Steve Diver wrote
>1. Why did the researchers suggest that natural sources of
nitrogen such as legumes (clovers, vetches, cover crops)
cause as much acidification as chemical nitrogen sources
(ammonium nitrate, anhydrous ammonia, etc.)???
This generalisation about plant effects does seem to need qualification.
Acacia spp actually raise pH while also fixing nitrogen. Hence, the value of
allowing/encouraging natural [here in Australia] regeneration of wattle
[Acacia] trees in an orchard when it is young. The wattles, a major pioneer
species here, grow quickly, most of them would if left alone fall over at
age 15-20, but if removed at 3 years or so will noto nly have done a major
job opening rock with invasive root systems, but also will (easily observed)
produce a nitrogen effect in the root area, for other trees/plants/pasture
for a couple of years at least. The rotting root systems also provide soil
benefit, apart from the nitrogen nodulation. These trees do not sucker,
unlike the nitrogen fixing pioneer Alder.
Sadly, these facts on the value of wattles are known better to permaculture
people than to conventional farmers here, who clear out wattles (woody
weeds) and plant pasture adding superphosphate, believing that the rapid
performance of the newly cleared land is due to the phosphorus when it is
probably almost entirely due to the clearance of the nitrogen fixers and the
sulphuric acid in the super bag.
I would think a major distinction should be made between use of legumes as
rotation, where they die or are dug back in, and constant maintenance of a
leguminous pasture. I have always had the (intuitive) view that no nitrogen
fixing plant with any brains, any more than any other organism, is concerned
about looking after others rather than itself. Maximum nitrogen effects are
likely to arise from allowing a succession away from the nitrogen fixer.
Keeping a nitrogen fixer in one place for a long time is likely to have
deleterious effects as with any other monoculture.
Which is not to say that I disagree with the following, which emphasises
that it is not the legumes per se but the cultural practices associated with
their use which impact on soil negatively or positively.
>If you think [Steve said] about long-term studies and real life situations
where
side by side farms are managed by organic or conventional
methods, I am pretty sure you'd find wide enough differences
in soil quality characteristics to disprove the statement that
nitrogen from legumes is just as harmful as chemical nitrogen....
especially when viewed from a whole farm perspective, there are
humus complexes and soil foodwebs that can bind and fix nitrogen
thus preventing its ability to contribute towards soil acidification.
Steve also wrote:
>Interestingly, compost is known to raise the pH of acid soils
closer to neutral, as well as lowering the pH of basic soils
closer to neutral. Compost helps soils adjust towards neutral
whether the pH is high or low.
...which is why it is useful to regard composting as essentially simply an
acceleration, centralisation and intensification of good soil building
techniques, with some further health benefits if high temperatures are
established.
One green manure technique for bringing available calcium into the
biological content of the soil, which is inexpensive on a small property, is
to spread inexpensive feed oats and cut the young (calcium scavenging) crop,
before it goes to seed.
Dennis Argall
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