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Spotlight on sedimentary soils

Spotlight on sedimentary soils

Spotlight on sedimentary soils

In everyday agriculture we use four umbrella terms for the soils we farm in New Zealand. These are ash, pumice, peat and sedimentary.

Within these umbrella terms are many different soil orders with vastly different soil properties, and within those orders are even more soil groups. To keep things manageable, the four umbrella terms are used and this terminology is found on soil tests.

Soils formed from sedimentary rock cover the vast majority of New Zealand and have a wide range of land uses. Each soil order within this larger umbrella can have vastly different soil properties.

However, all sedimentary soils are formed from quartz-rich sedimentary rocks as the parent material, such as greywacke, schist or granite. Sedimentary rocks are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans. They are then uplifted above the ocean into mountain ranges over millions of years and now form the basis of a lot of our hill country farms.

The vastness of their range and uses across the country means there is a good chance sedimentary soils are present on your farm.

Given the various soil orders within the sedimentary soil’s umbrella, the fertiliser management tools discussed here are generic.

Sedimentary soils naturally provide differing amounts of potassium from soil clay minerals for plant growth, and tend to have a lower range of soil test minimum/maximum than other soils. These soils also tend to have medium to low anion storage capacity (ASC). This means they require lower rates of maintenance or capital phosphorus fertilisers compared to other soil types such as ash.

Given the low ASC, sedimentary soils are responsive to sulphur fertilisers in the late winter or early spring, especially after a wet winter. Sedimentary soils can be naturally low in molybdenum, a trace element, even when pH is at optimum levels. This means periodic applications of this nutrient are needed to ensure optimal legume growth.

As farming underpins our economy, understanding the unique abilities of the soils being farmed contributes to a successful farming operation.

To arrange soil testing, talk to your local PGG Wrightson Technical Field Representative.