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. For 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. Therefore how frequently you need to irrigate (return period) and the maximum irrigation you can apply in one application (irrigation depth).

Q – Why did the A horizon and the B horizon not have children? 
A – Because they weren’t parent material.

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.

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 but 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 check out:
http://irrigationnz.co.nz/wp-content/uploads/2014/03/Final-Soil-Texture-and-Water.pdf

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 will also be discussed further on.

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

Visual Soil Assessment – A practical guide to assessing your soil quality

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

The VSA can be found online at: www.landcareresearch.co.nz/publications/books/visual-soil-assessment-field-guide/download-field-guide

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 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 as a result 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 are able to compare what you are viewing with examples. You will need basic equipment such as 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 subsoiler 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 on 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 of soils can be used to assist your farming.

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

"The Whole Shebang" - EM Mapping, Soil Moisture Measurement and VRI

The following is a very interesting case study about using all three elements the H2Grow companies bring to the table, informing the grower of soil variability and then adjusting inputs to match soil requirements.

The paddock studied is located at Greenvale Pastures near Methven. It was EM soil surveyed by Agri Optics NZ Ltd to provide data on field soil variability. This area was then divided into zones based on the deep soil profile results from the EM soil survey. HydroServices used the GPS points provided from the EM soil survey to ground truth the 6 different zones and then set up monitoring sites with a neutron probe in each as shown in the map below.

Figure 1: An EM soil survey map with neutron probe sites marked, for a Water Holding Capacity (WHC) trial conducted by Agri Optics and HydroServices.

As a result of HydroServices ground truthing, the full point and stress point was ascertained for the six different soil characteristics at each of the different EM zones. Table “A” shows the different water holding characteristics at different locations, along with the EM reading at that point and total area within the paddock in that zone.  As you can see from the table there was a large variation in water holding capacity between the different sites.

Table A	Full Point (mm)	Stress Point (mm)	DualEM  0-60cm	Zone Area (ha)
Site 1	187	139.5	4.77	0.6
Site 2	233	178	6.05	3.0
Site 3	228	178	5.77	5.4
Site 4	232	178	6.20	7.2
Site 5	181	134	5.44	3.6
Site 6	106	70	5.01	0.6

In the season of the case study the pivot watering this block did not have variable rate irrigation (VRI) installed. With HydroServices providing weekly monitoring throughout the season the irrigation applications were well managed using the conventional irrigation system. Through this the grower was able to keep water up to the lighter zones (zone 1 and 5) to stop any potential yield loss on those areas due to water stress.

At harvest time the yields where mapped and yield data processed by Agri Optics, enabling it to be viewed on our VA Gateway software platform. The map below shows yield variations in the crop of autumn-sown Phoenix wheat with the six locations marked. After harvest the yield in the different sites was then compared to see how they matched up relative to each other and to what the grower expected. 

Figure 2: A Yield map of Phoenix Wheat harvested after the first season of the trial, with the neutron probe sites marked on it.

From the results in table “B”, it can be seen that the heaviest parts of the paddock (where you would have expected the highest yield) underperformed when compared to the lighter soils which received the correct amount of water. 

Table B	Full Point (mm)	Stress Point (mm)	Wheat Yield (t/ha)	EM 0.60m (ms/m)	EM 1.5m (ms/m)	Zone Area (ha)
Site 1	187	139.5	17.56	4.84	10.95	0.6
Site 2	233	178	12.31	9.28	16.30	3.0
Site 3	228	178	13.26	8.18	15.02	5.4
Site 4	232	178	15.58	7.85	14.71	7.2
Site 5	181	134	16.48	5.18	12.52	3.6
Site 6	106	70	10.00	3.80	10.32	0.6

By over watering the heavier soils not only was the excess water potentially wasted with an increased risk for leaching nutrients but there was also a yield penalty in the order of 3-4 t/ha. At $400/t for wheat this would equate to a loss of $1200-$1600/ha.

The following winter Lindsay NZ (formally Precision Irrigation) retro-fitted Growsmart Precision VRI on the pivot, enabling the different soil zones to be irrigated with water application matched to soil characteristics and crop needs. Any variability in crop yield was then down to agronomic factors other than water. Due to watering in the different EM zones, yield variations and crop ripening evened out. This allowed management to concentrate on other factors which had influenced on the final yields.

Figure 3: The yield map from the season following Growsmart VRI being installed on the pivot, showing the variation due to differences in EM soil characteristics had been minimised.

For any more information about this blog or more information on EM surveying or to have your EM survey ground truthed by HydroServices please get in touch (to email click on the personal profile pictures on the right).

Chris Smith (chris@agrioptics.co.nz) - Agri Optics NZ Ltd

New Release: Irrigation Management Solution - FieldNET® Pivot Control

The H2Grow team like to ensure you are updated as new technologies reach the market and today's blog post is an example of this. Lindsay Corporation, maker of Zimmatic irrigation systems, recently announced FieldNET® Pivot Control, the industry’s first solution for upgrading almost any brand pivot to full remote irrigation control and monitoring capabilities.

Many growers today manage multiple brands of irrigation systems. This requires operators to understand various types of pivot control panels, which takes time to learn and could lead to operator error. Pivot Control can change the way growers manage their irrigation by providing a single consistent irrigation experience. “It is simple, it is very intuitive.  And the ability to basically tap into any system and have this product as kind of plug-and-play on there, that’s great. You can take a 30-year-old pivot and bring it right up to date with this unit,” said Jared Gardner, a grower who tested the product on his Zimmatic pivot and two other brands.

Unlike other remote irrigation control products, Pivot Control mounts at the pivot point and combines with GPS at the end of the pivot for increased precision and greater flexibility.  Growers can operate their pivot in the field at the control box or remotely using FieldNET - both elements automatically update together - minimising the chance for user error.  From a smartphone, tablet, or computer, growers can control their pivots and equipment, including pumps, injectors, and monitor sensors for pressure, flow, soil moisture, rainfall, temperature, and other devices in the future.  The system will send alerts to the growers’ phone to notify them of any irrigation issues. “Making all my pivots universally controlled through one device is great and having pump control is crucial,” said Gardner.

Pivot Control comes standard with 360-sector variable rate irrigation (VRI), offering twice the number of sectors available on most basic (sector) VRI products. Growers working with agronomists and crop specialists to analyse their unique field conditions can upload custom prescriptions to FieldNET.  “Once the prescription is created it takes less than a minute to upload,” said Agronomist Lamar LaPorte. “The biggest advantage is being able to scale the concept of VRI over a whole farm or multiple farms efficiently.  By eliminating the time consuming process of having to change each sector, it is a huge benefit.”  

Gardner says he has researched VRI and sees it as a next step. “Our inputs just keep getting more expensive so if we can utilise VRI to grow, that’s the next step,” says Gardner.

Pivot Control takes over the existing pivot, giving the grower full access to precision irrigation control and convenient monitoring. “There’s really nothing else like it on the market.  Pivot Control provides growers an affordable and easy way to own features that can more effectively and conveniently manage irrigation resources,” said Stu Bradbury, Lindsay NZ general manager.

For more information, visit www.myfieldnet.com/pivot-control or talk to your local Zimmatic dealer.

Posted by Sarah Elliot from Lindsay NZ.