Soil - Agroecology: The Ecology of Sustainable Food Systems - page 105

ion “bridges.” Under acid conditions these bridges form
by association of additional hydrogen ions with functional
groups such as the hydroxyl group (OH). An important
example is the binding of nitrate (NO - ) with OH + formed
following the dissociation of water molecules under acid
conditions. Because soil acidity influences electrical
charge on micelle surfaces and controls whether other ions
are displaced from soil micelles, it greatly affects the
retention of ions in the soil and the short-term availability
of nutrients, both of which are key components of soil
fertility.

Soils with a high concentration of neutral salt (e.g.,
NaCl or Na 2 SO 4 ) are called saline. In cases where sodium
is combined with weak anions (such as HCO - ), alkaline
soils develop, which have a pH generally greater than 8.5.
Soils with high levels of neutral salts are a problem for
plants due to osmotic imbalances. Alkaline soils are a
problem because of excess OH - ions and difficulty in
nutrient uptake and plant development. In some regions,
saline-alkaline conditions occur when both forms of salt
are present. Proper irrigation and soil water management
becomes a key part of dealing with these conditions.

S OIL A CIDITY AND P H

SOIL NUTRIENTS

Any experienced gardener or farmer is aware of the impor-
tance of a soil's pH, or acid-base balance. The typical pH
range of soils is between very acid (a pH of 3) and strongly
alkaline (a pH of 8). Any soil over a pH of 7 (neutral) is
considered basic, and those less than pH 6.6 are consid-
ered acid. Few plants, especially agricultural crops, grow
well outside the pH range of 5 to 8. Legumes are particu-
larly sensitive to low pH due to the impacts acid soils have
on the microbial symbiont in nitrogen fixation. Bacteria
in general are negatively impacted by low pH. Soil acidity
is well known for its effects on nutrient availability as
well, but the effects are less due to direct toxicity on the
plant than they are to the plant's impaired ability to absorb
specific nutrients at either very low or very high pH. It
becomes important, then, to find ways to maintain soil pH
in the optimal range.
Many soils increase in acidity through natural pro-
cesses. Soil acidification is a result of the loss of bases by
leaching of water moving downward through the soil pro-
file, the uptake of nutrient ions by plants and their removal
through harvest or grazing, and the production of organic
acids by plant roots and microorganisms. Soils that are
poorly buffered against these input or removal processes
will tend to increase in acidity.

Since plants obtain their nutrients from the soil, the supply
of nutrients in the soil becomes a major determinant of an
agroecosystem's productivity. Many nutrient analysis
methodologies have been developed for determining the
levels of various nutrients in the soil. When a particular
nutrient is not present in sufficient quantity, it is called a
limiting nutrient and must be added. Fertilization technolo-
gies have grown and evolved to meet this need. It must be
kept in mind, however, that the presence of a nutrient does
not necessarily mean it is available to plants. A variety of
factors — including pH, CEC, and soil texture — deter-
mine the actual availability of nutrients.
Because of the loss or export of nutrients out of the
soil due to harvest, leaching, or volatilization, fertilizers
must continually be added in large amounts to most agro-
ecosystems. But the cost of fertilizers as an input is
increasing, and leached fertilizer pollutes ground and sur-
face water supplies; therefore, an understanding of how
nutrients can be cycled more efficiently in agroecosystems
becomes essential for long-term sustainability.
As described in Chapter 2, the major plant nutrients
are carbon, nitrogen, oxygen, phosphorus, potassium, and
sulfur. Each of these nutrients is part of a different bio-
geochemical cycle and relates to management of soil in a
unique way. The management of carbon will be discussed
below in terms of organic matter; nitrogen in the soil will
be included in a discussion of mutualisms and the eco-
logical role of nitrogen-fixing bacteria and legumes in
Chapter 16. Here, as an example of an important soil
nutrient, we will examine the nutrient phosphorus.
Because the efficient recycling of phosphorus depends
principally on what happens in the soil, it can teach us a
lot about sustainable nutrient management (Figure 8.4).
Unlike carbon and nitrogen, whose principal reser-
voirs are in the atmosphere, the principal reservoir of
phosphorus is in the soil. It occurs naturally in the envi-
ronment as a form of phosphate. Phosphates can occur in
the soil solution as inorganic phosphate ions (especially
as PO 3- ) or as part of dissolved organic compounds.
But the primary source of phosphate is the weathering of

S ALINITY AND A LKALINITY

It is common for the soils of arid and semiarid regions of
the world to accumulate salts, in either a soluble or insoluble
form. Salts released by the weathering of parent material,
combined with those added in limited rainfall, are not
removed by leaching. In areas of low rainfall and high
evaporation rates, dissolved salts such as Na + and Cl - are
common, combined with others such as Ca 2+ , Mg 2+ , K + ,
HCO - ,and NO - . Irrigation can add even more salts to the
soil, especially in areas with a high evaporation potential
(Chapter 9), where added salts migrate to the surface of
the soil by capillary movement during evaporation. In
addition, many inorganic fertilizers, such as ammonium
nitrate, can increase salinity as well because they are in
the form of salts.