**1. Introduction**

Conventional tillage (CT, usually practised in every cropping cycle) in agriculture involves intentional soil manipulation using mechanical means for increasing water infiltration and storage of soil moisture to improve seed germination and root growth, suppressing weed population and mixing crop residues and organic materials.

However, the disadvantages of CT practices include the breaking of soil structure, which might lead to an increase in soil dispersion [1], wind and water erosion [2, 3], loss of conserved soil moisture [3, 4] and reducing soil organic carbon content [5] depending on the depth of tillage.

To reduce soil erosion, moisture loss, preserve soil organic matter, promote good soil structure and better nutrient cycling and plant nutrition, the Food and Agricultural Organisation (FAO) proposed three principles of conservation agricultural (CA) or no-tillage (NT) system. They are: (i) practising minimum soil disturbance for seed and fertiliser placement, (ii) permanently covering at least 30% of the soil surface with organic matter, i.e., crop residue or cover crop, and (iii) diversifying crop species [6]. CA is not a new concept, and it was first conceptualised to protect the soil by the farming community and scientists in the 1930s after the 'dustbowls' incident during a drought in the mid-west of the USA due to extensive cultivation. However, the CA only became more popular with the development of the improved seeding machinery in the late 1940s [5] and the widespread use of herbicides in 1960s in USA [7]. Currently, 12.5% of agricultural lands under the CA practices around the globe, and Australia has adopted CA practices at a wider scale than any other country [8–10]. The Australian grain grower survey in 2016 reported that around 80–90% of the strategically tilled soil does not receive any pre-sowing cultivation [11].

In current agricultural practices, many agronomic and soil constraints such as water-repellent surface soils [12, 13], soil pH and lime stratification have arisen [14], increase in subsurface soil acidity [15–17], nutrient stratification [18], some soil-borne pests and diseases such as slugs [19] or nematodes [20], and herbicide-resistant weeds [21, 22] due to the long-term NT practices. To manage these soil and agronomic constraints, occasional tillage (known as strategic tillage, ST) might be one of the ways, but there is a fear of reversing the benefits of long-term NT practices on soil physical, chemical and biological properties. While the benefits of CA or NT are overarching, the complete elimination of tillage from the agricultural system and its effect on sustaining the productivity of the broadacre cropping system still need to be investigated [23–33]. This book chapter postulates that ST would remain a key element for sustaining the productivity of broadacre cropping, especially in the arid and semi-arid regions of southern Australia.

### **2. Strategic tillage: pros and cons**

To avoid the conventional tillage (CT) practice, a needs-based occasional, usually every 3–10 years, deep tillage approach [34, 35] is gaining popularity, known as strategic tillage (ST). The occasional soil disturbance in a conservation agriculture (CA) or no-till (NT) system could minimise the risk of CT and amend the soil and agronomic constraints. Besides, ST can increase crop yield significantly over a period which might subsidise the cost of tillage operations and make the cropping system more sustainable and profitable than an NT system. However, there is fear that the practice of ST in a CA or NT system might affect the benefit of the long-term CA by affecting soil properties such as soil erosion and runoff, wind erosion, loss of soil aggregate, infiltration of soil water and soil organic carbon.

The impact of ST on soil properties, in a CA or NT system, needs to be more consistent, and still, there are opportunities to conduct more research and explore further. For instance, soil erosion and runoff may be accelerated by ST in an NT system. Usually, soil erosion and runoff depend on soil hydraulic conductivity and

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

other water infiltration properties [36]. Occasional tillage with a Mouldboard plough did not show any difference in soil hydraulic conductivity in a 35-year NT practice system [37]. Usitalo et al. [38] reported that the loss of dissolved phosphorus by runoff was 67% less in a CT system than in an NT system due to reduced runoff and improve infiltration in tilled soil. A recent review [36] reported that ST increased runoff in two out of five studies while decreased runoff in two studies and had no effect in one. Therefore, the ST in an NT system has a mixed effect on soil erosion and runoff and might also depend on other soil properties, such as soil texture and structure.

NT is very effective in wind-prone and semiarid areas in decreasing wind erosion by crop residue [39, 40]. The tillage buried the crop residue and enhanced wind erosion by increasing the emission of particulate matter in the soil (<2.5 to 10 um) [41, 42]. Considering dry soil aggregate stability, an important factor for soil erosion, no immediate effect of tillage was observed after 1 to 3 years on a 10-year NT system in loam, silt clay loam and clay loam soils [27].

The ST might impact soil aggregation, infiltration and soil water content. A review by Blanco-Canqui and Wortmann [36] reported that ST did not affect the wet and dry stability of soil aggregates in two out of three studies and decreased in one study. It has been reported that soil aggregates might decrease immediately after tillage but would reaggregate soon, within 7 to 12 months, if no further soil disturbance occurs immediately [43].

The impact of ST on soil hydraulic conductivity, soil infiltration and water retention are also mixed. ST might not have any effect [44], inconsistent effect [3, 45], or decrease [46], or increase [47] the soil water infiltration rate. It has been reported that frequent tillage impact more negatively than ST in long-term NT soil [48]. ST reduces soil cover, and it is often thought to decrease plant available water in soil through enhanced evaporation due to exposure to the sun and an increase in soil temperature [4]. It has been reported that soil temperature does not vary significantly between NT and ST [27]. Soil water content might decrease immediately after tillage [3], but it would recover quickly after rain. Therefore, the timing of the ST needed to be considered to minimise water loss.

The tillage operation usually impacts soil organic carbon (OC), where an NT will build soil OC on the surface [49]. ST would assist in removing OC stratification from the topsoil and a uniform distribution along the soil profile [14, 50]. Quincke et al. [51] reported an increase in soil OC in 0.10–0.30 m following an event of mouldboard plough in an NT soil. One of the major concerns is that the tillage might break soil aggregates and expose the protected soil OC to microbial decomposition. The effect of ST on soil OC on top 0.10 m was reviewed in 11 studies with 28 soils; out of which 22 soils showed no effect and 6 soils decreased the level of OC [36]. These results might indicate a limited effect of ST on soil OC content. However, even if soil OC decreases near the surface it has been reported that OC would build up below the surface due to the inversion or mixing of topsoil containing high OC with the subsoil that has low OC [14, 30, 46, 47, 51] as well as through enhanced plant root growth. Where a decrease in OC is reported due to the use of ST, an increase in soil nutrient status was also reported which attributed to the mineralisation of nutrients from the incorporated soil OC and improving crop growth and yield following an event of ST [36].

Another concern of the ST is its effect on the soil microbiological community. The review by Blanco-Canqui and Wortmann [36] reported a small or no effect of ST on soil microbial community in four studies. Dang et al. [3] reported no effect of ST within a few months of using a chisel plough on soil microbial activity or biomass in Australia. In contrast, Wortmann et al. [30, 52] reported a persistent reduction in soil microbial activity in 5 years in Nebraska, USA. However, the decrease in soil microbial activity did not affect the crop yield which might indicate that ST did not have any effect or minimal effect on the broader soil ecosystem [30]. Furthermore, Garcia et al. [53] reported that ST reduced root colonisation by arbuscular mycorrhizae but did not decrease the phosphorus uptake by plant roots.
