EM Values - What the data is telling you

Today we've got the second part of a 4 part series on EM Surveying and all it's uses. This week we've been into the EM Surveying over on the West Coast (check out our Facebook page if you want to see more) and it's certainly an important part of the job being out there doing the survey and seeing the physical aspects of the job to help make sense of the data and what it's telling you. Today we look at what the EM data does tell you...

An electro-magnetic (EM) sensor generates a constant electro-magnetic field that penetrates into the soil profile. It measures the bulk electrical conductivity of the soil profile. As we conduct an EM survey the sensor is taking readings at two different depths simultaneously. These two depths are known as the ‘Shallow EM’ and the ‘Deep EM’. The depths the DualEM reads depends on the height the machine is off the ground. With our EM setup we are reading the soil profile depth of 0-50cm for the shallow EM and the deep EM at a soil profile of 0-125cm. So the deep EM values are the same as the shallow plus another 75cm deeper. This is why the deep EM readings are always higher than the shallow as it is reading that extra 5cm.

Figure 1: Shallow EM survey values varying from 2-20 EM units (mS/m)

Figure 2: Deep EM of the same area with values ranging from 14-30 EM units (mS/m)

In this survey the same features are showing in the shallow EM and deep EM results, however sometimes this is not always the case the deeper profile can have a different underlying soil type that the shallow EM doesn’t pick up but the extra 85cm of deeper soil does and it changes the overall structure.

Generally speaking and depending on what part of the country you are in and the time of year the survey is carried out amongst other things, we would class a range in EM in the shallow profile of 1-3 units as low variability, 4-8 units as moderate variability and over 8 units range as high variability in the shallow layer/soil profile. In the deep EM/soil profile layer a range of 1-6 would be low variability, 6-15 moderate variability and over that high. It is often dangerous to generalise like that, but it gives you an idea of the type of ranges we look at, and as previously stated there are a lot of other factories that determine if the readings are low, medium or high variability. You also have to look at the distribution of the values as well, if the majority of the values are within a certain range and a few rogue values outside that but on a minimal area of the total, then the range in variation may not be as much as it first looks. How much the variability is costing you in terms of blanket irrigation applications compared to variable rate irrigation applications be it water, seed or fertilizer is a subject for another day!

For more information on EM Surveying please contact us at Agri Optics NZ Ltd.

Chris Smith.facebook

Tips, Tools and Technology for Efficient Farming - Part 1

During winter the H2Grow team ran a series of workshops throughout the South Island titled ‘Tips, Tools and Technology for Efficient Farming’. These workshops were very well attended and the team thoroughly enjoyed meeting everyone and the wide-ranging discussions that were had.

For those that were unable to attend we do not want you to miss out, so over the next few blog posts we will be posting notes of the key messages from each of the presentations. These are only condensed versions of the main points so if you would like further information or have any questions then please do feel free to contact the contributors directly by either clicking on the photo widgets to the right of this blog, or use the links provided.

The first set of presentation notes briefly cover the following topics:

1. Why should we care about farming efficiently?
• Nutrient management - why are we doing this?
• Irrigation and nutrient management - how to they fit together?
2. Soil moisture and water use efficiency

You will see there are two copies of the notes, one for Canterbury and the other for Otago as the notes relating to the regulations between these two areas differs.

Nutrient Management and Soil Moisture - Canterbury

Nutrient Management and Soil Moisture - Otago

Both topics were presented by Irricon Resource Solutions, so for more information please fee free to contact Keri Johnston or a member of the Irricon Team.

Know your Soil Better than your Bank Manager - Continued

Identifying Soil Texture

Soils are made up of particles of different sizes, the largest sand, followed by silt, to the smallest clays. Together these make up the soil’s texture. Soil texture has a direct impact on soil physical properties: porosity, water holding capacity and bulk density. Furthermore soil clay content determines soil chemical properties and the soil’s ability to hold onto nutrients.

This blog will discuss hands on ways to determine your soil texture, how texture relates to key soil physical properties and the role of clays in the soil. You can determine your soil texture at the same time as you carry out the VSA described in the previous blog post and together these practices will improve the quality of your information.

The change in a soil with depth, the cross section down through the soil, is referred to as the soil profile. It normally consists of a number of soil horizons (layers) each with different characteristics (texture and/or stone content). The picture below shows a soil profile with six distinct soil horizons. When scheduling irrigation you need to know information about the hydraulic (water) properties of each soil horizon that plant roots occupy within the soil profile to determine the amount of water available to the plant. This determines how frequently you need to irrigate (return period) and the maximum irrigation you can apply in one application (irrigation depth).

Example soil profile

Soil texture is an important characteristic because it gives a good indication of other soil properties such as water storage, drainage and nutrient supply. It is a stable soil property and is not likely to change with normal soil management. Soil texture can be estimated in the field by some practical tests involving the feel of the soil and these are outlined below. To determine the textures and get an idea for the ability of your soil to hold water it is beneficial to dig a pit and expose an open face on the soil profile so you can determine the different horizons visible down the profile. You should identify the soil texture of each of the horizons that plant roots are found to grow in, or down to about 60 cm.

Hands on method to determine your soil texture.
Found in the joint Irrigation NZ and Plant and Food resource - Click here to visit the webpage.

The graph below shows typical soil water holding capacities (WHC) for different soil textures in % or mm of water per 100 mm of soil depth. It also shows their typical permanent wilting points (WP) and field capacities (FC). The relationship between WHC, porosity and bulk density is straightforward. Sand has the largest particles, the lowest WHC and therefore the lowest porosity. This translates into the highest bulk density because less space is occupied by air. As shown by the WHC of silt and clay below, silt has a higher porosity and lower bulk density which is very similar to clay soils although clays tend to have the highest porosity. This is because clay is made up of lots of small particles which create lots of air spaces between them. Therefore clay also has the lowest values for bulk density.

Relationship between soil texture and soil water content.
Found in the joint Irrigation NZ and Plant and Food resource.

Another role of clay in the soil is in terms of nutrient management. The structure of clay's means that they tend to become negatively charged around the surface. This means that positively charged nutrients are attracted to the surface of the clay and, depending on the conditions, can move between this surface and the soil solution from where they can be taken up by plants. It is helpful to have an idea of how much clay your soil has because this will determine its ability to store positively charged nutrients such as potassium, calcium, magnesium, sodium and resist changes in pH. Clay also holds phosphorus by allowing it to be adsorbed into the clay structure; some clay's allow this more than others. This is important to note because when phosphate is adsorbed it is less likely to become available to the plant and more phosphate will need to be applied to the soil to avoid deficiency in plants.

For more information on soil texture and water holding capacity you will find a great resource by following this link.

Once you have an idea of your soil texture and water holding capacity mapping tools can be used to get an idea of the representation of this soil type across your whole farm. Simple mapping such as Google Earth images (see the Ground Truthing your Soil Variability blog) and S-Map (which will be discussed in a future blog post) are helpful resources. It is important to be aware that these are tools to increase your understanding but to provide the detail required for efficient farm management tools such as EM mapping and determining exact water holding capacity are greatly beneficial.

Blog post written by Nicole Mesman - BSc (Hons) Soil Science.

Know your Soil Better than your Bank Manager

A Practical Guide to Assessing your Soil Quality

The soil’s physical properties are vital to the ecological and economic sustainability of land. They control the movement of water and air through the soil, and the ease with which roots penetrate the soil. Damage to the soil can change these properties and reduce plant growth, regardless of nutrient status. Decline in soil physical properties takes considerable expense and many years to correct, and can increase the risk of soil erosion by water or wind.

The primary functions of the soil are to provide plants with air, water, nutrients and a rooting medium for growth and physical support (image sourced from the Landcare Research website) 

The Visual Soil Assessment (VSA) was developed by Landcare Research to give cropping and pastoral farmers a straight forward and time efficient checklist to use in the field to assess the state of their soil, primarily the physical soil quality.

The VSA can be found online here -> Visual Soil Assessment (VSA)

The VSA aims to help farmers identify changes occurring to soil physical properties so that they can assess the effect that these changes will have on their soil quality and the sustainability of their land management and long term profit.

Pictures in the VSA guide can be helpful when carrying out the assessment in the field (image sourced from: VSA Volume 1).

The assessment can be carried out quickly, reliably and cheaply with little equipment, training or technical skills. The scorecard below is to record those visual soil indicators used to assess soil quality. There is a similar scorecard for recording plant indicators. You are then able to compare the two sets of indicators to see if you have similar scores for both and if not why. For instance, is damage to soil quality not being seen in crops yet or are crops struggling to recover from previous soil damage?

VSA Scorecard (image sourced from: VSA Volume 1)

Below each indicator is a section in the online VSA booklet to refer to for assistance. Pictures are included so you can compare what you are viewing and refer to examples. You will need a spade, the score card, a surface to drop soil onto for a shatter test and a bin to contain soil. Each indicator is given a weighting and at the bottom of the scorecard you add the scores for the various indicators. Values falling within certain ranges are deemed “poor”, “moderate” and “good” quality. If your quality is poor or moderate it is suggested that you refer to Volume 2, also easily accessible from Landcare Research online. This volume contains tips on how to improve your soil quality or maintain it if it is already good.

Tips include:

• Cultivating at the correct moisture levels to avoid smearing of soil, formation of cultivation pans and reduced infiltration when the soils are too wet. 

  

  (image sourced from: VSA Volume 2)

• Use a sub-soiler to break cultivation pans and increase root growth
• Maintain soil organic matter levels to ensure porosity, drainage and root growth.

  

  (image sourced from: VSA Volume 2)

By utilising these resources, you will gain a better appreciation for the state of your soil and will be able to identify when changes are occurring and why. The VSA is a simple tool and when used regularly will help with building a picture of soil quality. There are a range of other resources that can continue from the VSA, further your knowledge of your soil and assist with management. SINDI, another resource for determining soil quality, will be discussed in a future blog post along with hands on ways to identify your soil type and S-Map, how its geomorphological (land formation) history can be used to assist your farming.

The blog post you have just read was written by Nicole Mesman - BSc (Hons) Soil Science.

Soil Moisture 101

Soils are made up of mineral matter, organic matter, water and air. The space between the soil particles are referred to as pores, air and water occupy these pores. Macro pores allow water to filter through the soil and then drain out the bottom. Micro pores store water that is available for plants to grow.

Soil texture is an important characteristic that influences water holding capacity, drainage characteristics and water infiltration rate. The finer the texture of the soil the greater volume of micro pores and therefore greater water holding capacity compared to coarser textured soils.

The total amount of water that a soil can store is referred to as the water holding capacity (WHC) of the soil. Coarse textured soils such as sandy and gravelly soils have a low WHC while silts and clays retain more water therefore have a higher WHC. WHC is usually expressed in miilimetres (similarly to rainfall) held per depth of soil e.g. Xmm/100mm.

Here are some common terms that you are likely to come across regularly on H2Grow and resources relating to soil moisture and irrigation scheduling:

Saturation – When all the macro and micro pores are full of water. If more water is added to a saturated soil it will either drain out the bottom, pond or run-off.

Field Capacity – Macro pores are full of air, micro pores are full of water. Silt and clay soils generally reach field capacity after 2-3 days of drainage from saturation, sandy and gravelly soils much faster. Field capacity may also be referred to as full point.

Stress Point – At this point the plant has to work to harvest the water from the soil, therefore plant growth is slowed and yield potential is reduced. The plant will survive beyond this point but will become increasingly stressed. Stress point is related to crop type, rooting depth and soil type. Stress point may also be referred to as trigger point or refill point.

Wilting Point – At this point although there is still water held in the soil the plant is not able to access it as it is held to tightly (hydroscopic water). The plant will therefore permanently wilt and die. Wilting point may also be referred to as permanent wilting point.

Water Holding Capacity (WHC) – Is a measure of the water that is extractable by plants. This can be calculated by taking the difference between the soil water at field capacity and at permanent wilting point. Water holding capacity may also be referred to as total available water or available water.

Readily Available Water (RAW) – Is a measure of the amount of water in the soil that supports optimum plant growth. This can be calculated by taking the difference between field capacity and stress point. As a general rule of thumb half of the WHC is readily available to the plant, therefore RAW = 0.5 x WHC.

Soil Infiltration Rate – Is the speed at which applied water can enter the soil. It is described as the millimetres depth of water infiltrated per hour (mm/hr).

Figure 1 below may help to illustrate the difference between saturation, field capacity and wilting point.

Figure 1

While this theory is all very useful, nothing beats seeing like in the real world. So I’d encourage you the next time you’re doing a paddock walk to take a spade with you and locate what appears to be the driest and the wettest spots in a paddock. Dig a hole in these two spots and compare the soil type/texture, the depth of topsoil, depth of the roots and other obvious visual differences. You will see posts over the next month that explain how to carry out a visual soil assessment and then how to apply this in your irrigation scheduling.

Posted by Sarah Elliot from Lindsay NZ

The 2015 EM Season has come to a close

As we fast approach the end of 2015 it is good to look back at the EM surveying season we have just had. Due to the very dry summer we didn’t get going until nearly the end of April which is about 3-4 weeks later than the season before. Stewart Darling join the team this year and many of you would of met him if we conducted any survey work for you. As Stu’s from Scotland he found the colder winter weather easier to handle than this already hot summer! We also changed our side by side Polaris for a larger capacity machine for greater capability in the field and improved downhill descent which comes in very handy at times in Otago!   We did a lot of testing with the new set up to make sure it was performing well before the season kicked off.

Field testing the EM rig on the new Polaris back in February 2015.

Like every season we have faced new challenges and found solutions and developed our protocols to take them into consideration. So next season we will be another step ahead of the previous year! We have met some varied and interesting new clients as well as continuing to work with our existing clients, helping all to achieve their goals with precision ag. through our services.

With the introduction of our AquaCheck soil  moisture probes this season we have been able to add another dimension to the service we offer, not only surveying  your soil’s variability and moisture holding characteristics but now also providing the means to monitor them very effectively.

Travelling past Mount Cook on one of many trips through the central South Island.

Moving forward to 2016 we will start surveying as soon as conditions allow; that is usually when irrigation has finished and we have had a couple of large rain events to negate the man-made influences of that seasons irrigation on the soil. This is usually anytime from late March to April depending on the year.  Our main concern is that our clients get the best data possible. This only happens when the conditions are right so please bare this in mind. If you want to book a survey in for a particular month to fit in with your on-farm management that is fine or if you just want to complete the  survey as soon as possible just let us know so we can keep in touch and start as soon as we’re able. For all enquiries be it for just more information on our services or a full quote please contact us at Agri Optics.

Wishing you all a fruitful and productive summer for what looks to be a very challenging time ahead. And hopefully you have the tools in place to help manage your water resource as efficiently as possible! Have a Merry Christmas and all the best for 2016 from the team at Agri Optics.

Chris Smith

Agri Optics 2015/16 Xmas Hours:

Closed Wednesday 23^rd December  & re-opening on Tuesday the 5^th of January. For any urgent matters over this time please call Jemma Mulvihill on 021796124. 

EM Surveying and Water Holding Capacity

In recent weeks we’ve been through the calendar of EMSurveying, A day in the life of an EM Surveyor and The value in ground-truthing your EM Survey in amongst other informative posts from Lindsay NZ and HydroServices. This week we’re going to shed some light on how an EM Survey by Agri Optics can be used to evaluate water-holding capacity of the surveyed area.

Predominately our clients ask us to conduct an EM survey for them to determine the differences in their soils with a view to varying their irrigation depths on the different soil types; however we can adapt the EM Surveys for many different uses, one of these being to evaluate water-holding capacity variation.

The DualEM sensor works by emitting an electro-magnetic field into the soil and measuring the returning conductivity of the soil. The conductivity of the soil in New Zealand is affected mostly by soil texture and the amount of water the different soil textures can hold (the more water the soil can hold the more conductive it is). The readings can also be influenced by salinity, however other than a few isolated areas in NZ we don’t have an issue with salinity affecting the readings. 

Figure 1: DualEM sensor being trailed behind our light weight Polaris

To fully relate the EM readings to water holding capacity (WHC), ground-truthing is needed to quantify the actual WHC at different locations as the EM Survey only measures relative difference of one area compared to another. As we have mentioned in previous blogs we create a map of EM zones and within that locate sample sites for each zone to be ground-truthed. The ground-truthing is then carried out by typically HydroServices using their neutron probe and our agreed protocols. They then provide us with the water-holding capacities for each different depth. From this we then create a map of WHC variation across the surveyed and provide a report back to the client of these additional maps with description on correlations of WHC and EM and recommendations on how these maps would be used to implement more efficient irrigation.  

In addition to getting accurate WHC maps and the associated report back the client can also use the ground-truthing sites to help site soil moisture probes and if the probes are installed before the ground-truthing is completed, the ground-truthing reading can also be used as one of the field calibration readings for the soil moisture probes.  

Figure 2: Water-holding capacity map created from an EM map

All of this information helps the client get the most of their EM data and make efficient use of their water by matching the water to the different zones and then monitoring moisture levels with their moisture probes. If the 2015/16 season is going to be as dry as forecast then making every drop count will be crucial. We not only conduct the EM surveying we can now also provide our clients with AquaCheck soil moisture probes to help manage your irrigation as efficiently as possible. Visit our website or give Jemma or Chris a call to discuss any of the above.