Re: more soil reading
Heres another note from a soils guy for those that like reading this kind of
stuff. I had to read it 5 times but some interesting thoughts
MB
Boy, that's a pretty tall order. In truth, there are established
ranges of 'optimum' levels for various nutrients and some soil
constituents, all dependent of course, on intended plantings as well
as the region of the world where you're growing, but it is more or
less impossible to identify one numerical range as ideal in all
settings. But at the same time, there are reasonably-well established
levels of nutrients which are thought to be adequate to support or
maintain plantings, again with the caveat that the levels will be
different for different plant types. (In a room with three people you
can probably start as many as a dozen arguments on the
topic of soil pH and what it is, what it means, and what to do with
it!)
Not to dismiss your good question, I simply don't know where to start.
If you are talking about a mineral soil where it formed, in the
southeastern U.S., and subject to past management effects, then a
simple soil test is generally adequate for the purposes of growing
almost all those things we can grow in this region. Mineral soils in
Alabama tend to be relatively highly weathered and thus relatively low
in native mineral constituents, so that fertility is generally lower
than is found in regions where the soil is not so highly weathered.
At the
same time, there are regionally-valid 'best range's for various
nutrients which are fairly well established.
Confused? I have hardly begun to scratch the topic. I can probably
answer specific questions more directly, whereas you've asked a
question which merits at least a dissertation if not an encyclopedia
to address the topic.
In general, the pH of a soil should be in the range of 6.0 to 7.0 for
optimum plant growth. (Azaleas, rhododendrons, hawthorns, and
camellias, among others, are plantings said to tolerate and even to
require a lower pH for good growth. At the same time, excessive
phosphorus in the soil coupled with low pH will likely result in
precipitation of iron, such that 'iron chlorosis' (yellowing of leaves
due to iron deficiency) may result. Chelated iron may be applied in a
foliar spray, and the plants will be greener tomorrow, but the
treatment is only cosmetic and the yellow tissue will develop again
with more growth. The path to take to correct the problem is to do
something to raise pH.) Ordinarily we recommend the application of
lime to correct low pH. The caveat here is that we make
recommendations for lime which are dependent upon the
plants-to-be-grown, such that the target pH for azaleas is pH 5.5 and
the target pH for alfalfa is pH 7.0.
Mineral nutrients are easily measured and we make recommendations for
application of primary nutrients, and sometimes for secondary or
micronutrients for specific crops and circumstances. A routine soil
analysis run in our lab does not include those secondary nutrients and
micronutrients, for some very good reasons. Firstly, we don't run
micronutrients for the reason that there is no calibration data
available to use to judge whether the amount we can measure is
deficient, adequate, or excessive. Secondly, there is a lack of
information on the relationship between measured soil levels of some
micronutrients and the levels observed in tissues of plants grown on
those soils, making it difficult to assert that a measured level is
adequate or not. Thirdly, there is the potential for over-application
of micronutrients to such levels that toxicity can become the
controlling
factor out there where the plants are growing. And lastly, there is
the assumption that poor performance is related to the lack of some
very small amount of a particular micronutrient, when in reality it
has proven impossible for us to demonstrate the deficiency of a
micronutrient as the source of poor performance. Generally, when the
plant is performing poorly the tissue levels of all nutrients are
skewed. (Sometimes we'll see elevated levels of all nutrients when
analyzing plants submitted because of a suspected problem. That may
simply be the result of a weakened plant's inability to utilize
nutrients in growth, and the nutrient uptake continues even while the
plant cannot use what it has currently in reserve. That makes plant
analysis interesting and challenging! But we sometimes do measure low
levels of some micronutrients in plant tissue, which can be helpful in
sorting out the source of the problem when general fertility and pH
are in that range said to be best.)
That is not to say that plant tissue analysis can't be used to
identify the source of problems, but sometimes, even usually, the
problem is more directly due to fertility of macronutrients (N, P, and
K), or to a pH that affects plant growth & metabolism, rather than due
to a shortage of something that should be there in only trace amounts
anyway.
Ordinarily, the limiting factors to plant growth are extremes of pH
and/or fertility. Lime and fertilizer can be used to correct either
of these as the likely source of problems, and our recommendations for
most
crops include a recommendation for micronutrients where we know that
the chance exists for a deficiency in the soil of that micronutrient,
for that crop.
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