1. Ross Monaghan & Cecile de Klein AgResearch, Invermay
Soil water and air
Ross Monaghan & Cecile de Klein
AgResearch, Invermay

2. Overview Balancing water and air in soils
Problems arising from too much water
Methods of measuring soil water content
Management practices for regulating soil water:air ratio

3. 1. Balanced soil…
Solids
Pores

4. The balancing act…. Too much water Too much air Anaerobic
Loss of nutrients
Greater susceptibility to structural damage
Too much air
Drought

5. Soil water and air for plant growth
Water for photosynthesis:
CO2 + H2O + energy → carbohydrate
H2O
Air (oxygen) for root respiration:
Carbohydrate + O2 → CO2 + H2O + energy O2
H2O

6. Soil pores Large pores (macro or transmission) Storage pores
Greater than 50 m dia.
Drain under gravity
Storage pores
0.5 to 50 m dia.
Hold plant available water
Residual pores (micro pores)
Less than 0.5 m dia.
Hold unavailable water

7. Soil pores
root
Soil
particle

8. Stages of soil water status
Saturated soil
All pores are filled with water
Field capacity
Storage and residual pores are filled with water
Wilting point
Only residual pores are filled with water

9. The bucket model of soil water…
Rain
Evaporation
Saturation
Field capacity
Water holding capacity
Soil water deficit
Permanent wilting point
Drainage

10. Soil water Field capacity Saturated Permanent wilting point root Soil
particle

11. ET slows as soil dries
Relative evapotranspiration 1
0.5

12. pF curves Permanent wilting point Field capacity
Soil water tension (log cm H2O or pF)
Field capacity

13. Water holding capacities of topsoil
Clay 16.3 mm water/100 mm soil depth
Silt loam 22.5
Sand 13.6

14. Problems with too much water2.
Problems with too much water
Nutrient losses
Drainage
Overland flow
Compaction/pugging
Greenhouse gas emissions

15. Rainfall v ET
Monthly rainfall and ET (mm)

16. Fate of surplus rainfall

17. Nutrient losses Drainage Overland flow Nitrate leaching
Phosphorus and ammonia losses

18. Soil compaction/pugging

19. Unaffected by compaction Slightly affected Moderately Severely10
Depth (cm)

20. Soil compaction → Reduces % of macro pores
→ Reduces soil drainage and aeration
→ Reduces plant growth
Pugging cycle

22. Greenhouse gas emissions
Greenhouse gas emissions in New Zealand
Nitrous oxide
14%
Carbon dioxide
40%
Methane
44%

23. Greenhouse gas emissions from soil
Nitrous oxide (laughing gas) N2O
Biological denitrification (NO3- → N2O)
Anaerobic process
High water filled pore space, i.e. low air filled pore space

24. Effect of soil water content
N2O emission (kg N/ha.day)

25. Methods for measuring soil water3.
Methods for measuring soil water
Gravimetric
TDR
Aquaflex
Neutron probes
Remote sensing

26. Aquaflex

27. 4. Management practices Rain (or irrigation) Evaporation - mulching
Water holding capacity
Avoid compaction
Maintain good structure
Provide good drainage

28. Minimize heavy traffic
Good drainage
Back-fencing
Feed-pads

29. Tools & info for farmers
Commercial test for soil compaction on-farm
E Lab Ltd
Best practices

30. Field drainage Fissure network Trench spoil 25 cm Mole channel
Pea gravel
45 cm
Intercepting tile line

35. Maintaining good soil structure
Stable aggregates (%)