Geoscience Reference
In-Depth Information
cannot fully exploit plant tissues that have higher proportions of carbon without draw-
ing on other sources of nitrogen. This has practical implications in the case of straw
that is incorporated into the soil after harvest, a point discussed further in
chapter
though the supplies of phosphorus and sulphur are not so limiting.
Humus is the final product of organic matter decomposition. It is a dark amorph-
ous material consisting of complex organic molecules which can be broken down into
humic and fulvic acids. There are, therefore, three interrelated organic fractions in
soil. First, there are the plant (and animal) residues which form the main source of
available nitrogen, phosphorus and sulphur for new plant (and animal) growth. Se-
condly, there is the microbial biomass which acts as a temporary store of such nutri-
ents, and thirdly, a persistent humus fraction which is highly resistant to further break-
down but which can release nutrients very slowly. Measurements have been made of
the age of humus by radio-carbon dating techniques; that is, by measuring the propor-
tion of the radio-active isotope
14
C left in the humus, and calculating the time since it
must have been taken up by the living plant as carbon dioxide from the atmosphere.
Such measurements give periods of several centuries, components such as humic acid
persisting for over a thousand years in some instances. To use a monetary analogy,
these three organic fractions - plant residues, microbial biomass and humus - might
be represented respectively by goods which are traded for cash, a deposit account, and
a long-term insurance policy.
The conversion of such stores of nitrogen, phosphorus and sulphur to available,
'cash-in-hand', nutrients is called mineralization; the mineral forms are the ammoni-
um cation (NH
4
+
), and the nitrate (NO
3
-
), phosphate (PO
4
3-
) and sulphate (SO
4
2-
) an-
ions. This important topic of nutrient cycling is developed in
chapter 6
. The carbon in
humus is oxidized to carbon dioxide and lost back to the atmosphere. Usually a steady
state is reached between gains and losses of carbon, but under waterlogged conditions
plant remains may accumulate as peat. It is worth remembering that the world's vast
deposits of coal, oil and gas represent the preserved surpluses of carbon built up by
plant tissues in former ages. On the other hand, if peat is dried out and cultivated, as
in the Cambridgeshire fens, the stores of carbon are quickly oxidized away again. We
are drawing on our capital here just as we are with fossil fuels.
The existence of distinct and persistent litter and fermentation layers character-
izes what are called mor humus soils. Mor formation occurs typically on well drained,
very acid soils under conifer woodland and heathland. The main deep-burrowing
earthworm species
Lumbricus terrestris
cannot survive in such acid conditions, and in
its absence the organic matter is not readily incorporated into the underlying mineral
soil. In contrast, in well drained, less acidic or calcareous soils, the feeding activit-
