What’s affecting your rainfall infiltration and retention?

Soil moisture is often the primary factor limiting plant growth from late spring to autumn.

Anecdotal evidence from farmers and scientific literature suggests that dry / drought conditions are being exacerbated on many farms due to valuable summer / autumn rainfall being lost as runoff, with soils unable to infiltrate rainfall fast enough. This is being driven primarily by compaction, bare soil and soil water repellency. However many farmers in Quorum Sense have observed significant improvements in rainfall infiltration and soil moisture retention as they incorporated regenerative principles in their management.

These stories inspired the ‘Catch the Rain’ project, funded by Beef + Lamb New Zealand, working alongside 35 farmers and a number of scientists to share knowledge and trial practice/system changes seeking to improve infiltration and soil moisture retention, as well as related soil health and pasture productivity outcomes. 

The information in this section draws from the interviews, workshops and trial results undertaken during ‘Catch the Rain’.

Farmers at a ‘Catch the Rain’ workshop sharing their knowledge and experience regarding rainfall infiltration and soil moisture retention.

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Key factors affecting rainfall infiltration and retention

Like most things in agriculture, the answer to this question involves a complex mix of physical, chemical, biological and management factors. Below are all of the key factors identified by farmers and scientists during a series of workshops in 2023:

*Factors in italics are those that farmers can influence

Landscape / physical factors

• Slope

• Aspect

• Topography (how and where water accumulates and flows)

• Water table

Weather-related factors

• Frequency and intensity of rainfall

• Wind

• Humidity

• Air temperature

Soil characteristics

• Soil texture

• Soil chemistry

• Soil structure

• Field capacity

• Soil temperature

Biological factors

• Plant species and roots

• Tree/shrub vegetation

• Soil organic matter

• Soil microbes

• Worms and invertebrates

Management practices

• Cropping practices (i.e. tillage, cover cropping)

• Grazing management and pasture covers

• Pasture species

• Fertiliser and chemical use

• Irrigation methods and management

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Example of a severely compaction soil under new pasture following a fodder beet crop. This soil had extremely poor infiltration rates.

Healthy soils should consist of ~50% air and water for proper function. Soil texture will affect how easy this is to achieve (ie. sand easier than clay) but farm management practices are the primary determinant over time.

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Implications for managing your water cycle

The key takeaway from these factors is that most of them (in italics) can be changed and influenced by farmers - farmers can manage their infiltration rates and soil moisture retention!

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Plant root exudates and associated soil microbes drive the majority of long term carbon storage. Image adapted by Merfield (2023) after Cotrufo et al. (2022).

Breakthroughs in soil science and microbiology in the last decade have revealed that plant root exudates, microbial exudates and and the remnants of dead microbes are the primary builders of soil structure and carbon - NOT plant litter, crop residues, compost etc. 

Therefore a farmers ability to influence and improve soil health and infiltration rates is primarily through increasing plant root exudates (the liquid carbon pathway). This is optimised by;

• maintaining a diversity of green living plants for as much of the year as possible, 

• maximising biomass production 

• optimising plant nutrition to increase photosynthesis (are therefore root exudates)

• managing for diverse and abundant soil biology

Minimising negative impacts such as pugging and compaction, cultivation and chemical disturbances is also important. 

Check out How to improve your soil health to learn more about way to increase the liquid carbon pathway

Managing above ground biomass via pasture covers, crop residues and trees/shrubs also helps slow the movement of water across the soil surface and protects the soil surface from the sun and wind, reducing evaporation. It may also lower transpiration by keeping soil temperatures cooler and reducing plant stress.

Check out What are diverse pastures and/or Regenerative grazing principles for ideas of how to better achieve these outcomes.

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In this article

Key terminology

Soil water holding capacity: The maximum amount of water able to be stored in the soil (saturation)

Field capacity: The amount of water stored in the soil after large pores have drained

(Plant) Available water holding capacity: The maximum amount of water a soil can provide to plants.

Permanent wilting point: The water content at which plants can’t get any more water out of soil (varies with species). 

Transpiration: The water ‘lost’ from plants as they respire. 

Evaporation: The water lost from the soil surface

Evapotranspiration: Transpiration + evaporation

Soil aggregates: Clumps of soil particles that are bound together by biological ‘glues’ from plant roots and soil microbes. Soil texture and aggregation determines soil porosity.

Soil porosity: Divided into macropores and micropores. In NZ macropores are > 0.06 mm and micropores are < 0.06 mm (really tiny!). Porosity determines how much air and water can flow through and be held in soils

Soil Water Repellency (hydrophobicity): Biochemical effect where soils ‘repel’ water (like potting mix or sand if left to dry) resulting in rain ‘running off’ or flowing through large cracks in soils. Some soil types are more prone than others. Also linked to poor soil health.