K. H. Tan

The geochemical nature and agricultural importance of commercial humic matter

Abstract The use of commercial humates in agriculture has been received with mixed reactions. Differences in origin between commercial humates and soil humic acids together with a lack of scientific data on the former are perhaps contributing to the controversy. Therefore, this investigation was conducted to determine the geochemical characteristics and effect on plant growth of humic acid (HA) and fulvic acid (FA) isolated from selected commercial humates derived from lignite and non-lignite deposits. Analysis were performed for C, H, N, carbohydrate, aromatic and carboxyl group contents. Spectral characteristics were determined by 13 C-NMR and infrared (IR) spectroscopy, whereas effect on plant growth was studied in the greenhouse with corn ( Zea mays L.) plants growing in the presence of commercial humates. The commercial humates were composed mostly of HA, and only very small amounts of FA were extracted. The HA fractions from the commercial humates were similar in C and H contents, but HA from lignite humate was higher in N content than HA isolated from non-lignite humate. Lignite-HA was also characterized by a higher carbohydrate content that non-lignite HA. The latter was more aromatic in nature. The IR spectra supported the presence of higher aromatic contents in HA from non-lignite than that from lignite deposits. Commercial humates were comparable with soil HA in their effect on increasing the growth of corn plants. It can be concluded, that the data suggested that HA from lignite deposits resembled HA in Mollisols, whereas HA from the non-lignite deposits was more related to HA from the Bh horizon of Spodosols.

Molecular weights and spectral characteristics of humic and fulvic acids

Abstract Gel-filtration of humic and fulvic acid with Sephadex G-50 yielded low and high molecular weight fractions with different chemical and spectral characteristics. In contrast to the low molecular weight fraction, high molecular weight humic acid was insoluble in H 2 O and had a strong infrared C-H absorption at 2,900 cm −1 . Both humic acid fractions had lower E 4 /E 6 values than fulvic acids, and no correlation could be found between humic acids partition coefficient (K d ) and E 4 /E 6 ratio. Low molecular weight fulvic acid was higher in K d and E 4 /E 6 values than high molecular weight fulvic acid. In this case, a positive correlation was observed between K d and E 4 /E 6 .

Characteristics of soil humic acid fractions separated by ultrafiltration

Abstract Different molecular weight fractions obtained by ultrafiltration of a humic acid (HA) extracted from a Mollisol were analyzed by chemical, infrared, electron microscopy, and isotachophoresis methods. The results showed that the different HA fractions did not differ substantially in characteristics from each other, although each fraction was composed of subfractions exhibiting different electromobilities. It appeared that fractionation of HA would yield molecular weight fractions as imposed by any exclusion limits used in the procedure. However, the elemental composition, infrared spectra, and electron micrographs showed that the fractions, separated by molecular weight limits, contained similar compounds. These results do not support the concept of humic acids as a heterogeneous mixture of compounds, but show instead that humic acids may possess a composition more homogeneous in nature than previously postulated.

Effects of humic and fulvic acids on release of fixed potassium.

Abstract Release of potassium fixed by expanding silicate clays is considered of practical importance in soil fertility. Humic and fulvic acids are expected to play a definite role in liberating this fixed K, because of their chelating power, but not much is known in this respect. The following investigation was conducted to study release of fixed K by montmorillonite and illite, using humic and fulvic acids isolated from the surface horizon of a Cecil soil (Typic Hapludult, Red Yellow Podzolic soil) as extractants. For comparison, extraction was also done with 1 N neutral NH4 -acetate, a mixture of 0.05 N HCl and 0.025 N H2SO4 or H2 O. Supporting analyses of clays were carried out with X-ray diffraction to establish changes, if any, in crystal structure due to fixation, release of K, or adsorption of humic compounds. The results indicated that humic and fulvic acids released some of the K fixed by montmorillonite or illite. In terms of percentage of the total K fixed, 9 to 28% were released by the various extractants. The percentages K released by humic and fulvic acids were similar from both montmorillonite and illite, but based on absolute values, humic and fulvic acids extracted less K (mg/100 g) from illite than montmorillonite. Although statistically significant at the 5% level of probability, the capacity of humic compounds to liberate fixed K was not different markedly from those of NH4 -acetate and the double acid mixture. Differences in pH of humic solutions had no influence on extraction of fixed K. X-ray diffraction analysis yielded curves showing an increase in spacing from 10.4 A for K-montmorillonite to 13.2 A as a result of extraction with the double acid mixture.

Dissolution of aluminum and iron phosphate by humic acids

Abstract An investigation was conducted to study the effect of humic (HA) and fulvic acid (FA) on the dissolution of aluminum phosphate (AlPO4) and iron phosphate (FePO4), to analyze the dissolution products, and assess their availability to plants. The rate of dissolution was determined by shaking 10 mg of Al‐ or FePO4 with 0 to 800 mg L‐1 of HA or FA solutions at pH 7.0 for 0 to 192 hours. The phosphorus (P) concentration was measured in the extracts by spectrophotometry, whereas the nature of P‐humic acid complexes was determined by 31P NMR analysis. Availability of dissolution products was studied by growing corn plants in aerated hydroponic solutions receiving treatments of 50 mg Al‐ or FePO4 and 0 to 800 mg L‐1 of HA or FA at pH 5.0. The results indicated that the amount of P released by HA or FA increased with time. Humic acid was more effective than FA in dissolving the metal phosphates. The 31P NMR analysis showed that the dissolution products contained free orthophosphates and minor amounts of P...

The nature of humic acid‐apatite interaction products and their availability to plant growth

Abstract An investigation was conducted to determine the nature of decomposition products resulting from the interaction between humic acid and apatite and assess their availability to plant growth. Interaction analyses were performed by shaking 200 mg apatite with 0 to 800 mg/L HA or FA solutions at pH 5 or 7 for 0 to 12 hr. Phosphorus concentrations were determined in the supernatants by spectrophotometry. The nature of P‐humic acid complexes was determined by 31P NMR analysis. Availability of these dissolution products was studied by growing corn plants in aerated hydroponics to which 200 mg apatite and 0 to 800 mg/L HA were added at pH 5 or 7. The results indicated that the rate of dissolution of apatite was parabolic in regression with time, and increased by increasing the amounts of HA or FA applied from 100 to 800 mg/L The dissolution reaction was influenced by pH, because larger amounts of PO4 3‐ions were detected at pH 5 than at pH 7. 31P NMR spectroscopy indicated the presence of P‐humic acid co...

Effect of organic matter on CEC due to permanent and variable charges in selected temperate region soils

Abstract Cation exchange capacities due to permanent charges (CECp) and variable charges (CECv) and at pH 8.2 (CECt) were determined in soil samples before and after removal of organic matter with H2O2 to study the effect of interactions between organic matter and clay on the different types of CEC. Five soils with differing organic matter contents and different clay mineralogies were selected and used, e.g. Dubuque, Tubac, Tama, Greenville and Houston Black series. The results indicated that the lowest CECp (2.9 mequiv./100 g soil) was detected in the Ap horizon of Greenville soil with a low organic matter content and kaolinitic mineralogy. The highest CECp (11.3 mequiv./100 g soil) in the A horizon was measured in the Houston Black soil also with low organic matter content but which contained montmorillonitic clay. Dubuque, Tubac and Tama A horizons, with mixed mineralogy, had CECp values (4.6–6.5 mequiv./100 g soil) between those exhibited by the Greenville and Houston Black soils. After removal of organic matter, the CECp of the Houston Black soil increased in value, indicating that the permanent charge of the montmorillonitic clay has been blocked by the interaction of the organic matter with the clay. This interaction between two soil components causing a decrease in CECp is known in plant science as antagonism. On the other hand, removal of organic matter from soils with mixed mineralogy resulted in a decrease in CECp. The interactions between organic matter and clay in these soils yielded clay—organic complexes with a synergistic effect on permanent charges, which produced the higher CECp values. In the Greenville Ap horizon, destruction of organic matter had no effect on CECp. Based on absolute values, the A horizons of the soils had low CECvs, with the Houston Black exhibiting relatively the highest value (24.0 mequiv./100 g soil). The Ap horizon of the Greenville soil had a CECv (6.7 mequiv./100 g soil), which was 3.5 times smaller than that of the A1p horizon of the Houston Black soil. When the percentage of the CECv contributing to the CECt was studied, however, it appeared that in the Greenville Ap horizon 70% of the CECt was attributed to the CECv, compared to 68% in the Houston Black A1p horizon. The conclusion was made that variable charges were equally prominent in the soils studied. In the absence of organic matter, %CECv in the A

Chemical and spectral differences in humic matter from swamps, streams and soils in the southeastern United States

Abstract There are differing opinions about the characteristics of humic matter from streams and soils. This study was conducted to determine and compare the chemical properties of humic matter from the Okefenokee swamp, Satilla river, Ohoopee river, and surrounding soils in the Georgia Coastal Plain, USA. Humic (HA) and fulvic acid (FA) were isolated from water by XAD resin and conventional NaOH methods and from soil samples by the NaOH method. The humic fractions were analyzed for total acidity, COOH, C, H, and N contents. Liquid 13 C NMR and infrared analysis were performed to determine the structural composition. The humic matter isolated from swamp and river water using XAD resin comprised of 72% FA and 28% HA, and that isolated using NaOH comprised of 60–70% FA and 30–40% HA. In contrast, humic matter extracted from soils with NaOH was 80% HA and 15% FA. The C and H contents, and spectral characteristics of HA (or FA) isolated by the XAD resin method from the swamp and river waters were similar. However, integration of NMR spectra yielded data indicating that HA from the Okefenokee swamp was more aromatic than that from river water. The conventional NaOH method yielded HA (or FA) with lower C, H and N contents, and smaller total acidity than the resin method. HA isolated by NaOH extraction from Okefenokee swamp was less aromatic than HA extracted using XAD resin, and structural differences, which may have occurred as a result of NaOH extraction, were detected by infrared spectroscopy. The conclusion was made that the NaOH method was an equally valid method as the resin method. No conclusive evidence was obtained to support the hypothesis that soil HA was more aromatic than aquatic HA. A strong signal at 55.8 ppm in the NMR spectra of black water HA, attributed to -OCH 3 , implied the presence of a common source component incorporated in the humic molecule of black water and soils.

Atomic force microscopy of humic acid

Abstract Structural analysis of humic acid extracted from a Cecil soil was conducted by atomic force microscopy (AFM) of monolayer thin films of humic acid deposited on mica strips using the Langmuir‐Blodgett deposition technique. The humic acid specimens display a fibre‐like morphology showing prominent hexagonal structures of benzene rings at a molecular level of resolution. The side chains attached to these ring structures were identified by infrared spectroscopy to be carboxyl and carbonyl groups as indicated by absorption bands at 1625 and 1720 cm‐1, respectively. The presence of C‐H alkyl chains were detected by bands at 2917 and 2850 cm‐1. This AFM observation provides the first direct evidence supporting the theoretical description of humic acid molecules consisting of benzene rings with various sizes of alkyl chains.

Composition of humic acids extracted under air and nitrogen atmosphere

Abstract Differences in nature and composition of humic matter, attributed to NaOH extraction under air or a N2 gas atmosphere, were investigated. Samples from the Ap horizons of a Cecil and Onaway soil, and lignite were shaken with 0.1 M NaOH in the presence of air or under N2 gas. The humic (HA) and fulvic acid (FA) separated were weighed, and analyzed for C, H, O, N, and S contents. Total acidity, carboxyl and phenolic‐OH group contents were determined by chemical analysis. Characterization was performed by 13C nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. The results indicated that differences in amounts of HA and FA, attributed to NaOH extraction under air and N2 gas, were small and no definite trend was noted for more HA and FA extracted under air. No differences in C, H, O, N, and S contents of HA and FA were also noticed as a result of the influence of air or N2 gas during extraction. However, HA and FA exhibited larger values for total acidities when extracted under a N2 gas at...