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September 2008
Welcome to the September edition of Fertile Minds, the monthly newsletter of Sydney Environmental & Soil Laboratory.
Spring is here and rain has fallen over much of NSW. The rain and the recent fall in the value of the Australian dollar could bode well for primary produce exporters.
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In this issue
- The Loam Ranger – Kikuyu yellows
- Salinity and structures
- Is camphor laurel mulch toxic?
- Soils and tree roots: improving conditions for trees
- Did you know ... ? – Citrus originated in Australia
The Loam Ranger – Kikuyu yellows
Dear Loam Ranger,
Each spring my kikuyu turns yellow and looks very unsightly. Why, and what can I do about it?
There are three possible causes of yellowing in kikuyu: nutrient deficiency, disease and cold temperatures.
If you apply fertilizer and the grass greens up, then the cause is probably nutrient deficiency (although nitrogen can mask disease). To be absolutely sure, take some tissue and soil samples and send them off to your preferred laboratory for testing.
If fertilizer doesn’t solve the problem, you will need to consider kikuyu yellows disease, a fungal disease for which there is currently no treatment. The disease starts off as a yellow patch that expands into a circular area. The kikuyu does not regrow in the patch, and is replaced by other grasses and broadleaf weeds. Kikuyu yellows usually appears in spring and advances through the growing season. The long-term solution is to grow tolerant cultivars.
But if the yellowing appears every spring and then goes away again, you are seeing the effects of cold temperatures. During winter the kikuyu will not grow (unless your winters are mild). Come spring, the days start to warm, prompting the grass to begin growing again. However, at this time of year the soil is still cold. This inhibits the bacteria that are responsible for releasing nitrate from precursor forms. So while the kikuyu is raring to go in the warm air, the bacteria are still slumbering in the cold soil. As a result, the kikuyu shows nitrogen deficiency. As the soil warms up during early spring, the bacteria become active once again and the nitrogen deficiency is resolved. The fact that the yellowing disappears again indicates that no treatment is necessary.
Salinity and structures
In the last article we looked at the causes of corrosion and scaling and how to test for them. In this article we go into depth about measurement and the interpretation of results, and measures than can protect structures from salinity.
Click here to read about how to protect structures from corrosion (1150 words, 5 minutes)
Is camphor laurel mulch toxic?
Camphor laurel (Cinnamomum camphora) is a stately tree native to East Asia. It was introduced into Australia possibly as early as 1822 as an ornamental, and quickly gained popularity as a shade tree. Unfortunately for the Australian environment, camphor laurel is a highly invasive species that is crowding out native plants and the animals that depend on them. This is helped by its fruits, which birds favour, distributing the seeds long distances.
Camphor laurel is a declared noxious weed on the far North Coast of NSW, and in other parts of the state no permission is necessary to remove it.
Because of the tree’s fast growth and extensive coverage, camphor laurel removal is a continual process in many parts of Australia. This generates much plant material that can be used as mulch (in addition to excellent timber for wood carving). But given that camphor laurel suppresses most plants beneath it, this raises the question of whether it is toxic to plants when shredded and used as mulch.
Click here for information on why camphor laurel mulch is probably not toxic (500 words, 2 minutes)
Soils and tree roots: improving conditions for trees
In the last article we looked at the five key requirements for tree health provided by soil: oxygen, water, nutrients, space to explore and anchorage. In this article we look at how we can improve life for trees by improving soil conditions.
Click here to read how to improve soil conditions for trees (700 words, 3 minutes)
Did you know ... ? – Citrus originated in Australia
It has long been assumed that the Citrus genus originated in Asia. But as recent work shows, Citrus actually originated in Australasia. An Australian taxonomist called David Mabberley, currently based at the Royal Botanic Gardens Kew, in England, has shown that Citrus comprises about 25 species, and that the earlier division of the genus into Citrus, Eremocitrus (Australian desert lime), Fortunella (kumquat), Microcitrus (Australian finger limes) and Poncirus (trifoliata rootstock) was incorrect. Molecular genetics – the study of the structure of genes – shows that these former divisions are not supported; that is, all 25 species are descended from a single common ancestor.
Of those 25 species, more than half originated in Australasia – Australia, New Guinea and New Caledonia. The rest originated in India, South-East Asia or China. This distribution shows that the genus Citrus originated in Australasia (possibly Australia itself) and spread from there.