Re: mulch

Trevor

the mention of Kevin Handreck's name prompts me to mention the two excellent 
books that he has written about soils, mulching, growing media 
etc...(Growing Media for Ornamental Plants and Turf; and Gardening Down 
Under). Whenever I have questions in the garden about these kinds of topics 
I always refer to what he has to say and it is normally really helpful. I 
was going to post an extract from his writing when we were last discussing 
compost a few weeks ago but tragic events cut that discussion short. 
However:

"Organic matter is the original slow release fertiliser. The nutrients in 
plant materials first become part of the bodies of microorganisms. Then, as 
these are eaten by others, some are released as 'wastes'. They are thus 
available for reuse by living plants...

In an ecosystem in which losses of nutrients are very low - in grasslands or 
forest, for example - plant growth can be sustained almost indefinitely 
through recycling. Here, the rate of release of nutrients can support 
adequate plant growth. (Some extra nutrients come from the decomposition of 
soil minerals.)

But if we want vigorous growth, we must increase the rate of supply of 
nutrients. The usual way of doing this is to use a fertilizer. If there is a 
high level of organic matter in the soil, however, the amount being 
decomposed each day may be able to supply enough nutrients. For this to 
happen, much of the organic matter in the soil needs to be residues of large 
amounts of young plant materials added over the previous years. The rate of 
decomposition of older organic matter - humus and woody materials - is too 
low to provide enough nutrients. At the other end of the scale, few 
nutrients are released from newly added plant materials until they have been 
decomposed for a few weeks or months. Composting shortens this lag time.

The rate of decomposition of organic materials varies enormously ... The 
time taken until only 10% is left might be months in a tropical rainforest, 
through three or four years in a cool moist pasture, to tens or hundreds of 
years in very cold, mostly waterlogged soils....(Ch 5 of Growing Media)

... In Chapter 5 we said that decomposing organic materials supply nutrients 
to plants. So they do, but in the early stages of decomposition, the supply 
of some nutrients can be greatly reduced. In order to use the organic part 
of the added materials, microorganisms must have a readily available supply 
of nutrient elements. They can get these from the added material or from 
reserves in the medium. All is well if the added material contain amply 
supplies of nutrients. Young plant materials such as lawn clippings, weeds, 
green manure crops and most animal manures are like this. All can be well, 
too, if the medium has large reserves of readily available nutrients - as 
from slow release fertilizers.

But if the added materials contain only small amounts of nutrients, the 
microorganisms must use nutrients dissolved in the water in the medium. They 
therefore compete, successfully, with any plants growing there. If the 
amount of materials added is large, plant growth is retarded. Nitrogen is 
the nutrient element in greatest demand, but supplies of sulphur and 
phosphorus may also be stretched (Ch 6 of Growing Media)"


Susan George
McCrae Australia






>From: tnottle@picknowl.com.au
>Reply-To: tnottle@picknowl.com.au
>To: Mediterranean Plants <medit-plants@ucdavis.edu>, 
>gastouri@ker.forthnet.gr
>Subject: Re: mulch
>Date: Thu, 29 Jul 1999 20:25:47 +0930
>
><< attach1 >>


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