[and the Argalls made a similar point]
>back up without any outside fertilizers. The crops pull what they need out
>of the nutrient rich subsoil and brings it to the surface where it can be
>used. Their crops continue to improve.
Nutrient pumping (by crops) can be a valuable buffer against topsoil
mineral loss to down-profile leaching in high rainfall areas. It can
even recover some minerals that would otherwise have been lost,
particularly calcium, magnesium, and potassium.
It can also be a form of mining the soil, especially in dryer areas.
That is how something like 60% of all the phosphorus in the top two
metres of Prairie soils in the Canadian west has been exported in the
form of wheat over the last 80 years or so. Phosphorus is not mobile
in the soil, so even in a wet climate any phosphorus being brought up
from the subsoil is being mined.
All the nutrients exported from the farm must ultimately be replaced,
or the system is not sustainable.
Conventional wisdom uses large annual inputs of generally inefficient
fertilisers --- only about 20% of the phosphate applied is available to
the crop, for example --- to compensate for annual crop removal of NPK.
Modified hydroponics, IMO.
Agronomically effective organic production creates a substantial
organic matter buffer against loss to leaching and erosion. Minerals
are periodically replaced in the form of more slowly-released
materials, made available to the system by an enthusiastic and healthy
microbial community, living in the organic matter and regularly fed
with green manure ploughdowns.
Biological farming combines the best elements of conventional and
organic approaches by supplementing the organic approach with judicious
applications of very modest amounts of carefuly selected soluble
fertilizers for improved crop response.
Whatever the model, however, the nutrients exported (or lost) must be
replaced, either by closing the loop (applying feedlot manure and human
sewage to crop land) or by mining geologic deposits of those minerals
and using them to replace what has left the system.
Personally, I'm not real excited about the sewage option, so I'd opt
for mined minerals. Fortunately, for most mineral nutrients this is not
a problem. Calcium and magnesium are available in virtually unlimited
supply in the limestones and dolomites found all over the world. At
current inefficient use rates ,the Canadian supply of potash alone is
sufficient for 4000 - 7000 years, depending on how the deposits are
measured. Micronutrients are used in such small quantities that they
are also not a problem.
The real long-term weak link in the system is phosphorus. At current
use rates (extraordinarily inefficient !), world supplies would last
another 80 - 350 years, again depending on whose numbers you use. The
only thing that mitigates the situation is the general immobility of
phosphorus in the soil --- it doesn't leach downprofile like the
cations. There's a lot I could say about improving the efficiency of
phosphorus use --- but won't --- because this is already a long post,
and since because this whole thread is in my area of expertise I've
already been showing my electronic face around here a lot in the last
couple of days.
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