>Granted that some sub-soils in some places (not just in a region of the
>country but, where geology is complex, maybe in different parts of the one
>farm) may have chronic deficiencies, or difficult pH, does not the
>availability of nutrients to plants depend on soil management, particularly
>the development of colloidal humus?
>
It is true that our first and foremost concern is to improve and maintain
soil structure. Soil structure is dependent entirely on biological
activity, the conversion of organic matter into energy (for soil
microorganisms to grow and reproduce) and "colliods", "humus" and such, in
other words secondary substances that bind together soil particles into
'crumbs' that produce a loose soil in with roots and grow freely and where
water and air can penetrate freely (roots need water *and* air to
metablolize properly). Achieving a good soil structure will solve most of
your problems.
If their are regional deficiencies, or more likely, imbalances among
nutrient elements then this process of creating soil structure may run into
difficulties. Plants may show any of a number problems including pests or
diseases. The mineral status of a soil--and we're talking about maybe 22
elements here, not just two or three or six--is part of creating an optimum
soil. (Unfortunately the idea of optimum soil isn't really addressed in
soil and plant nutrition studies.)
So, the answer to your question is "yes, but..."
>Put it another way: if the growing of a crop removes something from the
>soil, does that mean that something from a bag or a truck has to be added,
>or does it mean that the management of the soil and plant and animal
>production needs to be such as to maintain or improve soil quality, rather
>than see it go downhill? Is not this point - the enhancement of soil
>quality - the key factor in organic or sustainable farming, rather than
>measuring deficiencies and making choices between additives?
>
If harvesting a crop removes an essential element from the topsoil, then
this element becomes less available. If it was optimum at one point then it
becomes less than optimum after being removed. It has to be replaced, it's
as simple as that. If you take it out you have to put it back. There's no
magic in "soil management"-- its physics and biology. How can you replace
it? The most common way for many mineral elements to be restored is for
deep rooted plants to bring them up again from the subsoil and deposit them
in the top soil in leaf litter and other residues. If that element is
scarce in the subsoil then it will be scarce in the topsoil--it will be
quickly depleted in the harvest (perhaps, or by erosion) and will have to
be supplied from other sources--"a bag or a truck" or from another part of
the farm.
Most descriptions of soil mineral content are misleading. They are talking
about average amounts or minimum amounts necesary to prevent noticeable
deficiency symptoms. But the level of Ca or B or Cu necessary to prevent
obvious deficiency is usually quite different from the *optimumal* level of
these nutrients for soil and plant health and productivity. What is more
the optimal level isn't a fixed amount but varies according to the
proportion of other elements. "Deficiencies" can be caused by high levels
of an antagonistic element rather than by absolute absences of the
deficient one. Higher N levels in the soil require higher Mo and Fe, for
example. This is because these elements are used in enzyme systems the
plant needs to metabolize N. These same principles apply to animal nutrition.
How do you balance a soil optimally and know what elements to supply and
when you are wasting your money? This is the art. Things are more likely to
happen naturally on a diverse organic farm where the network of flows is
more complex. A lot depends on the daily attention of the farm family, just
noticing what is going on. But then questions arise and sometimes its not
easy to find answers. It seems like the technical people and scientists are
usually asking different questions.
The behavior of our animals can give us clue. The freer they are to move
around the more they can tell us about our soils etc. Sometimes animals
have natural licks where they like go. These may just be salt, but natural
licks probably have other mineral too. Getting these
analyzed can give us a clue as to what is missing in the animals diet, and
therefore in the soil (Geologically if there is a concentrated deposit in
an area of some mineral itspossible that this mineral is deficient in the
surrounding soils). Also watching what a cow eats after calving or when
they are sick can be instructive.
I have talked to graziers who use free choice mineral feeding. This is a
system where minerals are given in separate trays and the cattle (or llamas
or whatever) can choose their own mix. Observing what the animals do here
can reveal worlds about the soil and the seasonal availability of minerals
in the forage. Some graziers claim that this method allows very precise
adjustment of the mineral levels in the soil, as the cattle carry these
minerals and deposit them in their manure as they graze. I know one
grazier who believes that cattle will go and deliberately drop in patch of
soil that is deficient! Talk about sophisticated soil management, this
beats precision farming!
Anyway, talking to these people has convinced me we still have lot to learn
about soil fertility. The point is optimal mineral balance should translate
into healthy soil and plants and optimum quality foods.
Ronald Nigh
Dana, A.C.
Mexico, D.F. & San Cristóbal de Las Casas, Chiapas
Tel. y FAX 525-666-73-66 (DF)
529-678-72-15 (Chiapas)
danamex@mail.internet.com.mx
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