Abstract

Key Words

Humic substances, K-humate, calsulmag, soil acidity.

Introduction

Soil acidity within the rhizosphere is recognised as a major limiting constraint in crop productivity in Australia. Soils in the vineyard areas of Yarra Valley (south-eastern Victoria) are inherently acidic. Low soil pH is usually associated with an increased phytotoxicity of Al and Mn, and a deficiency of Ca and Mg (Leeper and Uren, 1993). Conventional liming treatment has proved to be largely ineffective in the short term due to the slow vertical movement of surface-applied lime (Pavan et al., 1982; Ritchey, 1996), and costly (Cassel, 1980), particularly within the root-zone (Noble et al. 1995, Bruce et al. 1988). However, treatment of subsoil acidity with Ca-rich organic materials such as fulvic and humic acids has shown some considerable promise (Van der Watt et al. 1991, Noble et al. 1995).

Humic acids and their salts which are derived from coal and other natural sources may provide a viable alternative to liming, to ameliorate soil acidity and improve soil structural stability. Research has shown that it is the humic fractions (humic acid, fulvic acid and humin) of the soil organic matter that are responsible for the generic improvement of soil fertility and improved productivity (Kononova, 1966, Fortun et al. 1989).

In this study, the potential for using commercially available potassium humates (K-humates) derived from Victorian brown coal to treat soil acidity was assessed under field conditions in an acidic vineyard soil.

Materials and Methods

Soils

The acidic vineyard soil used in this study is located at Dixons Creek in the Yarra Valley, in south- eastern Victoria, Australia. The soil has a characteristic fine sandy loam texture in the A horizon and is classified as a Grey Kurosol (Isbell 1996). It displays a strongly acidic (pH_H2O<5.5) B horizon. The soil is typical of the soils of economic importance within the south-eastern agricultural belt of Australia. As it is under continuous mechanised cultivation, structural decline is likely to occur, manifested for example in soil surface crusting, poor aggregation, compaction, and poor water availability to plants. Increasing levels of phytotoxic Al and Mn will also probably occur. Thus, a significant loss in crop productivity is expected. Standard methods of soil analysis (Rayment and Higginson, 1992) were used to determine the initial soil properties (Table 1).

Treatments and experimental design

Field plots (1x1 m) previously treated with a range of brown coal derived materials in 1999 (Issa 2002) were used in this follow-up study. Results from that study showed that of all the treatments tested, the 18% K-humate and 18% K-humate+Calsulmag treatments performed better. Therefore, these treatments were reapplied in March 2002 to assess cumulative effects of these materials on soil properties. An additional new treatment viz. 26% K-humate (a newly formulated product of interest) was also applied in March 2002 on new plots at the same vineyard. Control plots from the previous study (Issa 2002) were retained as the controls for this current study. The field trial thus comprised of 4 treatments and 4 replicates (16 plots) in a randomised block design. The treatments were surface applied at same rate in the rows between the vines.

Potassium humate

The K-humate used in this study is produced by alkaline treatment of Victorian brown coal and is commercially available in Australia (sold as a concentrated liquid soil conditioner). It is available in two concentrations (i) 18% K-humate and (ii) 26% K-humate. These products will be referred to as KH18 and KH26 in this paper, respectively, and the term “K-humate” will be used as a generic word for both 18% K-humate and 26% K-humate.

K-humate contains potassium salts of water-soluble humate (26% w/v) and fulvate (0.7% w/v). It contains 15% (w/w) total organic carbon and 1.6 mmol/g of acid functional groups. The solid-state ¹³C CP NMR spectrum of the humate salt isolated from the 26% K-humate preparation is shown in Figure 1. The tallest peak (0-60 ppm) can be assigned to alkyl groups, both straight chain and substituted. Carbonyl-C is observed in the 160-190 ppm region and O-substituted aryl C between 140 and 160 ppm. Resonances in the 100-140 ppm region of the spectrum are mainly due to aromatic-C. The distribution of carbon signals in the K-humate ¹³C-NMR spectrum gave the following results: aliphatic-C 44%, aromatic 32%, carbonyl 11%, O-alkyl 9% and the balance from methoxyl and N-alkyl type groups. It should be noted that not all the carbon in such humate samples is observable by ¹³C CP NMR and spectra need to be interpreted with caution (Smernik and Oades, 2000).

Figure 2 indicates the proton NMR of a freeze-dried sample of the 18% K-humate, and Figure 3 shows the Carbon-13 Solid State NMR spectra of the acidic vineyard soil (a) and a sample of the soil after treatment with K-humate-18 (b), Issa (2002).

Quantitative total elemental analyses of 26% K-humate gave the following results (% w/v): 4.80% potassium, 0.50% iron, 0.15% nitrogen, 0.15% silicon, 0.10% sulphur, 0.09% sodium, 0.07% chloride, 0.05% calcium, 0.04% phosphorus, and 0.60% total aluminium. This product also contains about 18.7% (w/v) humic acid fraction, 2.7% fulvic acid and zero humin fractions, within limits of experimental error.

Calsulmag (CSM)

Calsulmag is a coal derived treated fly ash product also available commercially as a light-coloured alkaline powder. It contains 19.5% calcium, 5.1% sulphur, and 7.0% magnesium on dry basis. In this field trial, Calsulmag was used as a lime substitute.

Soil sampling and measurements

Soil samples at different depths (0-3, 3-6, 6-10, 10-15 and 15-25 cm) were taken twice (i) before the reapplication of old treatments (KH18 and KH18+CSM) and application of new treatment (KH26) to obtain baseline data (March 2002) and, (ii) 7 months after application of treatments (October 2002).

The soil properties measured included electrical conductivity (EC) - 1:5 soil/water extract, and pH_Ca using 1:5 soil/0.01M CaCl₂ extract.

Table 1. Some chemical and physical properties of the acidic vineyard soil investigated.

Soil Property	Acidic Soil (DeBortoli)
CaCO3 % (w/w)	< 1
Texture	Very fine sandy loam
Coarse sand (0.20-2.0 mm) % (w/w)	1
Fine sand (0.02-0.20 mm) % (w/w)	40
Silt (0.002-0.020 mm) % (w/w)	29
Clay (< 0.002 mm) % (w/w)	24.5
Mean weight diameter	2.04
pH (H2O)	5.8
pH (CaCl2)	5.1
EC (dS/m)	0.08
TSS % (w/w)a	0.03
Total C % (w/w)	3.1
Total N % (w/w)	0.24
C:N ratio	13
Exchangeable cations (mmol (+)/kg)
Ca	66b
Mg	30b
Na	2.6b
K	2.5b
Sum of all cations	100
Ca:Mg ratio	2.2
ECaPc (%)	65
EMgPc (%)	30
ENaPc (%)	3
EKPc (%)	3

^a TSS = Total Soluble Salts; calculated from EC (EC*0.297 = TSS).
^b 1 M NH₄OAc (No wash).
^c Exchangeable Ca, Mg, Na, and K percentage, respectively.

Statistical analysis

The data were statistically analysed by a partly-nested analysis of variance (Quinn and Keough, 2002). The treated plots were used as units of repeated sampling (x 4 replicates), and were considered as random factors within the treatments. Time and depth were considered as within-subjects factors.

Figure 1. Solid-state ¹³C CP NMR spectrum of 26% K-humate.

Figure 2. ¹H Liquid NMR Spectrum of Freeze-Dried 18% K-humate dissolved in D₂O.

Figure 3. Solid State ¹³C CP/MAS NMR of a) A1 Horizon (0-25 cm) Acidic Vineyard Soil b) Acidic Vineyard Soil + K-humate (Issa, 2002).

Results

There were highly significant effects of the treatments on both the EC and pH. This was evidenced by a significant Treatment x Time interaction, Treatment x Depth interaction (for EC only), as well as Treatment x Time x Depth interaction as shown in Table 2. These significant interaction effects are printed in bold typeface in Table 2.

Relative to the control, the Treatment x Time interaction plots showed significant increases in both the soil EC and pH (P <0.001) for all the treatments, with the KH18+CSM treatment demonstrating the highest increases in EC and pH. Figures 4 and 5 are plots of the overall effects of Treatments over time as compared to control for the EC and pH. Baseline in Figures 4 and 5 refers to results from sampling data collected just before treatment application (March 2002) compared with results from samples collected 7 months later (October 2002) from the same plots.

Table 2 Summary of the partly-nested analysis of variance. Columns show the p-values for squared root of the (pH) and log(EC) for each effect in the analysis. Appropriate denominator Mean-Square (MS) is shown beneath each effect, along with numerator and denominator degrees of freedom. Significant results at P < 0.05 are shown in bold typeface.

Term	df	pH	EC
Between Subjects Test
Treatment	4	< 0.001	< 0.001
Plot (Treatment) MS	15	0.001119	0.000009
Within Subjects Tests
Time	1	< 0.001	< 0.001
Treatment x Time	4	< 0.001	< 0.001
Plot (Treatment) x Time MS	15	0.000989	0.000004
Depth	4	< 0.001	< 0.001
Treatment x Depth	16	0.380	< 0.001
Plot (Treatment) x Depth MS	60	0.00145	0.000006
Time x Depth	4	0.772	< 0.001
Treatment x Time x Depth	16	< 0.001	< 0.001
Error MS	60	0.000867	0.000007

Figure 6 shows results for EC baseline of the initial sampling prior to treatment, while the results for samples collected after 7 months are indicated in Figure 7. Again, the KH18+CSM treatment demonstrates the highest increase in EC over the KH18 and KH26 treatments. These significant increases in EC in the treated plots are probably due to the action of K-humate as a chelating agent of cations in the soil, as well as a direct addition of cations from the product itself, which is rich in potassium, magnesium and calcium. Calsulmag is also known to be a rich source of these cations, and when used in combination with K-humate will introduce more cations to the soil.

Figures 8 and 9 shows the plots of pH for the Treatment x Depth x Time interaction effects. As with the EC, the pH effects were also significant relative to the control and the baseline, and the trend down the soil profile were consistent with the lowering of pH as the soil depth increases. As shown in Figure 9, the KH18+CSM treatment demonstrated the highest increase in pH relative to the control and baseline (Figure 8), as well as across the various depths and soil profiles downwards.

Figure 4. Effects of Treatment over Time for pH_Ca. Error bar to the top right of graph is 1.0 standard deviation as calculated from the appropriate error mean square - Plot (Treatment x Time).

Figure 5. Effects of Treatment over Time for EC (μS/cm). The pooled standard deviation calculated the appropriate error mean square - Plot (Treatment x Time); magnitude is too small (0.181 μS cm-1) to show up on the graph.

Figure 6. Baseline initial EC results (sampled prior to treatment application).

Figure 7. EC results 7 months after treatment application.

Figure 8. Baseline initial pH results (sampled prior to treatment application).

Figure 9. pH results 7 months after treatment application.

Conclusion

Results from this field trial showed that commercial coal derived K-humate (18% K-humate and 26% K-humate) and Casulmag products are effective for use in ameliorating soil acidity, and for increasing the level of beneficial cations such as potassium, magnesium, and calcium in the soil.

Acknowledgment

We thank Mr David Robertson (Department of Primary Industries, Tatura) for his valued and timely assistance with formatting the graphs.

References

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