*3.1.1 ST for treating water-repellent surface soils*

Water repellence is one of the major soil constraints in southern Australia. It is estimated that around 10.2 Mha of arable land in the south-west of Western Australia is at risk of water repellence with 3.3 Mha marked as at high risk and another 6.9 Mha at moderate risk [58]. The main reason for water repellence is due to an increased OC content (hydrophobic carbon compounds such as plant waxes and other products from natural processes of plant biodegradation) in the surface soil [59–62], especially

#### **Figure 1.**

*Cause and effect relationship of multiple interacting soil constraints. For further information on soil constraints' cause and effects, see soil quality: Ebook series [54–57].*

#### *Strategic Tillage for Sustaining the Productivity of Broadacre Cropping in the Arid… DOI: http://dx.doi.org/10.5772/intechopen.112875*

in soils with low clay content, i.e., soil with a low particle surface area [60, 63]. The CA or NT practice has exacerbated the scenario by concentrating organic matter and its wax component in the top 0.05 m of the soil [55].

A set of agronomic practices such as furrow or on-row seeding, and repeated application of wetting agents can temporarily treat soil water repellence. Using ST such as mouldboard ploughing, spading, etc. can be a medium-to-long-term solution for treating soil water repellence, albeit at a moderate to high cost. As soil water repellence mainly occurs in sandy topsoil with less than 5% clay [55], the addition of clay followed by incorporation using an ST is a longer-term solution. Claying can be expensive; however, this cost can be cheaper if the clay source is within the same paddock, especially where texture contrast soils (e.g. Planosol and Solonetz in the World Reference Base for Soil Resources) are available. When clay is applied to treat soil water repellence, it is essential to ensure the total amount of clay (applied plus original clay in soil matrix) is around 7 2%. Any less than this amount could treat soil water repellence inefficiently. In contrast, a higher amount might enhance surface crusting and decrease plant available water in the surface soil, especially during plant emergence [55].

One-off deep cultivation, ST, aimed to mix topsoil with subsoil, was found effective in decreasing soil water repellence [64]. However, if the surface soil is extremely repellent, i.e., soil with a molarity of ethanol droplet (MED) value of >3 [55], there is a risk of making whole soil profile water repellence due to soil mixing as proposed by Steenhuis et al. [65]. Mouldboard ploughing has been reported to effectively decrease soil water repellence by burying the repellent topsoil and bringing wettable subsoil to the surface [66, 67]. Davies et al. [68] found rotary spading also effectively decreased soil water repellence in 12 field trial sites in Western Australia. A study by Davies et al. [69] compared different ST implements for mixing soil and treating water repellence and found that benefits from different implements varied in the order of offset disc < deep ripped < rotary hoe < rotary spade < mouldboard plough (**Figure 2**). However, a risk of mouldboard ploughing, or rotary spading is wind erosion which can be minimised by ploughing during a wet season and immediately seeding with a vigorous cereal or cover crop [68]. The duration of the benefits of ameliorating soil water repellence using ST varies between different ST implements. For example, the longevity of the soil inversion with a mouldboard plough has been reported up to more than 10 years [34] and about 3–7 years for the rotary spading [35].

#### *3.1.2 ST for treating subsoil compaction*

Approximately 12 million hectares of the south-western agricultural region of WA (75% of arable land) is affected by or susceptible to compaction (**Figure 3**). The annual associated loss of crop yield caused by compaction in WA has been estimated about \$AU883 million or \$AU36–87 per hectare per year [56]. One of the main objectives of ST is to remove soil compaction allowing plant root growing in the subsoil and accessing soil water and nutrients from the subsoil. Soil penetrometer resistance (also known as soil strength) and bulk density are widely used to measure soil compaction. A soil strength of 2.5 MPa is considered a critical value above which plant root growth is severely arrested [14].

According to Blanco-Canqui and Ruis [70], bulk density and penetrometer resistance did not generally differ between long-term NT and ST systems. However, it is important to note that the lack of soil penetrometer data availability and inconsistency with the measurement was mentioned as a cause of not having a distinct impact of ST

#### **Figure 2.**

*Impact of one-off cultivation of different intensities on soil water repellence of a Tenosol at Badgingarra as measured in the laboratory using the water-droplet penetration time. Capped lines are standard errors of the mean (*n *= 4). Adapted from Davies et al. [69].*

**Figure 3.** *Status of soil compaction in Western Australia. Source: Parker et al. [56].*

on soil compaction [36]. Moreover, the study by Blanco-Canqui and Wortmann [36] used ST implement that addressed only the top 0.20 m of soil which might not alleviate compaction in deeper soil, therefore, deeper tillage was recommended. Azam and Gazey [14] conducted deep tillage (excavation and filling back) in a highly

*Strategic Tillage for Sustaining the Productivity of Broadacre Cropping in the Arid… DOI: http://dx.doi.org/10.5772/intechopen.112875*

**Figure 4.** *Effect of strategic tillage (excavation and filling back) on long-term soil resistance. Adapted from Azam and Gazey [14].*

compacted soil (a soil strength of 3–4 MPa) in a low rainfall region of Kalannie, WA (35°42'S, 117°29<sup>0</sup> E) in 2018. They reported deep tillage significantly decreased soil strength (measured at field capacity) and the effect persisted for at least five cropping years (**Figure 4**). Davies et al. [33] reported that deep ripping was the most studied ST practice in southern Australia. Deep ripped soil maintained significantly lower soil strength than the unripped soil for 3–7 years depending on the soil type and rainfall region. Soil with less clay content was naturally recompacted faster than soil with more clay content [33, 56].

#### *3.1.3 ST for treating subsurface soil acidity*

In Western Australia (WA), around 50% of the subsoil (0.10–0.30 m) are acidic and have a soil pH (1:5 soil to 0.01 M CaCl2 extract) below the minimum target pH of 4.8 in the subsoil (**Figure 5**). Subsoil acidity is less widely distributed and shallower depths in other southern regions of Australia, but still remains as one of the main soil challenges for the farmers [16]. At low soil pH, the increase in the concentration of toxic forms of aluminium (Al) significantly limits root growth and crop yield [71]. Importantly, soil acidity also often occurs with other soil constraints such as compaction and topsoil water repellence [72].

Several soil amelioration approaches are available for Australian growers to manage subsoil acidity and compaction, the two most widely occurring soil constraints. *Approach 1*: the traditional approach whereby agricultural lime is applied (often multiple applications) on the soil surface. This approach takes many years to increase subsoil pH significantly and does not treat subsoil compaction [14, 16]. *Approach 2*: a

**Figure 5.** *Status of subsurface soil acidity in Western Australia. Source: Gazey et al. [17].*

more recent and increasingly adopted approach that involves the use of ST operations, whereby lime is spread and incorporated at deeper soil depths while loosening compacted soil by a deep ripper or overcoming water-repellent soils by inversion ploughing or rotary spading [33]. Literature suggests that physical tillage operations to treat compaction and non-wetting soils can opportunistically be used to incorporate lime [67]. However, such soil amelioration practices are found to remediate soil acidity only partially, hence yield responses can be variable as observed from various long-term field trials [34]. Scanlan et al. [73] suggested that if a tillage operation incorporates lime to the depths where the soil pH constraint occurs, an immediate payback on lime and cultivation might be possible.

From a long-term (25 years) field experiment with multiple rates (0–8.5 t/ha) and applications of lime, with or without ST, in Wongan Hills, WA (30.°85'S, 116°74<sup>0</sup> E), soil samples were collected in 2019 and pH was measured [74]. The result showed that the strategic deep tillage was effective in increasing the subsoil pH to the minimum target pH level of 4.8 (in 0.10–0.20 m and 0.20–0.30 m soil depths) within 3 months of applying ST compared to the NT practices, especially where higher rates of lime was applied (**Figure 6**). However, the ST did not show any significant negative effect in changing soil pH of the topsoil (0–0.10 m) compared to the NT [74].

The amelioration of soil acidity will consequently decrease the toxic form Al concentration. In fact, when soil pH (measured in 0.01 M CaCl2) increases to >4.5 pH units, the toxic form of Al almost disappears. An experiment by Azam and Gazey [75] in Kalannie, WA (35°42'S, 117°29<sup>0</sup> E) reported that lime incorporation increased soil pH to the recommended pH level of 5.5 in the surface and 4.8 in lime-incorporated subsoil horizons (**Figure 7**). Liming also decreased extractable Al concentration from very toxic range (≥30 mg/kg) to below the critical level for wheat (≤5 mg/kg). Loosening by excavation alone also decreased Al concentration, especially at 0.10–0.20 m depth (**Figure 7**).

The longevity of the effect of the lime application on soil acidity may depend on many factors (such as soil texture, rainfall, and moisture content) which might lure more research on that matter. Based on Australian data for ST, Conyers et al. [76] reported that the effect of the addition and incorporation of 2 to 3 t/ha lime should last for at least 10 years.

*Strategic Tillage for Sustaining the Productivity of Broadacre Cropping in the Arid… DOI: http://dx.doi.org/10.5772/intechopen.112875*

#### **Figure 6.**

*Soil pH under different liming strategies and tillage treatments in autumn 2019 in loamy sand at Wongan Hills, WA. The NT and DT indicate no-tillage and deep tillage, respectively. Different colours of the Bar indicate lime rates (0–8.5 t/ha). Statistics: The least significant differences (5%) for deep tillage x lime rates at 0–0.10 m depth = 0.36, 0.10–0.20 m depth = 0.60 and 0.20–0.30 m depth = 0.45. Adapted from Azam et al. [74].*

#### **Figure 7.**

*Effect of lime incorporation on soil pH (left) and extractable Al concentration (right). Horizontal error bars represent the least significant difference at P* ≤ *0.05. Adapted from Azam and Gazey [75].*

#### *3.1.4 ST for nutrient redistribution*

One of the downsides of conservation agriculture is nutrient stratification along the soil profile. It is a problem for mobile and non-mobile plant nutrients [77]. Broadcasting of immobile nutrients like phosphorus (P) with no or incomplete mixing by tillage will increase nutrient concentration in the surface soil and will pose a greater risk of nutrient loss through runoff and will be greatly responsible for nutrient eutrophication. The leaching of soluble nutrients is another cause of nutrient stratification in the subsurface layers. Soil mixing through tillage may reduce the risk of nutrient loss from topsoil layers [78]. Many studies also reported similar findings of an increase in subsoil nutrient concentration after tillage with a low concentration of nutrients [77–82] due to mixing of top and subsoil [83]. However, the benefit of tillage on nutrient redistribution will disappear over time through improved acquisition by plant roots [84]. McLaughlin et al. [85] rotary hoed a 250 kg/ha superphosphate at the top 0.10 m of soil under an annual pasture and the effect lasted for 7 years.

### *3.1.5 ST for herbicide-resistant weeds*

One of the main factors that accelerated the adoption of conservation agriculture (CA) was the development of herbicides. The long-term practice of CA resulted in the development of resistance against herbicides by many weed species [86, 87]. The advantage of ST is the physical mixing of soil which is likely to decrease weed emergence and bury weed seeds. The emergence and growth of weeds can be stimulated by shallow tillage, but deep tillage can potentially prevent weed emergence [35, 88]. A recent review by Mia et al. [89] reported that soil inversion with mouldboard plough (0.35–0.45 m operating depth) can decrease weed emergence by 50–99% [35, 90–93]. Soil inversion with a modified one-way disc plough, (0.30–0.40 m operating depth) decreased weed population by up to 90% [94]. However, in a recent study by Edwards et al. [95] reported an increase in the activity of some herbicides and an increase in phytotoxicity following ST due to decreased organic matter. Therefore, when ST is adopted, herbicides and their doses should be carefully selected.

#### **3.2 Effect of ST on plant growth and crop yield**

## *3.2.1 Effect of ST on plant root growth*

In the Wheatbelt region of WA, many paddocks have multiple soil constraints such as compaction, water repellence and subsurface soil acidity, resulting in limited and confined root growth within 0.20–0.30 m of the surface [96]. The amelioration of soil constraints such as soil acidity and compaction would have many benefits for plant growth for accessing subsoil moisture and nutrients [72]. Azam and Gazey, 2021 showed that amelioration of compaction and acidity in a 25-year NT paddock in Kalannie, WA (35°42'S, 117°29<sup>0</sup> E) by ST and incorporation of lime significantly enhanced root growth up to 0.70 m soil depth compared to control treatment (where root reached 0.30 m below the soil surface, **Figure 8**).

#### *3.2.2 Effect of ST on crop yield*

The amelioration of soil compaction, acidity, and other soil and agronomic constraints using ST and agricultural lime is expected to increase crop yield. The level of yield response from the adoption of ST will depend on whether a single or multiple constraints are addressed in a single intervention. Azam et al. [74] demonstrated using a Mouldboard (MBP) plough only for amelioration water repellent soil generated significant yield response for 7 years. However, incorporation of 2 t/ha lime using a MBP to a depth of 0.25–0.30 m significantly increased yield after 7 years and has continued to have a yield advantage over the next 7 years, compared to the MBP-alone and the

*Strategic Tillage for Sustaining the Productivity of Broadacre Cropping in the Arid… DOI: http://dx.doi.org/10.5772/intechopen.112875*

#### **Figure 8.**

*Wheat (*Triticum aestivum *L.) rooting depth and architecture in deep loamy sand at Kalannie, WA, 56 days after sowing under control (T0; a–c), removal of compaction only (T1; d–f) and compaction plus deep incorporation of lime (T4; g–i). Wheat root architecture for T0,T1 and T4 was imaged repeatedly in situ using a 360° scanner (CI-600, CID bio-science, camas, WA, USA) inserted in clear glass tubes (Rhizotron tubes, ICT international, Armidale, NSW). Photo adapted from Azam and Gazey [14].*

control [74]. Yield response will also depend on the severity and depth of soil constraints as well as the depth of ST. Davies et al. [33] 2019 reported an increasing yield response with increasing depth of deep ripping (**Figure 9**). However, a recent review of the effect of ST on crop yield by Blanco-Canqui and Wortmann [36] reported that about 80% of the studies (12 studies including 31 soils across different regions of Australia and worldwide) showed no effect, 15% of cases increased the yield. In comparison, there was a yield penalty in 5% of cases. The reason for such variable results was speculated as the variability of site-crop-year-management-specific. Another reason could be the use of shallow tillage activity (only the top 0.20 m of the soil), which does not ameliorate deeper subsoil compaction and acidity and allow plant root to explore further. Such shallow tillage operation might damage to the soil structure and increase the loss of soil water through enhanced evaporation rates [4].

A deep lime incorporation (re-engineering) experiment (see details [74]) was established in Kalannie (30°25'S, 117°17<sup>0</sup> E) in 2018 using 0, 1.5, 4.5 and 6.0 t/ha lime and monitored for three seasons. Five soil amelioration treatments were included, comprising an untreated control, removal of compaction only and removal of both compaction and acidity (by incorporating lime at 0–0.10, 0–0.30 and 0–0.45 m depths). In this experiment at Kalannie, the combined removal of compaction and

**Figure 9.**

*Relationship between ripping depths and changes in grain yield. The number of data points (comparisons) with 0.30, 0.40, 0.50, and 0.60 m ripping depths are 32, 29, 24, and 14, respectively. Adapted from Davies et al. [33].*

**Figure 10.**

*Grain yield responses due to soil re-engineering in Kalannie. Different letters indicate significant difference in re-engineering experiments at P* ≤ *0.05. Adapted from Azam and Gazey [74].*

acidity doubled the grain yield (**Figure 10**). Removal of compaction alone increased the yield of wheat but not of canola or barley. Grain yield exceeded the water-limited potential by 33─56% (as calculated using the French and Schultz equation [97] due to the amelioration of multiple constraints under standard agronomic practice [74]. Deep soil re-engineering allowed plants to produce 0.60–0.65 m deep root systems, while they were only 0.20–0.25 m deep for control [74]. Significant and rapid improvements in root growth and yield were achieved due to the increase in subsoil pH and a uniform and sustained decrease in soil resistance [74, 75].

*Strategic Tillage for Sustaining the Productivity of Broadacre Cropping in the Arid… DOI: http://dx.doi.org/10.5772/intechopen.112875*

## **4. Economic benefits of ST**

Currently the occurrence of multiple interacting soil and agronomic constraints is costing Australian farmers millions of dollars due to loss in crop productivity [54–57]. ST can manage these soil and agronomic challenges. However, use of ST always incurs an additional cost. The cost of tillage operation depends on many factors such as type of tillage equipment, type of tillage operation; deep ripping or soil mixing, strategic deep tillage, constraint type; single constraint or multiple constraints [33]. Yield response depends on crop type, soil productivity, rainfall, and other management factors. In calculating the economic benefits of ST, the longevity of operation for specific or multiple interacting constraints needed to be considered.

Among different tillage tools, deep ripping is mostly expensive in southern Australia, ranging from \$AU 45 to 1400 and depending on the type of tillage tool and constraints that need to be fixed (**Table 1**). Soil mixing is the cheapest one due to its shallow tillage depth capacity ranging from (0.20–0.40 m). The cost of soil inversion is higher than soil mixing ranging from \$AU 40 to 150 but would address more soil constraints.

The existing literature showed that soil mixing or inversion is the cheapest tillage method with maximum yield response and long-lasting benefits compared to the deep ripping method of tillage (**Table 2**). Although deep ripping methods show less economic return in the current literature, it requires more study and time to justify their contribution to yield. Because deep ripping up to 0.70 m is a new concept that requires careful attention to build the necessary equipment to reduce the operation cost at a cheaper rate. Deep ripping using a 'soil re-engineering' approach showed a greater yield increase potential in recent years (**Figures 8** and **9**) and require a longer observation before concluding its effect on longevity. Moreover, deep ripping in ST could address multiple soil constraints following the soil re-engineering approach.


*\* Soil constraints included (1) compaction, (2) Hardpans, (3) subsoil acidity (4) subsoil sodicity, (5) subsoil fertility, (6) water repellence, and (7) weeds. Adapted from Davies et al. [33].*

#### **Table 1.**

*Cost involvement in selecting tillage techniques and methods to specific/group of soil constraints.*


#### **Table 2.**

*Yield response from different tillage methods practised in various regions of southern Australia.*
