N-cycling, living mulches

jack rowe (75554.631@compuserve.com)
05 Feb 97 20:02:36 EST

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 Ive 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

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.