We Hope You Have A Green Christmas!

From all the H2Grow team we'd like to wish you a Merry Christmas and Happy New Year. We will be leaving the airways clear for a couple of weeks to allow all the Christmas messages and holiday photos to get through. And will be back in touch from mid-January.

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

Is Your Irrigator in Top Nick?

The spring weather has brought more rain to many parts of New Zealand compared to previous seasons which has meant many irrigators have barely got off the starting blocks. This extended downtime is a perfect opportunity to give your irrigator a bit of TLC which will help ensure it is running at full capacity when the heat comes on.

Many simple checks and servicing like lubricating joints, replacing oil in gearboxes and looking for signs of fatigue can often be carried out by farm staff. However, should you be unsure or think that you might have detected an issue don’t hesitate to contact your irrigation dealer.

Grafton Irrigation (Zimmatic dealer based in South Canterbury) have put together a handy checklist to guide you through the checks and maintenance that will help prevent mid-season issues. The checklist covers your intake, pump shed and mainline, and pivot, hard hose, soft hose, K-line and G-set (solid set) type irrigation systems. Print yourself a copy using the link below.

Pre-Season and Maintenance Check Sheets - By Grafton Irrigation

This post has been written by Sarah Elliot - thanks to Grafton Irrigation for your input!