Abstract
Surface soil acidity is an agricultural problem worldwide, but subsurface soil acidity is more problematic because of the difficulties associated with its amelioration. It takes several years for surface-applied lime to increase the pH and decrease soil aluminium (Al3+) concentration in the subsurface soil layer. The effects of surface-applied organic materials (OM) on soil pH and soil Al3+ in the subsurface soil layer can be observed a few days after its application under controlled environment conditions. However, this has not been studied in the field. Much of the research regarding the influence of OM on soil acidity has been examined by measuring change in soil pH after OM amendment of a soil, but this may not consistently reflect changes in soil chemistry that are associated with the decomposition of OM. In this thesis, a lucerne hay pellet (LP) was used as a consistent OM source to investigate the effects of surface-applied OM on subsurface soil pH and Al3+ concentration under glasshouse conditions and its effects tested under field conditions.
The effects of the surface-applied OM on subsurface soil pH and Al3+ concentration were investigated in a soil column experiment by applying LP (an equivalent amount of 15 t ha-1) of different sizes, i.e., ~2, ~1, ~0.5 and ground LP (GLP) 0.1-0.2 cm in the surface 0-5 cm with or without lime. The acid soil tolerant wheat c.v. Dart was grown in the soil columns for 35 days with soil moisture maintained at 100% of field capacity. Surface-applied LP, regardless of size, significantly increased soil pH and decreased soil Al3+ concentration to 40 cm depth of the soil columns by 14 days after LP application. In contrast, the addition of lime only increased soil pH and decreased soil Al3+ concentration to the depth that it was incorporated. The results suggested that the soluble alkalinity in LP had dissolved and moved down the soil profile, resulting in increased soil pH and decreased soil Al3+ concentration. It is also shown that the surface-applied LP combined with lime had higher soil pH in the subsurface soil layer than the surface-applied LP alone. This result showed that the soluble alkalinity in the LP combined with lime treatment moved down the soil profile more than in the LP treatment. However, the wheat dry weight was not affected by the surface-applied LP with or without lime relative to the control. This could be due to the use of acid soil tolerant wheat cultivar and maintaining favourable growing conditions in the soil columns (e.g., sufficient water and nutrients), which minimised the effects of soil acidity on plant growth in a short-term soil column experiment.
The contribution of fractionated components of GLP, which was used in the soil column experiment, i.e., water-insoluble (WIM) and water-soluble (WSM) materials to the observed changes in soil pH, Al3+ concentration and extractable phosphorus (Colwell P), were studied in a 55–day incubation experiment. Two soil types, Red and Yellow Chromosol soils, from two depths, 0–10 cm and 10–20 cm, were used. The addition of WIM and WSM significantly increased soil pH and decreased soil Al3+ concentration. However, the addition of WSM significantly increased soil Colwell P, but the addition of WIM did not. The soil incubation results supported the hypothesis that when GLP is incorporated in a surface soil layer, soluble materials (i.e. the WSM component) dissolve and move down the soil profile, resulting in increased soil pH, decreased soil Al3+ concentration and increased soil Colwell P in the subsurface. This study also showed that the soil pH at the end of the experiment decreased significantly compared with that at the peak. However, the soil Al3+ concentration did not increase as the pH decreased during the experiment. Therefore, the changes in soil pH following the application of GLP and its components: WIM and WSM, resulted from processes (e.g. nitrogen mineralisation and nitrification) other than the alkalinity concentration in the added materials.
The incubation experiment showed that the soil pH following the application of GLP, WIM, and WSM increased first, then decreased. The contribution of the processes to the change in soil pH following the application of these amendments was quantified by determination of the changes in soil proton ("H" ^"+" ) concentration due to the immediate chemical reaction of added materials, oxidation of organic anions (OA), nitrogen mineralisation and nitrification. It is shown that the changes in soil pH during the first seven days after the amendment was incorporated were mainly due to the immediate chemical reaction of added materials and the oxidation of OA. The changes in soil pH after seven days of incubation were due to nitrogen mineralisation and nitrification. It is showed that the decreased soil pH due to nitrification was greater than increased soil pH due to nitrogen mineralisation. These results indicated why there was a temporal change in soil pH after application of OM. Therefore, to increase the long term effects of the application of OM on soil pH, it is suggested to select the optimal materials, e.g., materials that contain a high concentration of alkalinity but contain a low concentration of nitrogen, to increase the contribution of the immediate chemical reaction and the oxidation of OA processes but to decrease the contribution of the nitrification process to the change in soil pH.
A field experiment was conducted over two years to examine whether lime, LP, or a combination of LP and lime incorporated to 5 cm depth in the surface of an acidic soil profile would modify crop yield, subsurface soil pH and/or subsurface Al3+ concentrations. The treatments were compared with an unamended control to which equivalent tillage management had been applied. The surface application of lime did not affect subsurface soil pH, Al3+ concentration, as well as wheat and canola biomass and grain yield relative to the control treatment. The addition of the LP or a combination of LP and lime decreased subsurface soil Al3+ concentration by 20 months after the amendments had been added, but did not increase subsurface soil pH. Wheat biomass and grain yield did not differ among the treatments in the first growing season. This period was impacted by drought and this was surmised to have constrained crop growth and prevent responses to the changes in subsurface soil chemistry observed after the amendments had been applied. In the second growing season, surface-applied LP alone or in combination with lime increased canola biomass by 27% and 29%, and grain yield by 50% and 66% relative to the control, respectively. The increased crop grain yield in the LP and LP+L treatments was due to the reduced subsurface soil Al3+ concentration, increased subsurface soil mineral nitrogen and soil gravimetric moisture content. Therefore, this study showed that the surface application of OM with or without lime was better than lime in the alleviation of subsurface soil acidity, resulting in increased crop growth and yield.
In conclusion, surface-applied OM could be considered as an alternative strategy for the management of the subsurface soil acidity due to the capacity of the soluble component to move down the profile to the subsurface layer and decrease soil Al3+ concentration. Increasing soil mineral nitrogen and capacity of increasing soil water holding capacity were other positive effects of the use of OM on crop growth in acid soils. Organic materials could be incorporated with lime to maintain the surface soil pH above 5.5 but also to ameliorate the subsurface layer. The investigation of the processes that contributed to the changes in soil pH provided a better understanding of why the increase in soil pH following the application of OM is normally transient. It is suggested that the contribution of nitrification to the decrease in soil pH could be minimised by incorporation of the OM that contains less nitrogen concentration in order to increase the long-term effects on soil pH.
The effects of the surface-applied OM on subsurface soil pH and Al3+ concentration were investigated in a soil column experiment by applying LP (an equivalent amount of 15 t ha-1) of different sizes, i.e., ~2, ~1, ~0.5 and ground LP (GLP) 0.1-0.2 cm in the surface 0-5 cm with or without lime. The acid soil tolerant wheat c.v. Dart was grown in the soil columns for 35 days with soil moisture maintained at 100% of field capacity. Surface-applied LP, regardless of size, significantly increased soil pH and decreased soil Al3+ concentration to 40 cm depth of the soil columns by 14 days after LP application. In contrast, the addition of lime only increased soil pH and decreased soil Al3+ concentration to the depth that it was incorporated. The results suggested that the soluble alkalinity in LP had dissolved and moved down the soil profile, resulting in increased soil pH and decreased soil Al3+ concentration. It is also shown that the surface-applied LP combined with lime had higher soil pH in the subsurface soil layer than the surface-applied LP alone. This result showed that the soluble alkalinity in the LP combined with lime treatment moved down the soil profile more than in the LP treatment. However, the wheat dry weight was not affected by the surface-applied LP with or without lime relative to the control. This could be due to the use of acid soil tolerant wheat cultivar and maintaining favourable growing conditions in the soil columns (e.g., sufficient water and nutrients), which minimised the effects of soil acidity on plant growth in a short-term soil column experiment.
The contribution of fractionated components of GLP, which was used in the soil column experiment, i.e., water-insoluble (WIM) and water-soluble (WSM) materials to the observed changes in soil pH, Al3+ concentration and extractable phosphorus (Colwell P), were studied in a 55–day incubation experiment. Two soil types, Red and Yellow Chromosol soils, from two depths, 0–10 cm and 10–20 cm, were used. The addition of WIM and WSM significantly increased soil pH and decreased soil Al3+ concentration. However, the addition of WSM significantly increased soil Colwell P, but the addition of WIM did not. The soil incubation results supported the hypothesis that when GLP is incorporated in a surface soil layer, soluble materials (i.e. the WSM component) dissolve and move down the soil profile, resulting in increased soil pH, decreased soil Al3+ concentration and increased soil Colwell P in the subsurface. This study also showed that the soil pH at the end of the experiment decreased significantly compared with that at the peak. However, the soil Al3+ concentration did not increase as the pH decreased during the experiment. Therefore, the changes in soil pH following the application of GLP and its components: WIM and WSM, resulted from processes (e.g. nitrogen mineralisation and nitrification) other than the alkalinity concentration in the added materials.
The incubation experiment showed that the soil pH following the application of GLP, WIM, and WSM increased first, then decreased. The contribution of the processes to the change in soil pH following the application of these amendments was quantified by determination of the changes in soil proton ("H" ^"+" ) concentration due to the immediate chemical reaction of added materials, oxidation of organic anions (OA), nitrogen mineralisation and nitrification. It is shown that the changes in soil pH during the first seven days after the amendment was incorporated were mainly due to the immediate chemical reaction of added materials and the oxidation of OA. The changes in soil pH after seven days of incubation were due to nitrogen mineralisation and nitrification. It is showed that the decreased soil pH due to nitrification was greater than increased soil pH due to nitrogen mineralisation. These results indicated why there was a temporal change in soil pH after application of OM. Therefore, to increase the long term effects of the application of OM on soil pH, it is suggested to select the optimal materials, e.g., materials that contain a high concentration of alkalinity but contain a low concentration of nitrogen, to increase the contribution of the immediate chemical reaction and the oxidation of OA processes but to decrease the contribution of the nitrification process to the change in soil pH.
A field experiment was conducted over two years to examine whether lime, LP, or a combination of LP and lime incorporated to 5 cm depth in the surface of an acidic soil profile would modify crop yield, subsurface soil pH and/or subsurface Al3+ concentrations. The treatments were compared with an unamended control to which equivalent tillage management had been applied. The surface application of lime did not affect subsurface soil pH, Al3+ concentration, as well as wheat and canola biomass and grain yield relative to the control treatment. The addition of the LP or a combination of LP and lime decreased subsurface soil Al3+ concentration by 20 months after the amendments had been added, but did not increase subsurface soil pH. Wheat biomass and grain yield did not differ among the treatments in the first growing season. This period was impacted by drought and this was surmised to have constrained crop growth and prevent responses to the changes in subsurface soil chemistry observed after the amendments had been applied. In the second growing season, surface-applied LP alone or in combination with lime increased canola biomass by 27% and 29%, and grain yield by 50% and 66% relative to the control, respectively. The increased crop grain yield in the LP and LP+L treatments was due to the reduced subsurface soil Al3+ concentration, increased subsurface soil mineral nitrogen and soil gravimetric moisture content. Therefore, this study showed that the surface application of OM with or without lime was better than lime in the alleviation of subsurface soil acidity, resulting in increased crop growth and yield.
In conclusion, surface-applied OM could be considered as an alternative strategy for the management of the subsurface soil acidity due to the capacity of the soluble component to move down the profile to the subsurface layer and decrease soil Al3+ concentration. Increasing soil mineral nitrogen and capacity of increasing soil water holding capacity were other positive effects of the use of OM on crop growth in acid soils. Organic materials could be incorporated with lime to maintain the surface soil pH above 5.5 but also to ameliorate the subsurface layer. The investigation of the processes that contributed to the changes in soil pH provided a better understanding of why the increase in soil pH following the application of OM is normally transient. It is suggested that the contribution of nitrification to the decrease in soil pH could be minimised by incorporation of the OM that contains less nitrogen concentration in order to increase the long-term effects on soil pH.
| Original language | English |
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| Qualification | Doctor of Philosophy |
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| Place of Publication | Australia |
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| Publication status | Published - 2022 |