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

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

1. Balanced soil… Solids Pores

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

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

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

Soil pores root Soil particle

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

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

Soil water Field capacity Saturated Permanent wilting point root Soil particle

ET slows as soil dries Relative evapotranspiration 1 0.5

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

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

Problems with too much water 2. Problems with too much water Nutrient losses Drainage Overland flow Compaction/pugging Greenhouse gas emissions

Rainfall v ET Monthly rainfall and ET (mm)

Fate of surplus rainfall

Nutrient losses Drainage Overland flow Nitrate leaching Phosphorus and ammonia losses

Soil compaction/pugging

Unaffected by compaction Slightly affected Moderately Severely 10 Depth (cm)

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

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

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

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

Methods for measuring soil water 3. Methods for measuring soil water Gravimetric TDR Aquaflex Neutron probes Remote sensing

Aquaflex

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

Minimize heavy traffic Good drainage Back-fencing Feed-pads

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

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

Maintaining good soil structure Stable aggregates (%)