>Legumes, root-growth die-back and same-season nitrogen contribution.
>
>Another interesting part of the fertility cycle which is valuable to natural
>gardeners is root die-back. Plants don't grow in even progressions (not much
in
>this world happens in even progressions!), but instead in cyclical growth
spurts
>based on a (probably-infinite) number of different cycles (daily, seasonal,
>wet/dry, etc...). One of the most useful of these growth cycles to growers
is
>the wet/dry cycle (the seasonal growth cycle is well-known and used in
seasonal
>rotation schemes).
>
>Plant rootlets (the tiny hairs growing out from the walls of root tips,
which do
>the actual absorptive work of the roots) have an ephemeral existence, and
must
>be replaced constantly. The rootlets grow only on the end inch or so of the
>growing root tips -- on parts of roots older than just a few hours, the root
>hairs have already died. These parts of the roots do not absorb nutrients
and
>water to speak of, but serve merely as conduits for the nutrients/water
absorbed
>by the hairs on the root tips. The root hairs are easy to observe on the
outside
>of actively-growing root balls, as on tomato transplants. When healthy, root
>hairs operate in a fascinating symbiotic relationship with mycorrhizal (root
>fungi) hypae.
>
>When it rains or the ground is otherwise saturated for more than about 4
hours,
>most of a plant's root hairs die from oxygen starvation. This is why squash,
>cabbages, etc. often wilt after a good rain, when we are expecting to see
them
>perk up. As soon as soil moisture falls below field saturation and air comes
>back into the soil, the root hairs and tips again begin to grow and the
plant
>quickly recovers its turgidity and begins a growth spurt. The growth spurt
>reaches a bell-curve-shaped peak while soil moisture and air are in balance,
and
>then starts to fall again as the soil begins to dry out (though the rootlets
can
>get water from mighty thin layers of water on the surfaces of soil
particles).
>The root tips and root hairs must be constantly growing to survive -- when
the
>soil dries out again, the root hairs quickly die. The plant has reached the
end
>of the current moisture-stimulated growth spurt and becomes semi-dormant
>(slowing metabolism and closing it's breathing pores) until it can get more
>rain. If this semi-dormant period lasts long at all, the root tips also
begin to
>die back. In this way plants can, and do, shed quite a bit of root mass
during
>each wet/dry cycle.
>
>The practical effect of this cyclical root die-back is that plants enrich
the
>soil with quite a bit of organic matter from their periodically-dying-back
roots
>even during their active growth season. If a legume is inter-planted in a
crop,
>nitrogen will be contributed not just to the succeeding crop when the legume
>dies, but also to the inter-planted crop due to the periodic root sloughing.
A
>study with inter-planted clover and ryegrass showed that 80% of the nitrogen
>taken up by the ryegrass came from the clover growing with it at the same
time
>(King, 1988). This means that leguminous living mulches can be significant
>contributors to a concurrent crop's nitrogen needs, and that land can be in
>production more continually than in a conventional mono-cultural rotation.
Total
>nitrogen produced by a leguminous green manure growing concurrently with a
crop
>would be expected to be more efficiently-absorbed by that crop, since less
time
>would elapse between the time the nitrogen was released and when the crop
would
>have access to it. Further efficiency through less gassing of nitrogen to
>ammonia, less leaching and loss to runoff, more continual feeding rates.
>
>Excellent, low-growing leguminous living mulch plants include the medics
>(Medicago spp.: Jemalong barrel medic is best I ve tried for cold, heat and
>drought resistance; Black medic is great; Spreader alfalfa, perennial and
>spreads by rhizomes, is a taller one) and the shorter clovers (white clovers,
>slightly-taller strawberry clovers). Living mulches might not be as good an
idea
>as dead mulches when growing under arid or semi-arid conditions, due to
water
>competition.
>
>According to the Land Institute, the higher the number of LEGUMINOUS plants
in
>an initial prairie seed mix, the higher the eventually-resulting total
number of
>species (LEGUMINOUS, GRASSES, AND BROADLEAF) which survive and become part
of
>the prairie.
Nice post, Jack. Please check your private email for a question about it.
I must add that some legumes exude nitrogen from their roots. Jim Duke
remarks on this in place in LEGUMES OF WORD ECONOMIC IMPORTANCE (or something
close to that title--PDBR). The roots don't wait to die to contribute
nitrogen to nearby plants. This apparently varies in degree at least from
species to species. There may be some relationship to mycorhizzal fungi
associates in all this too.
I've got a great photograph of these feeder roots AT THE SOIL SURFACE, right
under newspaper mulch that has been pulled back. The rootlets are surrounded
by earthworm castings plus residual plant debris. We published this photo on
the back cover of our soils issue of THE INTERNATIONAL PERMACULTURE SOLUTIONS
JOURNAL and folks who have a copy can read more info about what is going on.
However, you don't need those facts to know that the mulch/soil interface is
extremely important to plant health, that natural systems deposit nutrients
there, and the air, so important to roots as you point out, is close at hand
from both above and through earthworm "double digging" on all sides.
That picture is worth a thousand words.
For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since
1982), Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and now correspondence
permaculture training by email. Copyright, 1996, Dan & Cynthia Hemenway, P.O.
Box 2052, Ocala FL 34478 USA YankeePerm@aol.com
We don't have time to rush.