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.

Why my soil moisture sensor might be lying to me?

After choosing the type of moisture sensor you are going to invest in, the most crucial thing is to get the installation correct. It goes back to the old adage; rubbish in rubbish out, if you don’t get the installation correct everything that follows will at best be very marginal data.

Most probes are measuring a very small volume of soil within 10-20mm of the sensor itself, so good soil contact is imperative as well as a crop cover around the probe that is representative of the rest of the field being monitored.

If you are looking at installing a probe for next season or looking at maintenance on an existing probe then read on! These few basic does and don’ts will be of good use!

Don’t!

• Don't leave the excess cables on the ground – it is an accident waiting to happen!
• Don’t leave exposed cables for wildlife that want to see how tasty it is!
• Don’t site the probe on a ridge or in a hollow!
• Don’t site the probe in bare soil. Is there a crop growing over the probe site to give you a true representation of what is happening in the rest of the field? 
• Don’t site under the fence line

Don't leave cables on the ground

Do!

• Ensure you use good consistency of slurry around the probe to ensure good soil contact.
• Ensure you know the soil type your moisture sensor is located in and how that compares to the rest of the area you are monitoring.
• Make sure any tramlines or irrigation tracks miss the probe site by metres rather than millimetres!
• If your probe is near an electric fence, do ensure any metalwork is earthed.
• Do install the probe as early in the season as you can, so it has time to bed in and the crop over the top of it time to establish like the rest of the field.
• Do ensure a competent and trained person installs the probe with the right equipment to do so!
• If checking an old installation make sure there are no cracks around the probe site, the soil around the probe hasn’t sunk and the wires are in good order.

AquaLINK telemetry unit, away from AquaCheck probe out in the paddock

If you have any doubts about the site or installation of your probe, by installing it as early in the season as you can means that it can be moved and still have the winter to bed in again.

AquaCheck WEB, induvial sensor graph responding to irrigation and rain events.

Monitor your probe data and its response to rain or irrigation events, the beauty of the capacitance probes is that moving them is not an issue.

This article contains information from a post previously written by HydroServices but has been updated to include the experience from the Agri Optics team installing AquaCheck probes.

Improving Irrigation Efficiency for Only $50 cont.

Here is the much anticipated second installment from the Improving Irrigation Efficiency field day run by The Waihao Wainono Group and Morven Glenavy Irrigation. Dr Anthony Davoren, renowned Irrigation Consultant with Hydroservices, shares how drainage through the soil profile can be measured. With this key piece of information we can improve our irrigation management, and know when to turn the irrigator on (or off) to ensure all irrigation that is being applied is going to benefit the grass or crops we are growing.

Thank you to Dr Anthony Davoren, Waihao Wainono Group and Morven Glenavy Irrigation.

Reduce the Cost of Nutrient Loss with Precision Ag (Part 2 of 3)

In the last blog post we looked at nutrients and how Precision Ag can help with your Farm Environment Plans (FEP). This blog post looks at how an EM survey can help with identifying your soil types for your Farm Environment Plan.

An EM survey illustrates the relative variability in soil characteristics including soil texture that can be potentially related to water holding properties within that soil profile, this can help you manage water application through the use of variable rate irrigation technology. When combined with the use of soil moisture probes you have the data and technology you need to be able to retain nutrients within the soil profile itself. 

EM surveys can be ground-truthed to find the correlation between the EM value and water holding capacity (WHC).  From that you can create a WHC map and site-specifically place moisture probes to monitor the soil moisture levels within each identified zone.

Ground-truthing sites are identified within each zone (shown on the left). The graph illustrates the correlation between the EM values and WHC in the top 55cm of the soil profile for this paddock.

In the image above we can see the correlation between EM value and WHC at this site has an R2 of 0.97 (R2 quantifies goodness of fit. It is a fraction between 0.0 and 1.0, higher values indicate that the model fits the data better). We can then use the equation in VA Gateway, one of the PA software platforms supported by Agri Optics, to create a water holding capacity (WHC) map out of the EM values map.

The EM map converted into a Water Holding Capacity map

This water holding capacity map can then be used in conjunction with soil moisture probes and VRI to maintain the moisture levels between field capacity and critical moisture. This not only reduces any potential yield loss from moisture stress but it also ensures that you aren't saturating the soil profile, and therefore avoid leaching nutrients out of the root zone.

It’s all about balancing crop requirements, real-time moisture levels, rainfall (when it comes!) and application rates with irrigation return times as precisely as possible to keep everything at an optimum level.

An AquaCheck soil moisture probe graph showing soil moisture levels and how they are affected my irrigation or rain events on this soil profile.

As can be seen above by keeping the moisture between upper and lower readily available water levels you ensure yield isn’t compromised and eliminate leaching. The rooting depth used for the probe profile can be tailored to the crops specific needs on the moisture monitoring website.

Next time we will discuss how the EM maps and topography data can help you with your FEP.

Chris Smith

Agri Optics NZ Ltd

Winter Is Coming! Tips for preparing AquaCheck soil moisture sensors for winter.

Preparation for winter is key when soil moisture is involved. Once soil moisture probes have been removed from the paddock the first thing to do is plan where they will be installed for the coming season. Choice of paddock, crop and location are all important.

 Re-installing the probe in a similar location for the following season will allow for the comparison of moisture management from year to year. It can also provide insights into how soil moisture is used by different crops in a rotation. Re-installing the probe in a different paddock will require some thought as to where the probe should be placed. Ideally a few key things should be considered as to the location:

Plan:

• Choosing a location that is representative of the paddock. If an EM survey has been done then a location can be selected from the results. If no EM survey has been conducted then a location that looks, or has anecdotally been, representative of the paddock should be identified.
• Make sure that the site is located in the middle third of a pivot, isn’t under any towers or under the end gun and that nozzles that pass over the probe aren’t blocked. Generally placing the probe in the middle of a span is optimal.

Prepare:

• Run the pivot or lateral over the paddock if possible so that wheel tracks are easily identifiable.
• Ensure that tramlines can be identified. This is to avoid installing too close to the tramline. 
• Generally installing after the first spray or fertiliser application is ideal.
• Winter provides the best time for soil moisture probes to bed in. Therefore it is important that probes are installed as early as possible. By installing early it allows for the soil profile to have time to rebuild structure around the probe which is key for accurate data capture. On top of this winter time will provide an opportunity to identify the soil moisture field capacity.

A recent install ready for winter. Tramlines are visible in the background.

If the probes are not being re-installed prior to winter then the following will apply:

• Disconnect the probe from the AquaLINK 3G telemetry unit. Store the probe in a safe place. Particularly away from anything that may chew on the cable.
• Place the AquaLINK 3G telemetry unit on a windowsill or in the garden, somewhere it will receive sunlight. This is to ensure that the battery remains charged up over the winter.
• If the probe is not going to be used for a whole calendar month contact Agri Optics to get the connection deactivated and save on the monthly bill.

Remember the 6 Ps. Prior Preparation Prevents Piss-Poor Performance. Ultimately your soil moisture management will be improved due to accurate and timely data from probes that are installed in the right place and as early as possible.

My Soil Moisture Sensors Are Spot On – Yeah Right!

There is a misconception that all soil (moisture) sensors are precise and tell you the exact soil moisture content.  Not so.  HydroServices runs a trial site where a number of soil moisture sensors are installed for comparison – for both the “calibrated” soil moisture content and any long term trends.  A boring looking trial site because the sensors are installed toward the small gap in the trees.  In all there are 8 sensors – neutron probe, Decagon 5TM and GS1, Acclima, AquaCheck and two (2) Aquaflex.  The Decagon 5TM, GS1 and Acclima sensors are installed at 10cm, the shallow Aquaflex on a slope from 10-25cm, and the neutron probe and AquaCheck can measure at 15 and 10cm respectively.

Sensors are provided with a factory calibration, usually one for silt loam, clay loam and sandy loam soil types.  These are generic and may or may not truly measure the soil moisture content at your location because (for example):

a)    Your soil is unlikely to be the same as the generic soil type;

b)    The sensor is poorly installed (especially if there is not perfect contact between the sensor and the soil); and

c)    The soil is loose (cultivated) and perfect contact is not possible

While the traces of soil moisture content are sort of similar, none (with their generic calibration) read the same soil moisture content, as shown in the plot of all sensors.  (Note the GS1 Sensor is a recent addition and no data is available for the dates compared).

The only sensor that has been calibrated against true soil moisture content (gravimetric laboratory analysis) is the neutron probe.  Knowing that Field Capacity at this location and soil type should be about 40%; only the neutron probe and Aquaflex measure soil moisture content at this level.  The other three sensors measure field capacity 10% less than the true field capacity. Disconcertedly two sensors measure soil moisture content between irrigation events (the vertical rise in the traces) at or very close to wilting point – approximately 17-18%.  This is not the case; the pasture never died nor showed any sign of being close to wilting point.

What is to be taken home from the comparison?  If you want sensible and realistic soil moisture measurements the sensors must be field calibrated.  The simplest and easiest way to field calibrate is by neutron probe – click on http://www.hydroservices.co.nz/index.php?option=com_content&view=featured&Itemid=308 for more details.