Microsoft word - panui tips -describing the soil 2 2005.doc

If you dig a hole about a metre deep in the soil, you will see the ‘soil profile’ has a number of layers. Called ‘horizons’, these layers are quite different to one another. Taking time to study the different horizons can provide you with important knowledge about your soil. An important aspect of colour is mottling. As a soil weathers iron is released, and this forms rust At the surface there is usually a layer of fresh coloured iron oxides. Rusty mottles on a dark organic material – dead leaves, and surface roots, background indicate uneven weathering- that’s ok. and organisms feeding on these. This is called the Rusty mottles on a pale background indicate long litter layer (scientists label it the L horizon). term cycles of waterlogging and drying so beware The top layer of soil (Topsoil) is called the A horizon. This layer is richest in organic matter and Another important colour feature is gleying. Gleyed nutrients. It is usually very different to the zones of soil have a grey-blue sheen on top of original material from which it developed. You will normal background colours. This means that the soil find the soil is clumped into aggregates or peds. has recently been waterlogged- unless rainfall has Because it is rich in organic matter it is usually been unusually heavy gleying tells you to be careful The un-modified parent material (river silt or volcanic material maybe) is called the C horizon. Stones can have a big impact on what you use the In between the topsoil and the parent material is a soil for. Estimate the stone content visually- you layer of partly modified soil called the B horizon. want to know what percentage of the soil layer is Being low in organic matter it is normally lighter made up by stones. This figure can be very helpful, than the A horizon. It may have some soil especially if you are comparing your site with In many parts of New Zealand you need to be careful when inspecting soil profiles. Sometimes you will find ‘buried soils’ because flooding, volcanic You should be looking for signs of a healthy living activity or wind blown ‘loess’ has covered a soil that soil. The sorts of questions to ask are: was previously there. Also, we often have soil Is it easy to find roots and do they look developed from volcanic material that settled on eroded rock. The rock itself is not the parent material at all! Can you see earthworms (is that normal at this time of year)? Horizons differ in depth, colour and smell, stone content, biological activity, structure, strength and consistency and texture. These features can tell you a lot about the health, history and even the future of your soil. Soil texture is the amount of sand, silt and clay particles in the soil. Texture is different to structure, but does affect your soil in a lot of Deep brown colours come from organic matter and ways. For example, the amount of clay in the soil rusty colours come from iron. Very old or very is important as clay helps the soil hold onto nutrients such as potassium and magnesium. crops and how easy it will be to use machinery in the paddock after rain. Soil structure is the arrangement of the solids and spaces in the soil. It’s easiest to describe this by Stony soil structure resists damage during The “primary particles” in soil are organic matter, cultivation and if the soil is driven on. This is usually sand, silt and clay. These are usually joined into good. If the soil has poor structure and is bigger units called aggregates or “peds”. The size compacted, problems can occur. It is important to and shape of the aggregates is important- there look for strongly compacted layers or pans in your will be differences between the topsoil and soil. They affect root growth and water movement subsoils. For example, you wouldn’t want prismatic, and can make your crop more susceptible to drought (due to shallow root growth), waterlogging or diseases. You can check how easy it is to penetrate the soil by pushing a pencil or welding rod into the soil from the side of your hole- if you can’t push a welding rod into the soil with your hand then roots won’t be able to grow through it either! In badly compacted layers the water may not drain well either - this can be recognised by a hard dry layer between two wet layers. Even one badly structured layer can affect your crop. Below is a chart which may help in describing the Soil structure affects how well your crops will grow, how easy it will be to prepare the ground for Take a small sample of the soil (remove particles >2 mm if you can), then
Moisten it while rubbing between thumb and fingers (spit works fine!)
Stop adding water and rubbing it when there is a thin surface film of moisture that reflects light
How does it feel and sound when you rub the soil now? Work out the texture, as follows:
Feel and sound
Cohesion and plasticity
Texture class
Can’t be moulded into a ball
Gritty and rasping
Can almost be moulded into a ball - but
Loamy sand
falls apart if you flatten it
Slight grittiness/rasping sound
Moulds into a ball that cracks if you
Sandy loam
press it flat
Smooth soapy feel
Moulds into a ball that cracks if you
Silt loam
press it flat
Very smooth,
Plastic, moulds into a ball that you can
Clay loam
slightly sticky to sticky
squash without it cracking
Very smooth,
Very plastic, moulds into a ball that you
sticky to very sticky
can squash without it cracking
Clay textures are rare for topsoils in New Zealand, but textures in the mid range are common. You will often find silty clay loams (between silt loam and clay loam) and fine sandy loams (usually these are silt loams but with the recognisable feel and sound of fine sand). ‘Te Pànui Tips’ are simple fact sheets that cover topics designing organic crop production systems on the East Coast. Te Pànui is edited and produced by Page Bloomer Associates for Crop & Food Research under FRST Funded Project C02X0305 Science for This information sheet is intended to provide accurate and adequate information relating to the subject matters contained in it. It has been prepared and made available to all persons and entities strictly on the basis that Page Bloomer Associates Limited, its researchers and authors are fully excluded from any liability for damages arising out of any reliance in part or in full upon any of the information for any purpose. No endorsement of named products is intended nor is any criticism of other alternative, but unnamed product.


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