Indicators of Sustainability - Agroecology: The Ecology of Sustainable Food Systems - page 303

TA B L E 2 1 . 2
Indicators of Soil Health

Indicator Best Time to Test Healthy Condition

Earthworm presence Spring or Fall, when soil is moist >10 worms per cubic ft; many castings and

holes in tilled clods

Color of organic matter When soil is moist Topsoil distinctly darker than subsoil

Presence of plant residues Anytime Residue apparent on most of soil surface

Condition of plant roots Late spring or during rapid growth Roots extensively branched, white, extended

into subsoil

Degree of subsurface compaction Before tillage or after harvest A stiff wire goes in easily to 2x plow depth

Soil tilth or friability When soil is moist Soil crumbles easily, feels spongy when walked on

Signs of erosion After heavy rainfall No gullies or rills; runoff from fields is clear

Water holding capacity After rainfall during growing season Soil holds moisture well more than a week

w/o signs of drought stress

Degree of water infiltration After rainfall No ponding or runoff; soil surface does not remain

excessively wet

PH At same time each year Near neutral and appropriate for crop

Nutrient holding capacity At same time each year N, P, and K trending up, but not into “very high” zone

Source : Adapted from Magdoff F. and H. van Es, 2000, Building Soils for Better Crops . Second Edition. Sustainable Agriculture Network:
Washington DC.

technical, costly, and laboratory-based. They tell us a great
deal about the potential of any particular soil for farming,
or the impacts of various farming practices on the soil,
but they are impractical for farmers to use regularly. Farm-
ers prefer to describe soil health subjectively and qualita-
tively, using words related to how the soil looks, feels,
and smells. In this way they are able to assess character-
istics such as ease of cultivation, water holding capacity,
organic matter content, and potential for weed growth.
Soil scientists have been able to correlate these subjective
determinations with their quantitative analysis of soil
quality, and they have developed score cards for assessing
soil health on this basis (Magdoff and van Es, 2000).
Table 21.2 offers a fairly comprehensive set of soil
health indicators that can be tested easily on the farm.
Most are qualitative; only the last two require any testing
equipment beyond a stiff wire and a shovel.

From an ecological perspective, productivity is a pro-
cess in ecosystems that involves the capture of light energy
and its transformation into biomass. Ultimately, it is this
biomass that supports the processes of sustainable produc-
tion. In a sustainable agroecosystem, therefore, the goal
is to optimize the process of productivity so as to ensure
the highest yield possible without causing environmental
degradation, rather than to strive for maximum yields at
all costs. If the processes of productivity are ecologically
sound, sustainable production will follow.
One way of quantifying productivity is to measure
the amount of biomass invested in the harvested product
in relation to the total amount of standing biomass
present in the rest of the system. This is done through
the use of the productivity index , represented by the
following formula:

Total biomass accumulated in the system
N

Pr oductivity index PI ( ) =

et primary productivity (NPP)

T HE P RODUCTIVITY I NDEX

One important aspect of sustainability analysis is to
use a more holistic basis for analyzing an agroecosystem's
most basic process — the production of biomass. Conven-
tional agriculture is concerned with this process in terms
of yield. How the harvest output, or production , is created
is not important as long as the production is as high as
possible. For sustainable agroecosystems, however, mea-
surement of production alone is not adequate because the
goal is sustainable production. Attention must be paid to
the processes that enable production. This means focusing
on productivity — the set of processes and structures
actively chosen and maintained by the farmer to produce
the harvest.

The productivity index provides a way of measuring
the potential for an agroecosystem to sustainably produce
a harvestable yield. It can be a valuable tool in both the
design and the evaluation of sustainable agroecosystems.
A PI value can be used as an indicator of sustainability if
we assume that there is a positive correlation between the
return of biomass to an agroecosystem and the system's
ability to provide harvestable yield.
The value of the productivity index will vary between
a low of 1 for the most extractive annual cropping system,
to a high of about 50 in some natural ecosystems, espe-
cially ecosystems in the early stages of succession. The
higher the PI of a system, the greater its ability to maintain