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

It's starting to get dry...

Other than this slightly cooler snap we've had over the last couple of days you'd have to say spring is well and truly here! And with these nor-west winds (in Canterbury anyway) and warmer days things are starting to dry out and there's not much rain on the horizon. The seasonal weather outlook from NIWA suggests that we're in for a dryer than average season in most places (https://www.niwa.co.nz/climate/seasonal-climate-outlook/seasonal-climate-outlook-september-november-2018) and at this point I'd have to say they're about on the money. 

The joys of being a farmer or in the ag industry is that everything you do hinges on the weather, so we get really good (for the most-part) at managing timings and inputs and reading the signs to optimise what we do on farm. Now is no different. Whether you're an irrigated farmer or a dryland one now is the time to be installing your soil moisture probes if you haven't done so already so that you can accurately measure and manage your soil moisture and timings of related inputs on farm. 

Soil moisture probes allow you to know whats going on under your feet and make accurate and timely decisions to set yourself, your farm, your crops and your livestock up to perform to the best of their ability for the coming season. Soil moisture is one of the key drivers for plant growth so it's important that we know where we're currently sitting in terms of soil moisture levels so we can react to it accordingly. Decisions around fertiliser (and other input) timings, timing and amount of irrigation, stock carrying-capacity decisions etc can all be driven by more accurate information regarding soil moisture levels. 

If you want to find out more about some of the leading soil moisture probes in the NZ market have a look here: https://bit.ly/2OyeVj1 

And if you're wanting to get some installed for the coming season please pick up the phone and give the Agri Optics team a call now before you run out of time and you're left carrying a spade in the back of your ute or ruining the tip of your good pocket knife for the upcoming summer. 

All the best for an upcoming and prosperous season ahead! 

Cheers, 

Jemma

In the words of Rachel Hunter - it won't happen overnight, but it will happen...

Wasn’t the whole election one big roller coaster?  If we have learnt one this from this whole affair, it is that we are all very passionate about our water resources – not necessarily for the same reasons, but passionate none the less.

Regardless of who ends up running the country (at the time of penning this blog, Winnie was still  courting both the National and Labour parties), I think it is clear that public opinion will ensure that water management and associated policies will be addressed is some way, shape or form by the incoming government.  Given this, I think it is important (and timely) to look back and acknowledge where we have got too in relation to this, but also to look forward at where we can still go.  

The National Policy Statement for Freshwater Management (NPSFM) and Regional Plans

Most regional councils now either have notified or operative plans which outline their methods for addressing water quantity and water quality.  While no two plans are alike, the NPSFM requires limits are set, therefore, all plans do just that – they set allocation limits for surface and groundwater resources, as well as water quality limits.  It is the latter which takes various forms including property limits and/or catchment limits using Overseer, in stream water quality limits, and the development of the Good Management Practice (GMP) framework.    Many councils have also adopted the use of audited Farm Environment Plans to be able to monitor, measure, report and ensure that farms are meeting their environmental obligations. 

As with anything in life, good things take time.  Councils are in the process of rolling out and implementing these plans now.  The effects will not be immediate, but they need to be given a chance to actually work. 

It must also be remembered that for many areas, it is as much about maintaining the already good water quality that exists – this is not allowed to deteriorate.  For those few areas where improvement is needed, the plans bite much harder, and that it totally appropriate.

The fact that all of this has occurred seems to have been completely overlooked by many, and it’s not just politicians I’m referring too. 

How are we meeting our environmental obligations already?

Figures provided by Irrigation New Zealand show that since 2011:

·       

• $10 million invested in audited Farm Environment Plans;   
• $600 million invested by existing irrigators upgrading to modern, efficient irrigation systems;
• $18 million invested in precision irrigation technologies;
• $15 million invested in installing irrigation decision-making technologies;
• More than 24,000 kilometres of our waterways have already been fenced off to exclude stock at a cost of $220 million. 

As we continue to meet our environmental obligations, you can only expect these numbers to increase.  It is noted that this expenditure is all on farm, reinforcing the point that water quality will be addressed at the farm level. 

Looking Forward

Regional Councils need to continue to implement their plans. This may seem like an obvious thing to state, but it’s true nonetheless.  Consistent messages and enforcement from the regulatory bodies will be a must.

I believe that many farmers are on board with GMP (the on-farm practices) despite still being largely ignorant or merely confused by the new environmental regulations.  Education is still key to the success of this stuff, and that has to come from all involved – banks, valuers, real estate agents, farm advisors, customers… anybody involved with the farm.  And, many farmers are being innovative and taking up technology where it is available.  You only have to look at the statistics above regarding the investment to date in irrigation upgrades and precision irrigation technologies.  

And last, but not least, time, time, time.  To quote Rachel Hunter from her Pantene ad, “it doesn’t happen overnight, but it will happen”, and it is happening.  Get on board, and keep it up.

 By Keri Johnston, Irricon Resource Solutions

Phone 0272202425 or email keri@irricon.co.nz

The irrigation season is just around the corner…

With the days getting longer and the weather getting warmer (I’m sure it’s too good to be true!) spring growth will soon be kicking into gear and irrigation season will be just around the corner. Now is the time to be ensuring that you’re as prepared as you can be for the irrigation season.

If you’re an irrigated farmer now is the time to be thinking about how you’re going to schedule your irrigation throughout the upcoming season. The days of scuffing the dirt with your boot and having a dig with a spade are fast coming to their end with the need for on-farm soil moisture monitors such as the AquaCheck probe, to give some more accurate numbers to the soil moisture levels than a scuff of your boot on the soil. Having soil moisture probes installed on farm not only helps you make better irrigation decisions but it also gives you some hard and fast data to have when it comes to Farm Environment Plan (or the likes) auditing.

Soil moisture probes for use this coming season should be being installed now or over the next few weeks ideally. All continuous soil moisture measurement devices take a period of a few weeks to ‘settle down’ and give accurate readings post installation.

At Agri Optics we have a great soil moisture probe in the AquaCheck probe as part of our suite.

The key things to note on these probes compared others (other than their great price) are as follows:

·         They’re fully telemetered, giving you access to view up-to-date soil moisture data and make timely decisions based on current, actual data

·         They’re a vertical oriented probe that has multiple soil moisture sensors down their length, giving you a total soil moisture trace and soil moisture traces at each different sensor depth. This means that you can see how the soil moisture moves down through the soil profile and how effective you’re being with your irrigation management. The bottom sensor is also a good ‘check’ for drainage leaving the root zone

·         The AquaCheck probe has built-in soil temperature sensors – a good gauge to be able to better manage irrigation and fertiliser timings in the shoulders of the season in particular

·         They have the option of connecting to rain gauges to give accurate records of rainfall and irrigation at each soil moisture probe site

·         They have a short ‘settling’ time post install compared to most of their competitors, meaning that you’ll get useful data to make decisions off in a short time frame

·         They’re easy to install and uninstall, making them great for seasonal cropping situations

·         They’re very competitively priced

·         There are multiple depth option so that the depth of the probe installed can be matched to your farming system and requirements

·         In NZ they’ve got Agri Optics behind them, to help you, the farmer understand and interpret soil moisture readings and get the most out of soil moisture probes for irrigation scheduling

You can also view more information on the AquaCheck probes on our website: http://www.agrioptics.co.nz/portfolio/aquacheck/

If you’re interested in the AquaCheck probes or need a soil moisture solution for this season please don’t hesitate to contact one of the Agri Optics Team for some more information and a quote.  

All the best for an upcoming irrigation season & year ahead!

Cheers,

Jemma

A Guide to Making Sense of Soil Moisture Data

With an increasing amount of soil moisture monitoring sensors on offer in the market today there is growing importance on not only having sensors installed but actually understanding the information they provide. This blog is written to give some insight into the data they you might receive from one of these devices. The following traces are the output of an AquaCheck soil moisture sensor with 3G telemetry. The sensors measure soil moisture and temperature every 30 minutes. The data is available to Agri Optics clients from the AquaCheckWeb platform. For more info see our website http://www.agrioptics.co.nz/portfolio/aquacheck/

The key to getting the most out of your soil moisture sensor is to have an accurate field capacity (FC) and refill point for the probe site calculated. The most convenient way of identifying Field Capacity is to have the probe installed prior to the winter period. Typically there will be enough precipitation to allow the profile to recharge to FC. FC can also be identified by saturating the profile manually with a large quantity of water. The key points we are looking for when identifying FC is a repeated filling to saturation then drainage of the profile. The point where drainage ceases can be identified as FC. Night time events are more accurate as ET is not a factor.

Fig 1. Identifying field capacity

The next key feature to identify is drainage. Drainage is classified as the loss of soil water past the effective rooting zone. The effective rooting zone varies dependent on the crop. Once the depth of plant roots has been identified we can identify any drainage. For the graph below if the crop has a rooting depth of 600mm. The bottom pink line represents the sensor at 600mm. We can see the lift and subsequent drainage of soil water past the effective rooting zone of 600mm.

Fig 2. Drainage events

The third key piece of information that the AquaCheck package provides is the ability to set variable management allowable deficit (MAD) lines. These lines create the target “Green Zone” typically between 85% and 15% of readily available water (RAW). Using MAD lines leaves room for any rain so that any free rain water is not wasted as drainage. It also gives an indication when soil moisture is approaching stress point. The MAD is able to be adjusted to give a desired target soil moisture zone for crop and pasture growth stages e.g. establishment or harvest.

Fig 3. MAD Lines

The final bit of information that becomes available once the crop starts growing is the daily soil water usage. The staircase like moisture trace is showing us evapotranspiration and it allows us to see the impact that increasing crop biomass and increasing temperatures are having on crop or pasture water usage. Crop rooting depth can be identified by seeing how far down the water usage is occurring. In fig 2. above below the roots are drawing moisture down to 600mm vs a later spring sown wheat in fig 4. which is only drawing water to 400. Note the size of the usages. This relates to the root mass at the given depths.

Fig 4. Crop water usage

I hope these tips are useful when interpreting your soil moisture data and that it results in more efficient scheduling of your irrigation this summer. Irrigation New Zealand also has some more tips and info on their website http://irrigationnz.co.nz/news-resources/irrigation-resources/

Post By Nick

Legumes + Efficient Water Use = Great Results at Omarama Station

Omarama Station recently played host to the "Legumes in the High Country" field day, organised by Lincoln University and Beef + Lamb NZ. There was a good turnout of farmers and industry professionals to the farm owned and run by Richard and Annabelle Subtil, 2015 winners of the South Island Farmer of the Year competition. The focus for the day was the use of legume species in the high country environment with a short session on the use of irrigation and soil moisture monitoring in the arid environment that is the Mackenzie Country.

Omarama Station (Courtesty of Richard Subtil)

Omarama Station covers 12,000ha with a mixture of dryland high country and irrigated flats. The property has had significant development work undertaken and a number of centre pivot irrigators installed that irrigate 560ha. A large water storage pond has been constructed to supply water to the irrigation system.

Dr MS Srinivasan from NIWA gave the first presentation for the day at the site of the lysimeter that has recently been installed on the station. The lysimeter is the first in the Waitaki catchment and aims to build knowledge around drainage and soil water under the developing soils at Omarama Station. The site contains three catchment sleeves one of which has soil moisture sensors installed. Any drainage water from the site is measured which gives an indication of the soil moisture status and how drainage from the soil profile is taking place.

From a soil moisture point of view the lysimeter is important as the soils at Omarama Station have exceptionally variable fertility, structure and water holding capacity. Irrigation is not new to the area however the shift from border-dyke irrigation to more efficient spray irrigation has seen a massive change in the water use efficiency on extensive properties such as Omarama Station. Soil development under irrigation is an interesting concept and soils mapped on Omarama Station have shown to have varying levels of water holding capacity based on how long they have been irrigated for in the past. Investigation has shown that the depth of soil and the water holding capacity has improved under 30 years of irrigation. 

Irrigation at Omarama Station (Courtesy of Richard Subtil)

Agri Optics has installed three sub-surface AquaCheck probes that will complement the work being undertaken at the lysimeter site. This information will flow into the decision making process that is used around timing and quantity of irrigation water applied by the team at Omarama Station. 

Derrick Moot spoke on how selection of species was important to maximising water use efficiency in moisture deficient environments such as the Mackenzie Basin. As we know lucerne is a great fit into dryland high country systems. It has the ability to maximise the water use efficiency and has a high water to dry matter conversion ratio (kg DM/mm/ha). The selection of species going forward and the development of novel species all points towards maximising the efficiency of water use in dry high country areas.

Write up by Nick Evans

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

Variable Rate Irrigation and Soil Moisture Management - Introducing The Kowhais

Name: Tom Macfarlane

Farm: The Kowhais

Type: Intensive grazing

Location: Raincliff

Total Area: 800Ha

Total Effective Area: 655Ha

Area under VRI: 86Ha

AquaCheck soil moisture probes: 6

Annual Production: 1000 Lambs, 1500 deer and 1000 bulls

Average Pasture Production: 9500kgDM/Ha

Efficient meat production is the goal for Raincliff farmer Tom Macfarlane. After taking over the property 3 years ago Tom is striving to lift production whilst doing so in a sustainable manner. The Opuha River runs along the boundary of the property and this means that Tom is wary of the impact that his intensive farming operations may have on the environment.

Finishing 700-800 bulls before their 2^nd winter on farm requires a high level of feed intake and the bulls need to be growing every day. The techno system is intensive and requires daily management of multiple small mobs of animals. Juggling the stocking rate according to the feed supply and demand requires effective pasture management strategies to maintain quality throughout the spring and summer.

Water use efficiency is also high on the agenda at Macfarlane’s as water for the properties irrigation system is supplied from the often restricted Opuha Water scheme. Roughly 180 Ha is under irrigation on the property and his pivot and lateral, both with Growsmart Precision VRI, water an area of 86 Ha. Irrigation is also applied through K-Line, hard hose gun and Roto-Rainer systems which cover 31.8, 32.5 and 56 Ha respectively.

The Kowhais EM Zone Map to be used with VRI

The Growsmart Precision VRI really proves its worth when water restrictions kick in as Macfarlane is able to use planned moisture deficit management to water different areas of ground cover at reduced rates. When water is short the fodder beet is watered less frequently to free up more water for the higher quality pastures. Macfarlane also is able to reduce applications on heavier soils to spread his water further. All of this is enabled by the combination of EM Soil Surveys, AquaCheck soil moisture sensors and Growsmart Precision VRI.

The installation of AquaCheck soil moisture sensors has added to Tom’s suite of decision support tools. Tom is now aiming to forecast potential pasture production from soil temperature and moisture data received from the AquaCheck probes. This will enable him to better decide how many head of stock he will be able to carry and will aid in planning for the upcoming season.

The combination of AquaCheck moisture probes and a Growsmart Precision VRI system enables Tom to better manage his irrigation scheduling. The ability to see what is happening to soil moisture and applying the correct amount of water at the correct time has benefits in terms of sustainability and pasture growth. Pasture growth is critical in Macfarlane’s techno beef system where pasture utilisation needs to be kept as high as possible. With potential harvest efficiencies of up to 85 to 90% the cost of overwatering is not just limited to run off and nutrient loss. Pasture damage caused by heavy 18 month old bulls has a significant impact on the level of wastage and future pasture production. The AquaCheck probes will allow Tom to see how soil moisture is tracking and can alert him to when soil moisture rises above the pasture damage threshold. 

With one eye always on efficiency at The Kowhais the use of variable rate irrigation, EM soil surveying and AquaCheck soil moisture probes has given Tom Macfarlane the tools to help improve efficiency and productivity. Watch this space to see how things evolve at The Kowhais.

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.