A
better soil structure and improved
B
water-holding capacity
residue
C
active organic matter
(particulate, light fraction)
D
During the decomposition process,
growth-promoting substances are
E
produced (A), soil structure is improved
and water-holding capacity is increased
(B), nutrients are mineralized (during C,
passive organic matter
D, and E), and CEC is produced (C, D, and E).
(humus, organo-mineral complexes)
Direct Effects on Nutrient Availability
Indirect Effects on Nutrient Availability
1) Nutrients released during residue decomposition.
1) Growth-promoting substances. Substances produced by bacteria
Relatively high amounts of mineralization of available
promote better root growth and healthier roots and lead to the
nutrients are produced by a combination of rapid
exploration of more soil volume and more surface area for nutrient
decomposition plus previously accumulated POM or
interception and mass flow of nutrients to roots.
a high amount of added residues. Rapid decomposition
is stimulated by intensive tillage, good soil drainage,
2) Better soil structure and improved water-holding capacity. Better
coarse texture, and alternating wet and dry conditions.
soil structure may enhance root development and exploration (see #1).
Good soil structure and plentiful humus content contribute to higher
2) CEC and chelates produced during decomposition
amounts of plant-available water following rains or irrigation. This results
process.
in better plant growth and health and more nutrient movement to roots.
Figure 18.1. Influence of soil organic matter and its management on nutrient availability.
environmental quality. If a soil has good tilth, no
example, in the presence of too low or too high N levels—
subsurface compaction, good drainage, adequate water,
are not able to emit as much of the natural chemicals that
and a good supply of organic matter, plants should be
signal beneficial insects when insect pests feed on leaves
healthy and have large root systems. This enables plants
or fruit. Low K levels aggravate stalk rot of corn. On the
to efficiently take up nutrients and water from the soil
other hand, pod rot of peanuts is associated with excess K
and to use those nutrients to produce higher yields.
within the fruiting zone of peanuts (the top 2 to 3 inches
Doing a good job of managing nutrients on the farm
of soil). Blossom-end rot of tomatoes is related to low cal-
and in individual fields is critical to general plant health
cium levels, often made worse by droughty, or irregular
and management of plant pests. Too much available N in
rainfall or irrigation, conditions.
the early part of the growing season allows small-seeded
When plants either don’t grow well or are more
weeds, with few nutrient reserves, to get well established.
susceptible to pests, that affects the economic return.
This early jump start may then enable them to out-
Yield and crop quality usually are reduced, lower-
compete crop plants later on. Crops do not grow properly
ing the amount of money received. There also may be
if nutrients aren’t present at the right time of the season
added costs to control pests that take advantage of poor
in sufficient quantities and in reasonable balance to one
nutrient management. In addition, when nutrients are
another. Plants may be stunted if nutrient levels are low,
applied beyond plant needs, it’s like throwing money
or they may grow too much foliage and not enough fruit
away. And when N and P are lost from the soil by leach-
if N is too plentiful relative to other nutrients. Plants that
ing to groundwater or running into surface water, entire
are under nutrient stress or growing abnormally—for
communities may suffer from poor water quality.
204
Building SoilS for Better CropS: SuStainaBle Soil ManageMent