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Townsville Region Soils: introduction | clay soils |
SGAP meeting talk by Ross Coventry, 9/10/96, summarised by Doug Silke.

 Townsville Region main soil types:

1. Steep hilly
2. Foothills
3. Alluvial fans, especially Majors Creek, with a reasonable water store, water travels down under them along the rock to recharge U/G water.
4. Duplex soils, Add gypsum at 20 tonne per hectare
5. Solodised solodnet soils, (salty clays) pH>8.5 in B Horizon, physical properties poor, chemical properties poor. Sulphate of ammonia is the best and cheapest to raise pH.
6. Built up sediments, excellent soils, the best, and often very narrow.
7. Beach ridges.

 Organic matter here breaks down very quickly in our conditions, clay is important for retention of minerals.

Nature of soil colour (iron oxide), strongly indicates drainage situation:
1. Red, Haemotite... oxidised, well drained. Often found well up sides of the slopes.
2. Yellow, Garthite .. hydrated form og iron oxide. Found further down the slopes.
3. Grey,Iron leached out, poorly drained. Often found at the bottom of the slope. Ironstone on soil surface, or at top of B horizon, with grey soils indicate real problems

A much better representation of rainfall is "Rainfall exceeded 5 years in 10" rather than the average rainfall where flukey huge rainfall distorts the picture.

 

 Sodic soils, a feature of Townsville soils to top | to Townsville region info. main page | summarised by Doug Silke from referenced articles

Some 10% of Australian soils, 25% of Queensland soils and 60% of Townsville's soils are sodic.

Sodic soils are difficult to manage. They are the type of clay soils that are improved by the addition of gypsum. This is one of the cheapest and most effective ways to improve your soil, but always add gypsum at the rate and frequency recommended by suppliers, do not guess. When wet these soils are sticky and slippery.

In good quality soil individual particles of soil are stuck together in crumb size lumps creating larger spaces that fill with air; and smaller spaces that fill with, and store water. Roots of all plants positively asphyxiate and die without obtaining air. For almost all plants, air for roots must be able to circulate within air spaces in the soil. Only a small range of species tolerate and survive without this air... i.e. soils where pores fill with water for a long time when wet have "poor drainage".

In a sodic soil these individual particles disperse away from one another when wet, then condense in a closely packed way, eliminating the larger air spaces.

On drying the mess of individual particles on the surface of sodic soils form a dense "crust" that seedlings break through only with difficulty. The rest of the soil forms large hard solid clods (not accumulations of crumbs).

In sodic soils the infiltration of water is notoriously slow.

 

The 60% of Townsville's soils that are sodic, in the natural situation, actually only has sodic subsoils. The topsoil is loamy in the natural environment and varies in thickness from 10 cm to 70 cm thick. These soils are regarded as difficult to manage for landscaping. Since most land is flat, and water cannot drain through the sodic subsoil layer, in the wet season even the topsoil becomes filled with water. Surface ponding commonly develops. Where ever roots are immersed in water (for some time... 1 day for sensitive plants... or some days or weeks.. depending on the species characteristics) the loamy topsoil air pores fill with water, and the roots asphyxiate and the plant dies. The deeper the topsoil layer, the less severe are the problems since drainage is improved. Even slight slopes help drain water, unless replacement water also soaks down the slope. Improving drainage with pipes and drains is the key to this problem, but is expensive. These topsoils are poor in quality, wet topsoils are very boggy, and dry hard, breaking up to bulldust.

Loam needs to be imported for lawns, and the range of plants that can grow is quite limited unless drainage can be improved.

Especially in Townsville it is almost criminal that topsoils are often removed and not replaced, or buried during earthworks for housing developments, because the already difficult situation is made far worse for landscaping. In the first instance, clay drainage needs to be improved, probably by ripping and the essential gypsum application. But the long term key to improving the soil structure is organic material, organic material, and more organic material. Gypsum, but more permanently the broken down organic material, coagulates sodic clay subsoils into soil crumbs with the vital air gaps between. This is helpful for all soil problems. Even wet season topsoil saturation may be helped as water can drain out quicker (when there is somewhere for the water to go to).

All soils benefit enormously from organic content, perhaps this is the only way to make a permanent change, although the return of good soil health may take many years or decades. Never take organic material to the tip, mulch it instead. Grow as many plants as possible. It is good practice to assume that plant roots provide more organic material into the soil than is added by plant growth above ground. While mulching is vital for many reasons, growing living plants is an easier and much cheaper way to add organic material to soils, they effortlessly and continuously make organic materials from carbon dioxide in the air, only tiny amounts of fertiliser are depleted from the soil.

Sodic subsoils are alkaline, raising pH is difficult and may be prohibitively expensive except for small areas. Many plants do not like to grow in alkaline conditions. Especially many plants suffer iron deficiency and leaves become yellow. For these plants regular addition of suitable iron additives will be required. Where alkalinity is a problem as in sodic soils, it is best practice to only grow plants that are alkaline tolerant, perhaps except for a few favourites grouped together for special treatment. Grow plants from seed of species growing naturally in the same environment. If unsure when purchasing plants, drought tolerant plants generally have tolerance to alkalinity also (also species adaptable for coastal dune conditions, but drainage???).

summarised by Doug Silke from referenced articles

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References
Murtha G.G; 1982; Soils and Land Use on the Southern Section of the Townsville Coastal Plain, North Queensland; CSIRO
Murtha G.G, Reid R;1992; Soils of the Townsville Area in Relation to Urban Development, CSIRO.
McMillan, Rod;1993; A Student Handbook Soils and Growing Media