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Re: Prairie Soil Fertility

Alan,

Soil fertility is measured by taking proper soil tests. 
Minimum soil fertility levels have been established for most
crop plants.  Prairies in Illinois emerged on glacial deposits
and or loess.  The loess in Illinois generally had a great
deal of plant nutrients but not in a plant available form.  It
was not until the prairie vegetation grew for several thousand
years and deposited organic matter and then the soil ecosystem
essentially began to become established.  It was the several
thousand years of biological activity that transformed the
minerals in the loess to minerals that would readily release
plant nutrient elements such as phosphorus, potassium,
calcium, magnesium etc.  It was the result of several thousand
years of these processes that produced the prairie soils of
today.  If we were to visit the areas that are now Illinois'
prairies 15,000 years ago prior to the evolution of the
prairie vegetation, we would not have been too impressed with
the soil! One of the most limiting elements to grass growth
native or otherwise is nitrogen.  The nitrogen in the early
days of the prairie came from basically two sources
1) decaying plant and animal tissue and,
2) legumes

I believe that the colonization of the early prairies by
legumes is what "jump started" the amazing productivity of the
prairies.  The original parent material of our soils, glacial
deposits and loess, were and are low in nitrogen.  Legumes
transform atmospheric nitrogen to plant available forms.  Some
of the nitrogen is used by the legume for growth and some of
the nitrogen is left in the soil for other plants.  Legumes
are critical for any grassland system without them the grasses
just get by.  Prairie grasses evolved having to scavenge for
whatever nitrogen it could get.  That is why I believe the
prairie grasses have such phenomenal root systems.  One key
item that we're finding to be rather significant that has been
previously overlooked is a symbiotic relationship between a
group of fungi called mycorrhizae and plant roots.  This
relationship impacts phosphorus nutrition in plants even more
than the amount of plant  available phosphorus in the soil! 
Mycorrhizae actually explore soil material more extensively
than the plant roots themselves. In exchange for amino acids
and carbohydrates supplied by the plant hosts, mycorrhizae
provides phosphorus in a form that plants can utilize. 
Phosphorus is an important plant nutrient. Soils that have
been degraded by man have low populations of these fungi.
Experiments have shown by adding a little soil from an
established prairie to prairie seed mixtures at planting will
reintroduce this group of fungi to the soil and greatly
enhance establishment.   Once the fungi have a host plant they
reproduce and oila! they're back in business.  As I stated
above, one of the most critical components of any self
sustaining grassland ecosystem is the presence of legumes. 
Legumes depend on a bacteria which colonize the roots.  They
actually infect the root but in this case it is beneficial. 
It is the bacteria that actually performs the nitrogen
fixation.  The bacteria belongs to a genus called Rhizobia or
Bradyrhizobia.  Each group of legumes depend on a specific
strain and or species of Rhizobia.  For example clovers
require a different strain than do green beans.  There are
commercial preparations available for the common prairie
legumes.  Probably the easiest way to get the rhizobia is
again to go to an established prairie and find the legumes. 
Dig around the base of these plants and get some root tissue
with some soil.  You don't need much (like a cup full) so
there is no need to commit first degree plant slaughter!  A
garden trowel full of soil is all you need and the plant won't
hardly know it.  It is important to obtain this soil close to
the time you plan to plant your prairie seeds and plants.  Get
the soil a couple of days ahead of seeding and dry it.  When
the soil is dry, crumble it finely and disperse the soil
uniformly into the seed mixture.  Do not be tempted to add a
lot of fertilizer organic or otherwise.  Too much fertilizer
will stimulate weed growth and inhibit the colonization of
mycorrhizae and rhizobia.
The only thing to soil test for is pH.  Prairie grasses are
adaptable to a wide range of soil pH.  Legumes native or not
are not as easy to please.  If you find the soil pH of the
site to be below 5.5 some liming may be helpful.  If you
encounter this situation don't let any body tell you that you
will need to lime you soil to a pH of 6.8.  That is only true
when you want to raise high quality, high yielding alfalfa.  I
hope this helps you.

Brett Roberts
State Conservation Agronomist
USDA-Natural Resources Conservation
Champaign, Illinois

Alan_Rider@illinoispower.com wrote:
> 
> There have been several comments made recently about soil fertility and how natives 'best' survive in soil of low fertility.  On the other hand, we have been told that prairie soils are among the 'richest soils in the world'.  This seems to be contradictory.  Perhaps it depends on 'what part of the prairie you live', for example Illinois prairies should have better soils than western
> Nebraska due to the rainfall amounts.  My reason for asking is I am restoring former  areas of Highly Erodable Soil,  where there has been significant erosion and little if anything has grown there
intentionally planted or otherwise.  I have added large
amounts of grass  clippings and leaves and tilled them into
the soil.  I would appreciate comments etc. on this fertility
issue.  How much fertility is  too much?  How do you measure
fertility?   thanks
> 
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