How Soil Is Formed - Soil: Reflections on the Basis of our Existence - page 34

establish a foothold during the accumulation phase, building up the earth's nitro-
gen content. Phosphorus content is high at this point but then enters a steady
decline. This is because the presence of plant roots makes the soil acidic and
results in phosphorus leaching from minerals. If not absorbed by plants, the
phosphorus may be washed away by rain. It can also attach to aluminium and
iron-rich surfaces, which also takes it out of the equation. As phosphorus content
declines, plants start to economise on its use, retaining their leaves for longer
and adapting to a lower level of nutrients. This adaptation comes at the expense
of reduced growing power stemming from reduced efficiency in photosynthesis.
At this point, the vegetation will thin and once-tall forest may revert to scrub.
The rate of this process is determined by rainfall. At the Franz Josef glacier, it
has taken around twelve thousand years for phosphorus leaching to affect plant
growth. The result is an increase in coniferous trees and later on a dominance
of species-poor but stress-resistant scrubland. Soil formation over even longer
timeframes has been studied in some areas. Studies spanning several million
years are possible at Hawaii's lava flows, where the transition from nitrogen to
phosphorus deficiency can take several hundred thousand years due to lower
rainfall than in New Zealand.
The First Plants Needed Fungi
Phosphorus deficiency also played a key role when plants evolved from exclu-
sively aquatic to terrestrial organisms. The first plants grew on the sea line and had
creeping rootstalks, or rhizomes, rather than roots. Aquatic plants easily absorb
phosphorus dissolved in the water, but phosphorus on land is fixed and hard to
access. The plants that successfully absorbed sufficient phosphorus in this inhospi-
table environment gained a competitive advantage in terms of stronger growth and
a better survival rate. Here, they had fungi to thank. The fungi attached themselves
to the plant to gain access to carbohydrates. When confronted with a phosphorus
deficiency inside the plant they absorbed phosphorus from the soil, some of which
became a food source for the plant. Fungi that did not kill their host plant gained
continuous access to carbohydrates, giving them an advantage—and by trans-
porting phosphorus to the plant, they ensured a stable supply of carbohydrate for
themselves as photosynthesis requires phosphorus. This was how the mycorrhi-
zal symbiosis, one of the natural world's most successful collaborations, evolved.
Mycorrhizal fungi exist on between eighty and ninety percent of plants today.
Terrestrial plants would probably have not evolved were it not for the presence of
collaborative fungi.
Conditions for soil formation changed significantly when terrestrial plants
developed around four hundred million years ago. The plants produced vast
amounts of biomass that over time created the organic matter needed for soil to
develop. As mineral soil weathered, so the soil formation accelerated.