Term 2 – Week 5 – Soil Science


View of some terraces in the distance at Bannockburn, Kawarau River

This week we got stuck into soil science with Roger. There were several lectures and trips out and about to sample some different soil types. Surprisingly, this has been one of my favourite topics so far. I think that’s due to Roger’s wealth of knowledge about soils and horticulture.

So far we have covered the components that make up soil, soil development and horizons, structure, texture, nutrients and soil classification systems.


Roger + spade = soil science

Soil is made up of organic matter, minerals, air and water. Organic matter provides nutrients for soil organisms and plants, gives the soil structure and increases the water holding capacity.


The size of soil particles creates the soils texture. Sand is considered the largest particle, sandy soils are free draining and don’t retain water. Irrigation can be an issue. At the other end of scale is clay. Clay particles are tiny. There are micro pores between particles that hold onto water and nutrients making them unavailable to plants. Probably the easiest soil to manage is a mixture of all 3 types, sand, silt and clay. That’s called a loam soil.


Texture triangle

This chart works out what sort of texture a soil has. If your soil has 60% sand and 10% clay it is a sandy loam. As you can see it doesn’t take much clay in the soil for it to be considered a clay. A 50% clay and 50% silt soil is still called “clay”.


Different soil textures

Above are some photos from our outings to find different types of soil. At the top we have silt and clay soils.

Bottom left is an example of a gley soil. Organic matter sitting over the top of clay (O horizon over a C horizon). It has no drainage and is very water logged. This means the soil will be anaerobic, with none of the normal soil organisms to break down the organic material. Apparently a lot of the farmland in Canterbury was like this before the land was cultivated and drainage was sorted out.

Bottom right is an example of a semi-arid soil that has had irrigation. Semi-arid is probably the most common soil type in Central Otago. This soil develops where the climate is very dry. It’s low in organic matter (so lacking in nitrogen) and can have high salt accumulation since there has been little rain to wash it out.


The nutrients a plant needs are broken down into macro and micro. Macro are nitrogen, phosphorus, potassium, sulphur, calcium, magnesium, carbon, hydrogen and oxygen. Micro nutrients are boron, chlorine, copper, iron, manganese, zinc and molybdenum. Basically, without all these in the right quantities the plant will have poor growth and poor fruit. In some cases too much can be just as much of an issue as too little.


In this week’s propagation class we did lily scaling – the process of dividing lily bulbs to end up with more plants. They are dormant for winter now so can handle being dug up and broken to bits. You just snap the scales off at the base and pop them in a plastic bag with damp vermiculite. They get stored on the heat bed. You could do this at home using the airing cupboard. Here’s what the flowers look like:


Lily regale

We also dug up some of the peonies and divided them as well. They are quite freaky looking! It’s hard to know where to start.


Peony tubers ready to be divided

The flowers are gorgeous in the summer though, so it’s worth it. Here’s a photo of the peony garden I took at the end of last year:


Last week’s weather – a lot of minus temps in the minimum columns!



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