Soil Moisture Terms

In the last article we focused on soil moisture and how it is stored in the soil; adhesion, cohesion and capillarity. But how does this relate to the terms: saturation, field capacity and permanent wilting point?

When soil moisture is stored in the soil it is possible to measure both the amount (content V%) and the tension. The soil tension forms the basis of the following soil moisture parameters: saturation, field capacity and permanent wilting point.

Saturation

A soil is saturated when all pores (micro and macro) are filled with water and no air remains in the soil. At saturation there is free water in the soil profile. Gravity will cause water to drain from macro pores and saturation is therefore a temporary state.

Figure 1: Example of a soil reaching saturation point and the subsequent drainage period. 

This is how it appears on AquaCheck soil moisture plots. 

Field Capacity

When a soil is at field capacity, water is held by adhesion to soil particles and capillarity in micro pores. Field capacity is reached when rapid drainage decreases (Figure 1).

On your Vantage NZ soil moisture plots the field capacity is determined for each sensor depth, then summed to determine the l field capacity for the active root zone. This allows for soil texture changes throughout the profile and provides you with a field capacity unique to the sensor site.

Permanent Wilting Point

Evapotranspiration and drainage (to a much lesser extent) will cause the soil to dry below field capacity. During this process water is removed from all but the smallest micro pores. The permanent wilting point (PWP) varies depending on plant conditions, plant type and soil texture (Figure 2). Nevertheless, the soil water potential at which permanent wilting occurs is considered to be 1500 cba.

Figure 2: Illustration of saturation, field capacity and permanent wilting point for three different soil types. 

Available Water 

Available water (AW) is the amount of water held in the soil between field capacity and wilting point for a defined depth of soil and is expressed as V% or millimetres (mm). 

AW = FC - PWP

Readily Available Water

Not all the available water is equally (readily) available to plants. Water becomes more difficult for plants to extract the closer the water potential comes to permanent wilting point. This is because the reminding water is bound to the soil at increased tension.

Plants need to take up enough water to satisfy their transpirational demand and sustain optimum growth rates. For every kilogram of dry matter (DM) produced, a plant must transpire between 200 – 500 litres of water.[1] For plants to obtain this quantity of water from the soil, water needs to be readily available. Water is said to be readily available when plant growth is not restricted by water availability. Stress point is the point at which plants can no longer extract water at potential rates. On a soil moisture plot this will be demonstrated by a change in water use, i.e. a change in slope of the soil moisture trace (Figure 3).

Figure 3: A change in slope indicates a change in water use. This is how it appears on AquaCheck soil moisture plots. 

As water below the stress point is not readily available and not sufficient to meet potential daily plant demands, yield is lost. Water between the stress point and permanent wilting point is available to plants, but growth is adversely affected.

The key soil moisture parameters described above are essential in irrigation management. At Vantage NZ we strive to clearly determine and label these on your soil moisture plots (Figure 4) so you can make good irrigation management decisions. 

Figure 4: AquaCheck soil moisture plots clear labelling of key soil moisture parameters. 

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[1] McLaren, R.G. and Cameron, K. C. (2000). ‘Soil Science’, Sustainable production and environmental protection. Second edition, Oxford University Press. Page 99.

So, What is Soil Moisture

 

I recently heard a gardening segment on a NZ radio station. The gardening commentator was answering questions and providing advice on ‘how to irrigate your garden’. Her advice was: “Deep watering will encourage the roots to grow into the water table below. This is desirable as it allows the plants to be self-sufficient in accessing water.”

We all have our own perception of water and how it is stored in the soil, but the gardening commentator’s description isn’t an accurate description of what actually happens within the soil or what we are aiming to achieve through irrigation.

There are several processes at play when water is “stored” in the soil: 

·        cohesion - the attraction of water molecules (H₂O) to one another it causes water molecules to stick to one another and form water droplets;

·        adhesion – the attraction between water molecules and solid surfaces, in this case soil particles;

·        surface tension – as a result of the cohesive properties of water molecules and their attraction to other water molecules, a water surface behaves like an expandable film; and

·        capillarity – is a combination of cohesion/adhesion and surface tension forces and is the primary force that enables the soil to retain water and to regulate its movement.

In this article we will take a closer look at these terms and and apply the concepts to soil moisture storage.

To demonstrate or understand adhesion and cohesion, pick up a rock or stone, dip it into a pool of water, pull it out again. The water dripping off the rock is free water (lost to gravity, same as free water will be lost to drainage when soil is at saturation point). If you give the rock a shake you will free it of more water - this is the water “stored” by cohesion. The rock is still wet even after the shaking - the water left on the rock is “stored” by adhesion (Figure 1). Water is stored in this way on all soil particle surfaces, whether it be a clay, silt, sand or gravel particle.

Figure 1: Soil moisture is stored on soil particles like a film via adhesion. On this stone adhesion is demonstrated by dipping it into water solution containing blue dye.

Capillarity is the key to storage of water in the soil. It allows water to move upward (and through) soil pores against the force of gravity. The finer-textured the soil (silts and clays) the greater the ability to hold and retain water in the soil in the spaces between particles. The pores between small silt (less than 0.02mm diameter) and tiny clay (less than 0.002mm diameter) particles are known as micropores. Compare these to the larger pore spacing between larger particles, such as sand (0.2-2mm) and stones (larger than 2mm) which are called macropores. Micropores enable greater capillarity rise.

Capillarity can also be simply demonstrated by placing a dry sponge into water – it will progressively wet upwards through the sponge (Figure 2). The finer the sponge material the higher the water will wet the sponge.

Figure 2: Fine sponge placed into a dish with water solution containing blue dye demonstrating capillarity.

When we irrigate, we want the water to have the opportunity for adhesion and capillarity to take place; i.e. “coat” the soil particle surfaces with water and be retained in the micro pores by capillarity this is best achieved through low application rates and by matching the applied depth to soil moisture deficit.

Back to the garden commentator’s recommendation to practice deep watering and aim to push roots into a water table. Very few farmers/growers/irrigators will have a water table shallow enough for roots to reach the water table. When roots explore the soil profile, they form perfect contact with soil particles, via this contact they can extract the moisture stored on particle surfaces. Deep watering is accurate to an extent. We want roots to explore as much soil as possible as this allows them to access more water and nutrients. Roots will only grow in moist soil, so they’ll only explore the soil profile if it’s been wetted. However, it is unusual for the subsoil not to be moist enough for root growth as the plant advances through its growth stages. Irrigation should therefore only be aimed at wetting the soil within the active root zone.

Aquacheck sensors measure soil moisture at several depths. This depth profile is a very useful tool in managing your irrigation. It allows you to see if you are wetting the active root zone and whether the subsoil is wet enough to allow for root growth.

Jane Robb 

Vantage NZ Customer Support Specialist

"Action for Healthy Waterways" - What you need to know

Last year, the Government released its Action for Healthy Waterways discussion document. It outlined the much-anticipated proposed changes to our national freshwater management framework.  The discussion document resulted in over 12,000 submissions being received for consideration by a government appointed expert panel. 

The recommendations by the expert panel, and approved by Cabinet, were released at the end of May.  These are essentially a suite of broad policies.  They are final and therefore, no further opportunity exists to have input on them.  It would be fair to say that the recommendations overall result is a much more balanced, practical suite of changes, but there is still a lot of detail to come, as discussed later, and the odd quirk that wasn’t expected. 

The broad policies are as follows:

• Councils will be able to maintain water quality attributes below the national bottom line to “secure the benefits” of the existing structures in the Waikato, Tongariro, Waitaki, Manapouri, and Clutha hydro schemes.
• Limits will be defined and how they are to be expressed in planning documents.
• Water quantity limits must be linked to ecosystem health outcomes.
• Territorial authorities will be required to manage effects of urban land development on freshwater bodies and coastal marine environment.
• Clarification of what Te Mana o Te Wai means and how it is to be implemented, both nationally and regionally.
• Councils will be required to actively involve tangata whenua in council processes for policy and plan development and decision-making.
• Regional council policies and plans must include mahinga kai as a value.
• Amendments to ensure regional authorities manage all aspects of ecosystem health (not just water quality and quantity).
• New attributes with national bottom lines:
• Macroinvertebrates
• Submerged plants in lakes
• Dissolved oxygen
• Suspended sediment
• Deposited sediment
• E. coli at swimming sites during the bathing season
• New attributes without national bottom lines:
• Fish species
• Ecosystem metabolism
• Dissolved reactive phosphorous
• Existing national bottom lines for nitrate and ammonia toxicity attributes will be strengthened to protect 95% of species from toxic effects. Exceptions to this will be allowed in specific areas of the Pukekohe and Lake Horowhenua catchments, due to contribution to national food security (vegetable production).
• From mid-2020, technical standards, methods, and requirements for activities affecting streams and wetlands will be prescribed. This will include vegetation clearance, earthworks (including for drainage), and changes to water levels. Includes surrounding vicinity. Resource consents will be required for most of these activities.
• From mid-2020, minimum design standards for new weirs and culverts to provide for fish passage. Passive flap gates will be a non-complying activity. Regional councils will be required to gain information on current structures and adopt work programmes to address barriers to fish migration.
• Until 31 December 2024, resource consents will be required for:
• land-use change of more than 10 hectares to dairy
• land-use change of more than 10 hectares from woody vegetation or forestry to pastoral farming
• increases in irrigated pasture for dairy farming above 10 hectares
• increase in winter forage cropping area above annual highest 2014/15 – 2018/19
• increase in dairy support activities above highest annual 2014/15 – 2018/19
• From July 2021, there will be a national maximum of synthetic nitrogen fertilizer application of 190kg of nitrogen per hectare per year for dairy, dairy support, sheep, beef, deer farming. Dairy farmers must report applied amount to councils.
• From winter 2021, if you are winter grazing on areas that exceed the following thresholds, you will be require a resource consent:
• less than 50 hectares or 10% of property area (whichever is larger) is used for winter grazing
• minimum setback of five metres from waterways
• average slope of paddock 10 degrees or less
• Farm plans will be required for:
• pastoral farming totalling 20 hectares or more
• arable farming totalling 20 hectares or more
• horticulture totalling 5 hectares or more
• an agricultural purpose prescribed in the regulations (not yet determined)
• any combination of the above uses totalling 20 hectares or more.
• Water users with consents to take water over 5 litres per second will be required to measure water use every 15 minutes and provide electronic records to councils daily.

One thing that is notably missing is the national bottom lines for Dissolved Inorganic Nitrogen (DIN).  This was proposed to be a limit of 1 mg/L and caused a huge amount of debate.  It has been kicked for touch and the need for a DIN limit will be reassessed in the future.  Also kicked for touch is water allocation and iwi rights and interests.  This is not a surprise – a complex, fraught debate that no government to date has had the balls to address. 

The nitrogen fertiliser cap was one of the quirks.  What they are hoping to achieve with this, I am not sure.  You can reduce its use, but to make up the feed shortfall, supplementary feed is used, which is a form of imported nitrogen, so you reduce nitrogen in fertiliser only to replace it in imported feed.  Very non-sensical.

So, what happens next?  Some of the specific regulations are still to be drafted.  This includes the National Policy Statement for Freshwater Management and National Environmental Standards.  Councils must give effect to these new documents by 31 December 2024.  It is indicated that there will be consultation with stakeholder groups (for instance in relation to the requirements for mandatory farm plans with freshwater modules), but the regulations are due to be presented to Cabinet for consideration in July, so there is not a lot of time.  As if often the case, the devil can be in the detail, so there may still be some sting in the tail in the drafting of the regulations.  

Watch this space. 

Keri Johnston - Irricon Resource Solutions

Now Find H2Grow on Social

As Albert Einstein once said, “Life is like riding a bicycle, to keep your balance, you must keep moving.” 

With that in mind the H2Grow team have reflected on how we initiate conversations and share knowledge, and have decided to extend our presence to social media where so many meaningful conversations are being had by food and fibre producers and consumers. 

So come and join us on Facebook and Instagram, search for @H2Grow.NZ - we look forward to seeing you soon!

Note we will still also be posting longer, more in-depth information on our blog, and available for workshops and field days at request. Just click on the email links to any of the team if you would like to know more. 

Keri, Jemma and Sarah

Maximising the Value of Irrigation

The H2Grow Team are excited to introduce Carolyn Hedley as our guest contributor, it is with great pleasure that we can share with you her valuable expertise. Carolyn is a Soil Scientist with Manaaki Whenua, based in Palmerston North, and lives on a small Kairanga farm with husband, Mike. Carolyn has combined her interests in soil science, proximal soil sensing and precision agriculture with on-farm studies of precision irrigation and soil carbon mapping. She has led several nationally funded projects in irrigation and soil carbon, including current leadership of the MBIE funded programme “Maximising the Value of Irrigation”.

Maximising the Value of Irrigation  -  Carolyn Hedley

Early in the new millennium I found out about EM mapping and in 2004 published a method in the Australian Journal of Soil Research to rapidly EM map soil variability on a basis of soil texture. I realised that EM mapping was a really useful new technology to rapidly survey soil variability. The EM map had picked the difference between a Kairanga silt loam and a Kairanga clay loam, and this had management implications for the farmer because the heavier textured soil would compact sooner when grazed in wet conditions.

I could see great potential in this new technology and so embarked on a PhD in proximal soil sensing and this is when I started to relate the EM map to soil available water holding capacity and realised how useful this could be for irrigation scheduling. But critics commented that irrigation systems cannot irrigate to such a complex pattern (example shown in Figure 1 below). Enter Stu Bradbury and George Ricketts, who had worked with me on some EM mapping projects when they were students at Massey University. There was an engineering solution to this problem – control the sprinkler system on a pivot to irrigate to any pattern – which led to the development of the Precision VRI system. Precision VRI, the world’s first true variable rate irrigation system, turned the heads of the global irrigation giants and as a result Lindsay Corporation acquired the technology development company founded by Stu and George.

Figure 1: Available Water-holding Capacity map derived from an EM map for a 100-ha area irrigated by a VRI linear move irrigation system

There was still work to be done though and a proposal put to the Ministry for Business Innovation and Employment received six years funding in 2013 to further research methods to improve management of irrigated land. Now in its final year, the “Maximising the Value of Irrigation” programme has been able to refine methods to use proximal sensor data to create prescription maps for precision irrigation. It has developed soil and crop sensing methods that can inform in near real time the prescription map, and a prototype scheduling tool has been tested with participating farmers as a smart phone app. The in-field sensor monitoring methods have been used to support Lindsay further refine the software control features for the Precision VRI system, which is remotely managed through the FieldNET platform.

Ekanayake, J. and Hedley, C. (2018) Advances in Information Provision from Wireless Sensor Networks for Irrigated Crops. Wireless Sensor Network, 10, 71-92. 

Research into different soil management methods has identified correct tillage and soil surface management methods to store more water in the soil and reduce irrigation requirement and water losses. A spatial framework to run the APSIM model has been created to test the effect of different irrigation scenarios on yield, drainage and water use efficiency. Spatial-APSIM simultaneously runs the model for up to 1,400 grid cells for one irrigation system to compare results of different irrigation scenarios at spatial resolution < 50 m, over several decades.

The MBIE Programme “Maximising the Value of Irrigation” is now working closely with its industry advisory group to ensure that its findings are communicated effectively and to find ways to integrate new tools and support improved management of irrigated land in New Zealand.