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Soil fertility "Myths" - do plantations
cause soil damage?
This is a ongoing debate and will be updated regularly. Any contributions can be sent to sawac@soft.co.za
Evidence
rejects myths I say "myth", because all the evidence shows they don't. In 1914, the famous New Zealand botanist Leonard Cockayne wrote: "It is evident that the growing of pinus radiata, instead of weakening soil fertility has the very opposite effect." Forest Research Institute scientist Nick Ledgard said much the same thing recently when addressing the Mid Dome Wilding Tree Committee at Lumsden. Fear was expressed that the removal of the offending plantations (of lodgepole and mountain pine) would resume the erosion they had been planted to stop. He thought this unlikely. In his experience of similar situations elsewhere, the build-up in fertility under the trees would lead to an explosive invasion of browntop and other plants immediately the trees are gone. The myth seems to have originated in Europe, where it was common practice for farmers to collect forest litter for livestock bedding, thus over the years transferring much of the plantation's nutrients to their farm. Trees grown for wood differ from agricultural crops in one significant way – the crop is largely carbon extracted from the air by photosynthesis and it contains very little in the way of nutrients derived from the soil. These are held in the leaves and other parts of the tree which are left on-site after logging. By contrast, any form of agriculture puts a heavy strain on soil nutrient reserves because these are held in the parts of the crop which we harvest – meat, potatoes, grain, leaves, fruit and so on. The difference is made greater by the difference in root depth. Most agricultural crops use only the top 10cm of the soil but tree feeding roots go down 40-50cm. Thus over the years of the tree's growth, nutrients are brought up from deep down and then returned to the surface by leaf fall. This is what gives crops their kick-start when planted after the trees are gone, and also why farmers use fertilisers while foresters usually do not. From: The Southland Times
The article in the Southland Times concerns soil fertility in timber plantations. I find the article misleading, specially if applied to the industrial plantation model.
I
am referring to large scale mono culture timber plantations of alien tree
species.
Such
plantations (managed) covers an expanding 1.5 million ha of South Africa.
In
addition to this considerable threat to biodiversity, a rapidly expanding 1.6
million ha of unmanaged, alien invasive timber "jungles" exist.
I
have no doubt that successive rotations of ANY crop will deplete soil
nutrients and lead to the introduction of chemical fertilizers.
Timber
crops would take longer to deplete soil nutrients, as their roots penetrate
much deeper than most food crops. (The author of the above article mentions a
tree feeding root depth of 40-50cm, I suggest the man go dig up a mature
eucalyptus or pine tree!)
The
article implies that it is the leaves and other debris not harvested which
contains most of the soil nutrients, but it has to be pointed out that it is
this debris which is usually burned off after harvesting occurs.
It
should also be considered that healthy soil does not rely only upon the
availability of dead organic materials, but also the presence of decomposers.
A
superficial comparison between indigenous grassland and timber plantations
reveal an alarming lack of biodiversity in last mentioned. Unbiased scientific
analyses will confirm this phenomena.
Biodiversity
is essential to the health of living soil.
Ref.
"Mutualistic biodiversity networks: the relationship between soil
biodiversity, and their importance to ecosystem function and structural
organization" available at: http://www.angelfire.com/sk/monkeypuzzle/mbionet.html#Ecosystem_models
Lack
of biodiversity in mono culture timber plantations occur due to the
following:.
If
it is true that timber plantations will lead to a long term depletion of
soil nutrients it should be obvious that this is unacceptable, and changes
should be implemented as soon as possible.
There
is a lot of evidence to show that chemical fertilizers have a negative effect
on the environment and these should be avoided at all costs.
Possible
solutions could be for the industry to plant the trees further apart / thin
out existing plantations, to allow more sunlight, and more biodiversity. This
could have positive long term effects on the health of the soil. Some pine
tree species (Pinups elliotti?) allows more light. Such properties should be
encouraged and implemented throughout.
Industry
should also consider devoting a section of their operations to organic compost
building to supply future fertilizer demand.. This would entail collecting
organic material from under the plantation trees and exposing it to conditions
conducive to organic breakdown (sunlight / water). Compost building should be
practiced at various sites throughout the plantations.
I
do not believe that the possibility of plantations leading to the long term
depletion of soil nutrients can be called a myth.
Philip
Owen
SAWaC
It
is interesting to note that in south Africa, rural communities in the Tzaneen
area have long realized that there is a positive soil nutrient build up in
Eucalyptus plantations. The communities have developed a system where they
move in to felled plantation blocks straight after clear felling and grow a
cash crop for the next year or two. They will attest to these areas having
better fertility than areas where trees have not being grown. Plantation
managers have realized that this is a win-win situation as the community
members help in weed control and protection of new seedlings in blocks that
are re-planted.
This
is certainly circumstantial evidence that a tree crop may be beneficial to the
soil, even in a mono-cropped situation such as in South Africa. This is not
totally surprising, and is the basis for the ancient practice of slash and
burn agriculture. Much of the agroforestry practices being investigated by
ICRAF are based on the fact that trees can potentially be beneficial to soils.
A practice termed relay cropping uses a fast growing tree as a lay crop for
two to three years in a long term rotational cropping system.
One
thing would seem clear, tree crops are likely to be less detrimental than most
annual crops where soil structure is changed by compaction, nutrients are
extracted through harvesting of grain, soil fertilization and chemical
pest control is greater, and soil erosion is often very high.
I fully agree with Philip Owen that the report from the Southland Times cannot be the full truth. Unless, of course, we can be supplied with the scientific data to substantiate the claims. Trees, like any other crop grown in successional rotations, will use soil nutrients, provided that a substantial proportion of the plant material is periodically removed from the site. Anyone who has ever moved a lawn should know this fact. In the case if timber trees, the loss of soil nutrients is supposedly more substantial because a very large proportion of these elements are locked up in the wood and bark. Anyone who has ever lit a fire will know that more ash is produced by wood than by leaves. It is true that trees can bring nutrients from deeper soil layers to the surface in their transpiration stream. It is also possible for such nutrients to be washed by the rain water from the foliage, thus fertilising the soil below the tree canopy. This is the reason why certain grasses, for example Panicum maximum, prefer to grow under tree canopies in the bushveld. This also explains why trees are essential for retaining the productivity of busveld. The bulk of a tree's roots is nevertheless fairly superficial and absorb nutrients mainly from the topsoil where usually the bulk of soil moisture and microbial activity is to be found. I suspect the phenomenon described by Graham von Maltitz is due to the waste left behind after a plantation is felled---unless, of course, available data may point to another explanation. It is most likely a temporary phenomenon comparable to the age-old slash and burn technique; if that is indeed the case, then it is not sustainable in the case of quick rotational crops in the long run. I would also not doubt that the use of selected trees in agroforestry may have a positive effect on soil nutrients. One would logically assume that the crucial factor resulting in soil impoverishment by commercial plantations is the periodic export of the nutrients locked up in the wood. If this is so, then there is no way how this action can result in soils retaining their fertility in the long run, unless fertiliser is eventually applied. Perhaps somebody can provide us with references to papers in which the impact of plantations on soil nutrients is being dealt with in a scientific way. Braam
van Wyk
Phone: +27 12 420 2545/4044/2006 |
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