Issues for More Sustainable Soil System Management 359its volume, bringing up nutrients from lower horizons of the soil to distribute on
the surface and in upper layers, and reducing leaching, to list some of the most
prominent.
Any one of these processes, if occurring to an extreme, can have deleterious
effects, much like the overuse of inorganic fertilizer. The complexity of natural
systems includes mechanisms and feedback loops for curbing excesses. Agricultural
practices that fit into these flows and these mechanisms, not truncating this com-
plexity, have better prospects for sustainability than ones that seek to set their own
parameters independent of what existing systems would support. This is the chal-
lenge presented by the second paradigm.
2 Nutrients in a Soil System Context
Agricultural systems lose carbon and nutrients through the off-take of crops, but
there are multiple mechanisms for restoring elements in deficit. Plants have been
‘exploiting’ soil resources for millions of years without depleting them because of
efficient cycling and little nutrient loss. Depletion of available nutrient supplies in
soil systems has been more a consequence of their management and off-take of
nutrients through harvest than of natural processes.
Nutrient constraints need to be understood and remedied in terms of the sup-
ply of ‘available’ nutrients. Most soil systems have large stocks of nutrients in the
soil that are currently ‘unavailable’, being bound up in recalcitrant chemical com-
plexes or physically inaccessible. The issue for agricultural practice becomes
whether rates of utilization of these ‘unavailable nutrients’ can meet production
needs and expectations and are sustainable.
- Carbon is continuously restored to the soil through processes of photosynthe-
sis and root exudation and through litterfall. The share of photosynthate
exuded into the rhizosphere is difficult to measure and certainly varies, but
plants commonly put about 10–20 per cent of the carbon they acquire from
the atmosphere into the soil (Pinton et al, 2001). The amount of litterfall and
crop residues returned to the system varies with system composition and man-
agement. - Nitrogen (N) also from the atmosphere is fixed by organisms in, on and around
plant roots and even on their leaves, in what is referred to as the phyllosphere,
so N is restored to the soil through multiple pathways. Nitrogen is certainly
abundant; the question is whether sufficient amounts in available forms can be
maintained in the soil to meet crop needs, offsetting losses through leaching
and denitrification as well as crop removal. Nitrogen fixation is not limited to
leguminous species. Also the contributions that protozoa and nematodes make
to N available in plant root zones have seldom been given the attention this
process deserves.