Re: Soil Quality Attributes (fwd)
---------- Forwarded message ----------
Date: Mon, 27 Feb 1995 23:42:32 +0000
From: Max Turner <M.Turner@massey.ac.nz>
Subject: Re: Soil Quality Attributes
Ann Clark wrote:
>I have been observing the discussion on soil quality and would like
>to add a query. An issue which has been taken up with fervor by my
>colleagues in soil science has been the notion that potential for
>nitrate contamination of groundwater is **reduced** by using chemical
>rather than organic sources of N.
I think it is fair to point out that up until rather recently the concerns
of proponents of "organic" or "conventional" systems were more about
productivity than efficiency. Losses of N from the different systems are
either known or suspected but not widely studied because these losses are
negligible from a production standpoint. My research has shown me that many
of our organic-rich systems are in fact very high fertility systems, having
been developed using in-situ organic methods or imported nutrients from
off-site sources. With high organic matter comes the opportunity for losses
especially as the "equilibrium" is disturbed.
>The logic presented is that organic matter mineralizes in
>ways/rates/timing which a) is not controllable, and b) is not easily
>predictable, and hence, c) has the potential to continue to pump out
>labile N long after the crop has been harvested. On this basis, they
>discount the merit of long-held organic farming wisdom, namely, "feed
>the soil" as a source of immobilized and slowly released mineralized
>nutrients to feed the crop. The next dimension to this logic is that
>perennial forages, which enhance soil OM, are hazardous to the
>environment because when they are plowed under, they mineralize and
>release copious amounts of N - with again, the potential to pollute.
Whether N will be lost will to some extent be determined by the C:N ratio of
the environment in which decomposition takes place. Ploughing will usually
engender losses by increasing the potential for increased mineralisation.
But the main point is really that no system is "tight" for N cycling, and
with high organic matter of low C:N ratio, the potential for leaching loss
undoubtedly occurs. Loss of nitrate from both types of systems will be lost
by the same processes. We routinely measure significant leaching losses of N
from soils with, or without, fertilizer additions.
However it is another matter to say that fertilizer is a more controllable
source of N, at least in New Zealand where our climate and high organic
matter soils make control extremely difficult. At high rates of N addition,
losses would be a normal expectation and difficult to avoid.
>However, a great deal of the rationale behind organic field crop
agriculture is to keep >the land covered with a live - and predominantly,
growing - crop year-around.
>This practice, in addition to supplying a sink for labile nutrients,
>also keeps the soil colder longer and enhances the synchrony between
>such T-sensitive processes as microbially-based mineralization and
But I would guess that you could still measure losses of N, even under a
cover crop, since the N cycle is fairly leaky (i.e. PROVIDING WATER MOVES
THROUGH THE SOIL AS DRAINAGE).
>I would be interested in the comments of SANETers on these issues,
>particularly as they pertain to the definition of soil quality, and
>ultimately, the attributes of the "ideal" soil that we should be
When losses from organically-farmed systems are eventually measured I
venture to suggest that we will have even more of a dilemma as to what
really is an ideal soil and which system best creates it. It seems to me
that N losses, which are a bi-product of the system, increase with
increasing productivity of a system. It may be a corollary that to minimise
losses we need to reduce the overall productivity of particular Loss-prone
soils. Would this then be "ideal"?
(reply to SANET list as well as to Ann by personal email)