>> Thirty-seven years of data collected from a plot at the University
>> of Wisconsin-Madison's Arlington agricultural research station is
>> yielding alarming results: acidification from excess fertilizer is
>> wearing out the soil.
>A couple of points come to mind:
>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.)???
>2. The report also noted the importance of calcium, magnesium,
>and potassium; cation exhange capacity; and liming.
>This brings to mind the notion that eco-farming advisors who've
>been emphasizing base saturation and high calcium lime all these
>years may have clued into an important principle of good soil
>management way ahead of their time.
I have not read the original paper yet, so what I say should be seen
more as an observation triggered by the comments than as a dierct
response to them.
It seems at least reasonably logical that natural sources of nitrogen
will have somewhat the same effect as chemical analogues, especially in
high rainfall areas when nitrification rates are high. All nitrates are
soluble --- at least I can't remember a single metal that would
precipitate nitrate out of a solution --- so high nitrate situations
will result in calcium leaching, given enough water.
Removing calcium from a soil has some fairly unfortunate effects on its
structure, usually reflected as a greater susceptibility to compaction.
Compaction robs yields, and as yields disappoint, what is the response
of most farmers (and Extension folks) ? --- more nitrogen --- which
strips more calcium, and off we go. I think this is one of the
least-appreciated vicious cycles in all agriculture, and there's no
compelling agronomic reason why it can't happen with leguminous
nitrogen or manure nitrogen, especially if the nitrogen is being
cranked up to push the yields. [editorial comment: just what we need,
eh? more #2 yellow corn ...]
The "destruction of CEC" part of that article is intriguing. One
possibility might actually be only indirectly related to nitrogen. The
tendency in Wisconsin has been to keep soils absolutely awash in
soluble potash, while largely ignoring calcium. Apart from potatoes and
a couple of other horticultural crops, calcium is added to the soil
only to the extent that it is incidental to liming intended to adjust
pH. In high nitrate soil environments much of the exchangeable calcium
will be stripped off colloids, but that (in itself) shouldn't damage
the exchange site.
Here's where the potassium may come into play. Those potassium ions can
sneak between the layers on the clay particles and (especially with
certain types of clay), pull the two clay layers together around the
ion, and lock the whole thing up. Those exchange sites will be
effectively lost to the soil from then on. So I'm speculating that the
nitrogen (by stripping calcium) *enables* potash to tighten up the
edges of certain clays, making their exchange sites unavailable for
The effect may be particularly pronounced in conventional agriculture,
owing to that system's marked tendency to sacrifice organic matter for
short term yields (which may also feed into this whole loop by
increasing the compaction effect). Typical organic matter has a CEC of
about 200 meq/100g, or roughly ten times that in a clay loam.
Consequently, 2% OM in a soil is responsible for about 4 meq of CEC. On
the good organic farms there is enough OM in the soil that the portion
of CEC attributable to the clay fraction is of reduced functional
Most university researchers try to hold equal all factors except those
under study. In the case of the UW work it leads me to suspect that in
the "organic" systems potash levels were kept just as high as on the
conventional side. If potash is actually a significant part of the
problem it would be quite likely that organic sources of nitrogen would
have something of the same effect as the "chemical" sources.
I'd have to read the details of experimental design and see the data
before I went any farther down this path, but I don't think it's an
outrageous stretch to suggest that calcium and potash management may
have at least as much to do with the observed phenomena as the nitrogen
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