1. Removal of a subsoil constraint. When does it pay? Imma Farre, Bill Bowden, Stephen Davies & Dennis van Gool Department of Agriculture and Food Western Australia

2. Outline  The issue  Aims of the work  Methodology  Results  Conclusions  Take home message

3. Introduction - Subsoil constraints lead to a reduction in root growth, which results in reduced yield. - Constraint removal can have positive or negative effects on yield, depending on season, location, soil type and constraint level. - Simulation modelling can be used to give an indication of the level of risk associated with removal of subsoil constraints. - Aim: to assess the impact of soil type, location, season and severity of the constraint on the yield response of wheat to removal of a non-specific subsoil constraint.

4. Methods APSIM-Wheat simulations Loc: 30 locations in WA Soil types:loamy duplex (heavy) loamy sand (light) Years: 1957-2006 Constrained root growth at 20-40 cm depth: (non-specific constraint) unconstrained typical severe extreme

5. Results. Root depth (cm) mean rooting depth, 1957-2006 level of constraint LocationSoil typenonetypicalsevereextreme MingenewLoamy duplex102957023 MingenewLoamy sand1841709724 The different levels of subsoil constraints caused a reduction in final root depth (cm) Mingenew, loamy sand, 1999

6. Yield response to removal of constraints

8. Assuming amelioration cost $40/ha and return $250/t grain Return to amelioration

9. Average yield response to constraint removal Loamy duplex Typical Loamy sand Typical

10. Frequency of positive responses to constraint removal Loamy duplex Typical Loamy sand Typical

11. Loamy duplex Typical Loamy sand Typical Loamy duplex Severe Loamy sand Severe

12. Conclusions Wheat yield response to constraint removal is on average positive, but can be negative. The information on probability of positive response becomes important in decision making. Factors that have the major impact on the size and frequency of the response: 1.Soil type 2.Severity of the constraint 3.Season

13. Conclusions In summary for this study the following factors are associated with positive, variable or negative responses to constraint removal: Likelihood of response PositiveVariable or Negative Constraint is severe or extreme (Root depth < 30 cm) Mild or typical constraints Higher rainfall locationsLow rainfall locations Wetter seasonsDrier seasons Light soils (Deep roots are critical to access a available water) Heavy soils (more plant available water in top soil layers)

14. Take home message Greater benefit if: Severe and extreme constraints Typical constraints & High yielding locations & Wet seasons Light soils Greater risk if: Mild constraints & Low yielding locations & Dry seasons & Heavy soils

15. Thanks  Paper available  - Hard copies  - email: imma.farre@agric.wa.gov.auimma.farre@agric.wa.gov.au  - Online Proceedings of the Australian Agronomy Conference 2010