INFLUENCING SOIL PORES FOR MORE PLANT AVAILABLE WATER

From time to time I hear comments about “how quickly it’s dried out”.  Farmers can experience going from a situation of ample plant growth and OK conditions on the farm, to desperately needing rain, over a short period of time.  Has it always been this way and can we insulate ourselves from this seeming cliff face nature of moisture availability and growth?

If we understand how water is held in the pores of the soil and how plants extract this moisture, it will go a ways to helping us understand how we might positively influence soil moisture.  So let’s first understand a few terms relating to soil moisture levels (the irrigators among you will likely be all very familiar with these).

Field capacity – this is the amount of water remaining in a soil after the soil has been saturated and allowed to drain for approximately 24 hours.¹

Permanent wilting point – the minimal point of soil moisture that the plant requires not to wilt.²  It is the water content of a soil when most plants growing in the soil wilt and fail to recover their turgor upon rewetting.³

Available soil water – this is the difference between field capacity and wilting point – the amount of soil moisture retained in the soil and that can be extracted by a plant.

The available soil moisture is held in the spaces between soil particles, or soil pores.  Consider three sizes of soil pores:

• Macropores or larger pores – Drain quickly due to gravitational forces.  When these pores are full of water, a soil is waterlogged and there will be no plant growth.  These spaces are caused by soil cracking, soil aggregation, plant roots and by the activity of the macrofauna of the soil.
• Mesopores or medium pores – hold the main plant available water of the soil.  They consist of both moisture and air – which allows the plant to access moisture for growth, but without being waterlogged.  The more mesopores in the soil, the greater water holding capacity there is.
• Micropores – Water that generally only microbes can access.⁴

The pore numbers and sizes will vary with soil type (sand, clay & silt percentages), chemical balance, compaction and also soil biological health.  “A sandy soil may have insufficient organic matter to bind the sand grains into larger aggregates. In this case, the soil will have many large pore spaces and very few small pores. The plant roots will have plenty of air, but water will drain freely through the soil with very little storage.  On the other hand a compacted, heavy clay soil will have many small pores and few large pores. Plant roots will suffer because water is tightly bound in the small pores, the soil is poorly aerated, and drainage is poor.”⁵  So there is much influence from the inherent nature of the soil type and from compaction, but this is also influenced by the biological health of the soil.

Organic matter and soil pore makeup

Mesopores are very important as they are where the majority of the plant’s moisture (needed for growth) is held in the soils.  The sandy soil from the example above lacks enough mesopores to offer sufficient soil water storage for highly productive plant growth.  Increasing organic matter levels will help to increase mesopores of the soil.  Organic matter “contributes to the stability of soil aggregates and pores through bonding or adhesion properties of organic materials, such as bacterial waste products, organic gels, fungal hyphae and worm secretions and casts”.⁶  Better aggregation means more pore spaces and greater water holding capacity.

Soil moisture access by fungi

Also to be considered is the impact that the soil microbiology (particularly fungi), have on the plant’s ability to access soil moisture in dry times.  Remember that moisture held in the micropores is mostly only available to microbes?  So if our soils are microbially active, then the plants will be able to access more moisture (via the conduit of fungi) from the micropores, than if the soil life wasn’t present.  “Mycorrhizal fungi acquire water from pores too small for roots and root hairs to access, and at distances from roots and root hairs.”⁷

When plants have used the moisture stored in the larger mesopores, the remaining soil moisture is either held very closely/strongly to the soil colloids, or is stored in the micropores of the soil.  Here it may be accessed by the hyphae of fungi (particularly mycorrhizal), and can be transported back to the plant roots by the hyphae which bridge across the now moisture deficient macropore gaps. ⁸

Accessing the moisture held in the micropores may enhance drought tolerance and help to prevent the seemingly quick ‘cutoff’ of moisture supply to the pasture or crop plants that many farmers experience and that I referred to in my introduction.  “This dry-down period also represents a period of high root mortality, and mycorrhizae may enhance root survival during these periods.”⁹  This will only be true however if we create the right environment for the fungi.

What can we do to improve soil pore structure?

We can do two things to encourage better soil pore structure (apart from the traditional amendments to soil structure via chemistry):

Increasing the organic matter levels and the biological activity of our soils will aid in increasing the number and spectrum of pore sizes in the soil.  This will mean more places in which moisture can be stored. A greater spectrum of pore sizes should also mean that the seemingly quick cut off of moisture availability that can be experienced is softened.

The other thing that can be done is to increase the soil biological activity.  Increased fungal levels will help to access soil moisture from the small micropores that the plant would otherwise not be able to access.

The photo at the top of this blog is an image from some lighter soiled, poorer country on our farm. There is an amazing difference in the pasture here during a dry time – with no influence from leaking water from pipes etc in the green area.  Is what we are seeing here a reflection of soil biology?  I’m not really sure. We’ve seen patches like this before (but not this dramatic), which become more abundant and bigger as soil health improves with thoughtful, conscious grazing and with the influence of the ‘edge effect’.  Have you experienced something like this – why not share it too?

References

1. Plant and Soil Sciences eLibrary.  http://croptechnology.unl.edu/pages/informationmodule.php?idinformationmodule=1130447123&topicorder=3&maxto=13&minto=1
2. Permanent Wilting Point. Wikipedia. https://en.wikipedia.org/wiki/Permanent_wilting_point  4^th Nov 2015.
3. Soil Hydrology AEM. Northesast Reagional Certified Crop Adviser (NRCCA) Study Resources. Cornell University.  http://nrcca.cals.cornell.edu/soil/CA2/CA0212.1-3.php 4/11/2015
4. Singh, B. Global Centre for Land- Based Innovation – Soil Health Workshop, Narrabri. 12-13^th Oct 2015.
5. How do Properties of soil affect plant growth?  Victorian State Government. http://agriculture.vic.gov.au/agriculture/dairy/pastures-management/fertilising-dairy-pastures/how-do-the-properties-of-soils-affect-plant-growth. 5^th Nov 2015.
6. The Importance of soil organic matter. Chapter 5 – Creating drought resistant soil.  Food and Agriculture Organisation of the UN. http://www.fao.org/docrep/009/a0100e/a0100e08.htm#TopOfPage
7. Allen, Michael F. Linking Water and Nutrients through the Vadose Zone: a fungal interface between the soil and plant systems.  Journal of Arid Land 2011, 3 (3): 155-163. file:///C:/Users/User/Downloads/Linking+water+and+nutrients+through+the+vadose+zone%253A+a+fungal+interface+between+the+soil+and+plant+systems.pdf
8.  Allen, Michael F. Linking Water and Nutrients through the Vadose Zone: a fungal interface between the soil and plant systems.  Journal of Arid Land 2011, 3 (3): 155-163. file:///C:/Users/User/Downloads/Linking+water+and+nutrients+through+the+vadose+zone%253A+a+fungal+interface+between+the+soil+and+plant+systems.pdf
9. Allen, Michael F. Linking Water and Nutrients through the Vadose Zone: a fungal interface between the soil and plant systems.  Journal of Arid Land 2011, 3 (3): 155-163. file:///C:/Users/User/Downloads/Linking+water+and+nutrients+through+the+vadose+zone%253A+a+fungal+interface+between+the+soil+and+plant+systems.pdf