprised of four parts viz., characterization of HAs, incubation study for assessing iron chelating property, incubation study for assessing biosurfactant property of HA and evaluation of the effect of HA on improving rice productivity. The salient research findings emanated from the experiments are summarized below. Humic acid (HA) used in the study (HA solid formulation and HA liquid formulation) was initially characterized based on physical observation, elemental composition and spectrometric studies. The HA (solid) and HA (liquid) had a pH of 9.58 and 9.2, EC value of 1.04 dSm-1 and 0.46 dSm-1, respectively. Analysis of C, H, N and S revealed that the HA (solid) and HA (liquid) contains 36.18 per cent and 1.79 per cent of C, 2.56 per cent and 11.47 per cent H, 0.76 per cent and 0.12 per cent of N, and 0.35 per cent and 0.06 per cent S, respectively. The CEC value of solid HA and liquid HA was 181.40 cmol (+) kg-1 and 35.40 cmol (+) kg-1, respectively. The C:N ratios of solid and liquid HA formulations was 47.61 and 14.92, while the E4/ E6 ratio was observed to be 3.69 and 4.82, respectively. The solid HA was used for soil application and the liquid formulation was applied as foliar spray. The incubation study for assessing iron chelating properties of HA was carried out at submerged condition for 6 weeks and sampling was done at weekly intervals. The analysis after 4th week of incubation revealed a significant decreasing trend in Fe availability ranging from 229.3 mg kg-1 to 179.3 mg kg-1 with an increase in HA application levels from 2.5 kg ha-1 (T1) to 15 kg ha-1 (T4). A similar trend continued in the 5th week and 6th week and the treatment T4 (15 kg ha-1 of HA) could efficiently chelate Fe resulting in a lower available Fe content of 103.6 mg kg-1 in the soil. Compared to initial value (278.0 mg kg-1), T4 and T3 showed a decrease of 62.72 per cent and 54.32 per cent in available Fe content in soil, respectively. The treatments in the study significantly influence Fe fractions such as water soluble Fe, exchangeable Fe and organic matter bound Fe. The value of Fe fractions viz., water soluble Fe, exchangeable Fe, carbonate bound Fe, iron-manganese oxides bound Fe, organic matter bound Fe, residual fractions and total Fe under the treatment combinations ranged from 3.41 mg kg-1 to 3.92 mg kg-1, 67.52 mg kg-1 to 87.11 mg kg-1, 23.54 mg kg-1 to 34.30 mg kg-1, 2373 mg kg-1 to 2569 mg kg-1, 281.7 mg kg-1 to 311.4 mg kg-1, 7300 mg kg-1 to 7676 mg kg-1 and 10331 mg kg-1 to 10462 mg kg-1, respectively. A significant positive correlation was observed for available Fe with water soluble Fe, exchangeable Fe and carbonate bound Fe whereas, it was negatively correlated with organic matter bound Fe and iron-manganese oxides bound Fe. The incubation study could identify that HA application @ 15 kg ha-1 was the most effective in chelating Fe, followed by T3 (HA @ 10 kg ha-1). A second incubation study was carried out to assess the biosurfactant properties of HA under field capacity. Water stable aggregates, water holding capacity, bulk density and porosity ranged from 47.48 per cent to 52.74 per cent, 22.54 per cent to 26.59 per cent, 1.41 Mg m-3 to 1.51 Mg m-3 and 38.03 per cent to 41.31 per cent, respectively. Similarly, clay dispersion ratio and dispersion ratio ranged between 31.24 per cent to 44.05 per cent and 39.52 per cent to 46.14 per cent, respectively. Soil physical properties like water stable aggregates, water holding capacity and porosity were significantly improved by the application of HA @ 15kg ha-1 followed by HA @ 10 kg ha-1. The field experiment was conducted in two seasons to evaluate the effectiveness of various doses of HA identified from the incubation studies, in combination with chemical fertilizers, lime and FYM on productivity of rice crop (var. Bhagya). The experiment was carried out at ORARS, Kayamkulam during the Kharif and Rabi seasons (2023-24). Biometric observations like plant height, number of tillers per hill, root length, root dry weight and number of active roots per hill were significantly high in the treatment T5 (soil test based recommended dose of fertilizer (RDF) + FYM + HA @ 15 kg ha-1 + foliar application of 0.2 % fortified HA) during panicle initiation and harvest stage in both Kharif and Rabi seasons. Humic acid supplemented treatments (especially T5, T6 (75% soil test based RDF + FYM + HA @ 15 kg ha-1 + foliar application of 0.2 % fortified HA), T9 (soil test based RDF + 50% FYM + HA @ 15 kg ha-1 + foliar application of 0.2 % fortified HA) and T10 (75% soil test based RDF + 50% FYM+HA @ 15 kg ha-1+foliar application of 0.2 % fortified HA)) could bring about a significant decrease in number of iron coated roots per hill, content of iron coatings and number of decayed roots per hill in comparison with treatments receiving RDF, lime and FYM as per POP (T1) and soil test-based RDF + FYM (T3). Visual scoring of Fe toxicity in rice in each treatments revealed the ability of HA in alleviation of Fe toxicity in rice in both maximum tillering to panicle initiation stages. The Fe toxicity score of zero was observed in T5, T6, T7 (soil test based RDF + FYM + HA @ 10 kg ha-1 + foliar application of 0.2 % fortified HA), T9, T10 and T11 (soil test based RDF + 50% FYM + HA @ 10 kg ha-1+foliar application of 0.2 % fortified HA) at panicle initiation stage of both season crops. With regard to grain yield, the treatment T5 (soil test based RDF + FYM + HA @ 15 kg ha-1 + foliar application of 0.2 % fortified HA) was found to be superior in both Kharif (3.43 t ha-1) and Rabi seasons (3.32 t ha-1) which was followed by the treatment T9 (soil test based RDF + 50 % FYM + HA @ 15 kg ha-1 + foliar application of 0.2 % fortified HA). Straw yield was also observed to be high in the treatment T5 which was on par with T6 and T9 for Kharif crop and the treatment T5 was on par with T6 , T7, T8, T9 and T11 for Rabi crop. Soil analysis data from both seasons revealed that the treatment T5 had the highest pH, EC, primary nutrients, secondary nutrients and B content. Highest values of Fe, Mn, Zn and Cu were recorded in the treatment receiving RDF, lime and FYM as per POP (T1 & T2) and soil test-based RDF + FYM (T3 & T4). The carbon fractions viz; soil organic carbon, total organic carbon, soil microbial biomass carbon, water-soluble carbon, very labile carbon, labile carbon, less labile carbon and non-labile carbon were enhanced in HA applied treatments with the highest value recorded in T5 treatment. The combined application of HA and chemical fertilizers had pronounced effect on various Fe fractions such as water soluble Fe, exchangeable Fe, carbonate bound Fe and organic matter bound Fe. A significant increase in organic matter bound Fe was observed in HA applied treatments, of which T5 registered the highest value. Available Fe in the soil was positively correlated with water soluble Fe, exchangeable Fe, carbonate bound Fe and negatively correlated with organic matter bound Fe and iron manganese oxides bound Fe. Effect of treatments on soil physical properties were statistically on par with each other. However, soil supplemented with treatment T5 had the highest porosity and the lowest bulk density values among all the treatments. Plant analysis of both Kharif and Rabi season crops revealed that significantly higher N, P, K and Mg content in straw and grain was observed in the treatment T5, while the highest Fe, Mn, Zn and Cu were recorded from the treatments receiving RDF, lime and FYM as per POP (T1 and T2) and soil test based RDF + FYM (T3 & T4). The Fe partitioning study on the root, straw, grain and glume also revealed that higher amount of Fe is partitioned and deposited in the roots, followed by shoot. Application of HA in combination with chemical fertilizers and FYM brought about significant changes in microbial parameters. The count of bacteria, fungi and actinomycetes was significantly higher in T5 treatment. Similarly, dehydrogenase activity was also found the highest in T5 followed by T9 treatment. The economic analysis of various treatments considered in the study revealed that the treatment T5 had the highest value for net income followed by T9 treatment. Similar trend was observed in case of benefit-cost ratio also and the highest value of 1.66 (Kharif) and 1.63 (Rabi) were observed in T5 treatment. From the above experiments, it can be concluded that the combined application of soil test based RDF + FYM + HA @ 15 kg ha-1 + foliar application of 0.2 % fortified HA had superior effect on mitigating Fe toxicity, increased nutrient availability and better physical properties. Humic acid has the potential to alleviate Fe toxicity in Onattukara soil, which indirectly leads to improvement of rice productivity is evident from the present study. Supplementing humic acid to the soil has proven to enhance the carbon content, nutrient status, yield attributes in rice and Fe toxicity tolerance. Hence, the application of soil test based RDF + FYM + HA @ 15 kg ha-1 + foliar application of 0.2 % fortified HA can be recommended for alleviating Fe toxicity, augmenting soil fertility and improving rice productivity in the sandy soils of Onattukara.