Ground Truthing Your Soil Variability

To build on the previous blogs discussing and identifying soil variability, it is essential you determine what causes the variability observed or mapped; for example those drier areas in the field or differences identified on historic Google Earth maps or variations identified on an EM survey.  The variability illustrated by these “surface” observations can be caused by one or more soil characteristics buried beneath the surface.  You need to find out if it is variability resulting from soil moisture, soil chemistry (e.g. pH), residual chemicals (e.g. previous herbicide use), compaction (e.g. an old gate way or heavy traffic when the area was wet), depth to gravels (or sand) and other characteristics. 

Nothing beats a hole in the ground or measurements that will depth profile your soil.  Start with a spade – it’s the least expensive option.  Alternatively, take the tractor or little digger out and dig a hole in the areas of interest when the paddock is about to be cultivated, get “down and dirty” and observe the differences.  The two examples (while a bit extreme in terms of excavated depth) are on the same property about 350m apart and no matter how you identified the variability at the surface, the holes tell you why.  Make some notes about the soils – you don’t have to be a soil scientist to gather useful information.  Measure the depth, feel the soil, dampen it and run through your fingers (what does it feel like?), describe the colour, note the depth of roots – nothing complicated or mind bending.

Notes: 200-250mm sandy silt loam (smooth to feel but a little gritty as well), then gravels and sandy gravels, dark brown topsoil, roots 250mm deep, hardly any roots in the gravels.

Notes: 150-200mm red brown silt loam (very silky to feel when dampened), grey sandy silt with orange mottles and streaks, not too many roots in top soil and none in the sandy silt.

You are now well on the way to sorting out the cause of what you’ve seen at the surface.  The soils are different (clearly).  Time to apply some science to the differences.  The gravelly soil will have low water holding capacity – inherent because stones have less surface area to hold water than the same volume of sand or silt.  The sandy silt soil has lots of orange mottling, a sign that drainage is poorer here than the gravelly soil.  

How can you measure the differences in soil moisture characteristics?  I’ll delve into this in a later blog.

Posted by Dr Anthony Davoren from HydroServices Ltd

Characterising Your Soils

Identifying the variability of your soils on not only a farm but also paddock scale is important if you wish to manage your inputs to meet the soils requirements in a sustainable and efficient manner.

This time of year, if not slightly earlier before it gets too dry, it is very easy to see on dryland properties the different water holding characteristics of soils from the state of the crops growing in them. Marking these is crude but still useful way of noting differences in soils, even better if a simple hand held GPS device is used.

Above: very apparent differences in soil characteristics. A white clover paddock in early January, in the fore ground the crop has burnt off but in the background still in mid flower.

The use of yield maps is a great way of identifying a crops reaction to different soil characters in a specific climatic year, but the crop type and weather patterns that season should also be recorded as different crops react in different ways to different climatic conditions. The more years of harvest data you have the better the bigger picture of variability you will get.

Another very useful tool to identify variation if you can’t hop up and have a fly over your place is to utilise Google Earth and its ‘Historical Imagery’ toolbar to go back through previous aerial images of your farm and visually identify different areas of interest. While quite a few recent satellite images on Google Earth may show you irrigated and non-irrigated areas, particularly in Canterbury, looking back through historical ones will give you more of a picture of natural soil variation and how it comes through into what’s grown on the surface. In most cases significant variation can be seen. The ‘Historical Imagery’ icon is a little clock like icon and can be found at the top of the map window. 

Here is a comparison between what Google Earth shows is the variation in a paddock at different times and the EM Map. You can clearly see different soil & crop patterns running through the paddock that line up well both on Google Earth and in the subsequent EM Survey completed 2 years later.

To accurately capture the in-field and farm scale variability in soils an Electro-magnetic (EM) Survey of your property can be carried out. This measures the soils conductivity, which is influenced by the composition of clay, silts and sands in the soil at a given point; were clay gives a higher EM reading than silt which in turn has a higher EM than sand. These readings are logged across a paddock with sub-metre GPS accuracy which also gives you a 3D surface map of the area. This not only helps define the soil characteristics beneath but is also a very useful management tool in its own right, showing where water will move in a rain or irrigation even. After identifying different EM zones you can use them as they are or ground truth the zones to put actual water-holding capacity figures to the variability; either way you can then start to manage your different soil characteristics.

Characterising your soil precisely is really important to enable greater efficiency in irrigation management, pasture management, fertiliser management, effluent management and a whole bunch of things that are done on farm in relation to your soils. By a more precise understanding and management of your soils, this can further enhance the efficiency of your farm. Additionally, characterising your soils is important for understanding and managing your soil moisture which Tony Davoren is going to talk about in the next blog.

We’ll go into more detail on the characteristics of different soils and what this relates to in terms of EM Surveying and vice versa in future posts. 

Soil Moisture Basics Defined

For this first post I will be introducing some basic soil and soil moisture principles. Many of the terms defined here will be used through the posts on this blog so here I hope to provide some background for those of you who are not yet familiar with them.

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 particular soil can store is 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.

The following terms define certain soil moisture characteristics:

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

It is worth mentioning that 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. Later posts will discuss how to use this information to help determine the soil water characteristics of your soils.

Posted by Sarah Elliot from Lindsay NZ

Welcome to the H2Grow Blog!

We would like to welcome you to the H2Grow blog where we look forward to sharing with you some words of wisdom and examples of efficient and effective irrigation practices. It is hoped that growers will be able to gain direct economic benefits while also improving the outcomes for the environment and sustainability of farming industries as a whole.

The H2Grow blog contributors boast a range of experience in irrigation management, soil and soil water assessment, precision agriculture, irrigation technologies and farming operations. Many of the posts will include advice from other industry experts and input from growers demonstrating best practice and achieving great results. The H2Grow team includes:

Dr Anthony Davoren (HydroServices) - MSc in Earth Sciences (University of Waikato), Ph.D. in Engineering Science (Washington State University). Tony established HydroServices Ltd in 1983 and specialises in irrigation management, irrigation requirements, soil water assessment, and surface and ground water resources. 

Jemma Mulvihill (Agri Optics New Zealand Ltd) - B AgSci with honours (Lincoln University). Whilst an undergrad Jemma also completed a year abroad in the US at Colorado State University where she came across precision agriculture. Jemma and her parents Craige and Roz Mackenzie launched Agri Optics New Zealand Ltd in 2010, filling the gap in the ever-changing technology driven ag market in NZ.

Sarah Elliot (Lindsay NZ) - BTech with honours (Massey University), Kellogg Rural Leadership Scholar. Sarah combines her engineering background with her passion for agriculture in her current role at Lindsay NZ. Lindsay NZ (formally Precision Irrigation) developed the award winning Precision VRI variable rate irrigation technology. In her spare time Sarah helps her husband Hamish on their farm in South Canterbury. 

The H2Grow team are happy to offer their time to share their expertise in regards to efficient irrigation should you have a group that would be interested. You are more than welcome to contact any of the contributors directly using the email links (click on the photos to the right) to discuss the options further. 

Enjoy!

Posted by Sarah Elliot from Lindsay NZ on behalf of the H2Grow team