Crust formation

Crust formation and surface sealing were observed to occur during the first soil wetting event when water was applied with very low energy via the Cornell infiltrometer 2 cm height. Multiple techniques including penetration resistance, crust density, unsaturated hydraulic conductivity and steady infiltration rate indicated that around 85 % of the maximum crust formation occurred after the 9th rainfall and/or irrigation event equal to about day 14 after bed preparation under normal irrigation practice at Houston’s farms. This finding is supported by Mellis et al. (1996) and Fohrer et al. (1999) who also reported that soil crusting formed between the 1st and 10th rainfall events.

The SEM analysis indicated that soil crusts consisted of two distinct layers, in which the upper layer had a porosity of 3.5 % and 290 µm thick while lower layer had a porosity of 15 % and 1800 µm thick. These findings are in agreement with McIntyre (1958) who reported that soil crusting consisted of two thin layers namely upper layer 100 µm thick and washed – in layer 3000 µm thick.

7.6

Management of soil crusting

The variation in soil crusting at Houston’s farms offered the opportunity to explore how chemical, physical and mineralogical properties influenced aggregate stability and crusting. The survey of soil properties and aggregate stability across the five farms found that the type of soil properties associated with aggregate stability differed between the three methods used for measuring aggregate stability. Aggregate stability determined by RS was related to soil properties associated with aggregation, namely ECEC and the proportion of polyvalent cations (Ca2+, Al3+), whilst aggregate stability determined by WS was more closely related to soil properties associated with

General discussion Page 161

monovalent cations and organic carbon (MCAR, EPP). Clay dispersion was related to factors active in re-aggregation and flocculation, namely pH, quartz content and different measures of particle size (clay, silt and sand).

Options for improving aggregate stability and preventing soil crusting appeared to be limited as aggregate stability was most closely associated with inherent soil properties, which cannot be readily changed. Results from the previous survey (Chapter 3)

indicated potential for Ca+2 and organic based products to reduce crusting. These were included in two replicated field trials together with a range of available commercial products that were claimed to be able to improve soil aggregation or reduce crusting.

7.6.1 Gypsum

The positive relationship between aggregate stability and polyvalent cations such as exchangeable Ca2+ indicated potential to improve aggregate stability through application of products such as gypsum or lime. Gypsum was applied as a single amendment (0.5 kg/m2) in the preliminary experiment (Chapter 4) and at two rates of 0.25 kg/m2 and 0.5 kg/m2 and in combination with paper waste, phosphoric acid and wire mesh in a second experiment (Chapter 5). Results indicated that the gypsum significantly reduced soil crusting relative to the control, especially in combination with the paper waste and phosphoric acid. This reduction in soil crusting was attributed to increased flocculation, resulting from Ca+2 having greater charge density than

monovalent cations (Hanay et al. 2004, Bennett et al. 2014), and increased electrolyte concentration, which reduced clay swelling (Chan 1995).

7.6.2 Soil carbon and paper waste

SOC was only moderately and somewhat inconsistently associated with aggregate stability while labile carbon was consistently poorly associated with aggregate stability.

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This finding is in contrast to the extensive literature that strongly relates aggregate stability to SOC, or measures of labile carbon (Tisdall & Oades 1982, Chenu et al. 2000, Loveland & Webb 2003). We postulate the lack of a relationship between labile carbon and aggregate stability, and the poor to moderate relationship between SOC and

aggregate stability was due to the presence of recalcitrant forms of carbon, which were not actively involved in soil aggregation.

Extensive literature has demonstrated the ability of organic carbon waste and compost to increase soil carbon and aggregate stability (Tisdall & Oades 1982, Tejada & Gonzalez 2003, D'Hose et al. 2014). However, at the Houston’s farms, use of animal waste based compost is not allowed due to strict food safety protocols. As such we sourced news print-paper waste as an organic soil amendment. The paper waste was applied as a single rate at 8 kg/m2 in the preliminary experiment (Chapter 4), and at 1.0 kg/m2, 2.5 kg/m2 and 7.5 kg/m2 or in combination with gypsum and or phosphoric acid in the second experiment (Chapter 5).

Results indicated that paper waste was the most effective product for reducing soil crusting as measured by hydraulic conductivity, crust density and penetration resistance. This reduction in crusts using the paper waste treatment also significantly improved seedling emergence and crop yield. The reduction in soil crusting in the paper waste was attributed to its high carbon content (50 %), high Ca+2 content (3292 ppm) and physical presence in soil, which may have reduced raindrop impact or slowed rates of wetting. Results indicated that the greatest reduction in soil crusting was achieved when the paper waste was incorporated into the soil surface along with phosphoric acid and gypsum.

General discussion Page 163 7.6.3 Phosphoric acid

Phosphoric acid was applied at a single rate (75 ml/m2) in the preliminary experiment (Chapter 4), and at 80 ml/m2 and 160 ml/m2 and in combination with paper waste and gypsum in the second experiment (Chapter 5). Results indicated that phosphoric acid improved aggregate stability, crust density and soil hydraulic conductivity, especially when included with the paper waste. This reduction in soil crusting and increased aggregate stability were attributed to dissolving surface soil carbonates and formation of soluble Ca and Mg phosphates that act as aggregating agents.