Re: grass clippings as mulch

I guess I must politely disagree with Susan's statement in this circumstance.

It is my understanding, both from my reading in matters horticultural, and from
personal experience with all kinds of mulch including fresh pine shavings, wood
chips and high quality hay, that organic matter of any kind when just placed on
the surface as mulch causes little or no nitrogen 'drawdown'.  This is largely
because there is no significant mechanism for transfer of nitrogen up out of the
soil into the mulch.  The mulch may, if very high in carbon relative to nitrogen,
intercept small inputs of nitrogen from above moving into the soil (as bird
droppings, spent spider dinners, lightening-fixed nitrogen, air pollution etc.)
until the mulch can be broken down, at which time the inputs are released.  So,
even if grass clippings were low in nitrogen, they should cause no problem if
placed on the surface.

But grass clippings are very high in nitrogen.  There is more than enough nitrogen
in them to allow complete decomposition without any additional input needed. In
fact, if grass clippings are too high a proportion of a compost mixture it can
cause problems with ammonia release as the overabundance of nitrogen is not needed
by the microorganisms.  The municipal composting operation in the small city near
us has had this problem since grass clippings are the majority of their
'greenwaste' in the summer months.

The clippings have essentially the same nitrogen content whether dry or wet, since
the N is bound in cell proteins and such.  As they decay, this nitrogen is
released. This decay can be mediated by lots of different organisms from mammals
through invertebrates to fungi and bacteria.  The drying just changes them enough
physically  that they usually do not mat down.  This is especially the case in dry
climates like central Spain and interior California.

Our vegetable garden nowadays is under permanent mulch, and is not tilled at all.
Every year we go out and trowel in the transplants, then place a layer of  several
thicknesses of newspaper on the surface and then cover that with flakes of hay,
peeled off of rectangular bales and placed as tiles all over the area. Usually we
get alfalfa-grass mix hay, but some years it is just various pasture grasses cut
green, dried and baled, intended for livestock feeding.

As the hay is wet by the sprinklers (all overhead water), the hay flakes expand to
make a fluffy layer several inches thick, often as deep as 20 cm to start.  By the
next year, that has all disappeared and only a thin strewing is left on the
surface, and weeds are again germinating. The decomposition of this hay layer
provides most of the nutrients needed by the vegetables.  Some years and with some
crops, a slight addition of ammonium sulfate  is sprinkled on and watered in
sometime during the plants' growth.  We do not add organic matter to the soil
directly, but let the worms come up and drag it down in for us.  The soil has
become porous and friable in a way I would not have believed possible. (This is a
stony,red, kaolinitic soil, with high content of iron and aluminum clays and a
dense clay pan at a few feet of depth)  I think the grass clippings should
function in exactly the same way as the hay.

I have never been aware of an allellopathic problem from anything I have used, and
perhaps it is because, as was mentioned in another post (Jerry's I think--I've
already deleted it from my machine),  the substances don't persist very long.
Concerns about allellopathy are more often expressed regarding living plants and
their root exudates and leaf leachates. Living grass roots are especially
inhibitory of trees and shrubs, I have read.
-
Gary Matson     Far Northern California
http://www.plantstogo.com
Adapted Plants for Hot Summer Climates

Damian Martin wrote:

> However, I'm still unclear about this nitrogen business...
>
> Gary says...
>
> > Other than that, drying the clippings first converts them into
> > essentially
> > very short-cut, high quality hay, and eliminates sticking and matting in
> > most cases.  Tremendous nitrogen release as they decompose
>
> whereas Susan warns...
>
> > another problem might be the so called 'nitrogen draw down' effect
> > (basically as raw materials rot/compost they use up nitrogen in the process)
>
> So does this mean that with dry clippings nitrogen is released *to* the
> soil but that fresh clippings draw nitrogen *from* the soil?
>
> Finally, do dry clipping still contain those worrisome allelochemicals?
>
> Damian
> Talavera, central Spain

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