Geoscience Reference
In-Depth Information
a couple of weeks when the fi ne roots have matured, the external fungi fi bers cease
to function, the molecules of glomalin enter into the soil, and the second period of
glomalin action begins. Glomalin's primary aim was the reinforcement of the exter-
nal fungi fi ber walls, and when this aim disappears, glomalin remains in the soil
applying its fi xing property upon its next partner - the soil particles. With the half-
life of glomalin being at least 40 years, many reports of research frequently state
that it continues to exist for about 100 or even more years. Its resistance against
decomposition is much higher than that of humins.
Various reaction groups on glomalin's molecular surface account for its tena-
cious ability to glue fi ne silt particles and clay minerals together. Moreover, it makes
linkages with other organic molecules. Through these and other attributes, glomalin
becomes a very active glue to bind microaggregates together to form macroaggre-
gates. Historically until the very end of the twentieth century, the presence of glo-
malin and its advantageous binding contributions to aggregation remained a
mystery.
We now know that glomalin was discovered 17 years ago by Sara F. Wright, a
scientist of the
USDA
Agricultural Research Service
.
Soon after its discovery, many
research papers were published on aggregate fi xation by glomalin when changes
occurred in land use or tillage technology. With the dominant role of glomalin in
soil aggregation and soil protection being confi rmed, it is now beyond any doubt
that glomalin is much more important than humins and humic acids for maintaining
and sustaining aggregate stability.
Here, we must remind ourselves that earthworms also play a vital role contribut-
ing to the existence of soil aggregates. Taking a rough average of data, one earth-
worm swallows in one day the same weight of soil as is its own weight. Their
excrements are fi rst-class aggregates. We should also remember to honor Gilbert
White (1720-1793) who documented the unique contributions of earthworms to
soils more than a century before Charles Darwin's historical topics. At that time,
concepts of soil fertility were at their infancy and soil structure was not known.
Historically, man's management of soils for agriculture has generally had a bad
infl uence upon soil structure. To his credit, he sometimes practiced a fallow system
when the land was neither plowed nor used for one or more years to enhance the
fertility of his farm for the next time that he tried to grow a crop. We know now that
the soil fertility was enhanced mainly by the restoration of humus content including
the rise of glomalin. In such cases, the deteriorating crumbled soil structure was
strengthened and the accessibility of plant nutrients was renewed. On the other
hand, owing to fi nancial pressures and economics, fallow periods were often ostra-
cized with the frequency of cultivation intensifi ed. This intensifi cation gradually
decomposed both humus and glomalin without suffi cient opportunity for their
replacement. The quality of soil structure decreased and machinery used for tillage,
harvesting, and transport of harvests fi nished the act of soil structure destruction.
The compacted soil with its pseudo-aggregates and clods was further damaged by
erosive rains when water did not infi ltrate into a muddy, sludgy surface soil. When
the soil dried out, a compacted crust formed on its surface. All these negative con-
sequences have reduced yields and deteriorated the ecologic environment (Fig.
6.2
).
