13/03/2021
I found this quite interesting
Soils of Whanganui
In an earlier article I touched on the different soils of Whanganui, and suggested that in all cases, soil management should be based on building up organic matter and protecting the soil. While this is true, it is useful to discuss each soil in turn, and discuss its particular features, strengths and weaknesses, and how it should be treated.
As can be seen on the simplified map, much of Whanganui is dominated by sand. This ranges from raw sand, often disturbed by earthworks to remove modified top layers that contain organic matter. Before the introduction of plants able to stabilise sand, it drifted freely across the city, as far as Pukenamu / Queens Park, and the western side of Rotokawau / Virginia Lake. Most of Castlecliff, Gonville, Springvale and Otamatea is built on sand.
Sand is challenging to grow on in some ways. It is actually simply ground-down rock, and has little or no ability to hold water or nutrients. This means that the addition of organic matter, to build up the soil structure, is critical to success. Keeping a permanent mulch on the surface helps to conserve scarce moisture. There are some advantages to sand – it is very free-draining and well-aerated, and can warm up quickly in spring. In fact, given the choice, I would rather grow gardens on sand than clay.
However, the positive factors encourage the breakdown of organic material, so the development of a productive soil out of sand is an ongoing exercise. So often, if you want a flower or vegetable garden on sand, it is best to develop raised gardens, fill them with topsoil brought in, and maintain the condition of the bed with periodic additions of compost or mulch.
Clay, which largely comprises the soils of Durie and Bastia Hills, is pretty much the opposite of sand. Clay is formed of tiny crystals, which have a strong ability to retain both moisture and nutrients. However, clay is usually poorly drained and aerated, which means that plant roots are unable to access these resources (roots do a lot of work, and require air from the soil to do this).
There are two ways to manage the drainage and aeration issues with gardens on clay. The first is by providing drainage; however, for several reasons this is largely impractical. The second way is ….. wait for it! …. by building raised gardens. The lifting of the garden above the water table allows the movement of water and air through it. The addition of organic matter helps to build the structure that a raw clay soil lacks, and improves aeration, draining, and nutrient retention.
The third main soil in Whanganui is Westmere silt loam, which dominates much of St Johns Hill and the river terraces of Aramoho and Whanganui East. Silt loams are an optimal mixture of sand, silt and clay, with a generous proportion of organic matter, which is what provides the dark colour.
Silt loam is pretty well the ideal soil to have. Having said that, gardening a silt loam still benefits from …. you guessed it! ….. raised gardens and the addition of organic material. I’m lucky to have silt loam in my garden, and practice both of these methods. In fact, I’ve just come inside from sweeping gum leaves out of the gutter by my berm, and spreading them onto my garden!
The main soils of Whanganui
- Phil Thomsen is a life-long gardener, who grows fruit, vegetables and ornamental gardens at his Whanganui home.
