The Birth of Soils - Soil: The Skin of the Planet Earth - page 59

without causing an observable density change in the suspension. In addition to the
classical hydrometer, more sophisticated methods are used, e.g., measuring the den-
sity of suspension by X- or γ -ray attenuation and laser light scattering or by using
laser light to count the individual particles.

5.3 Why the Topsoil Is Black or Gray Brown

5.3.1 Soil Humus

The birth of soils is not thinkable without the existence of living bodies. Up to now
we devoted a lot of time to the physical and chemical transformation of the mineral
part of the thin top skin of our planet Earth while temporarily paying no attention to
the existence of plant life. Its existence is closely linked to soils, and real soils are
full of living organisms nourished either directly or indirectly by plants and their
decaying and decayed remnants. Knowing how to repay these living soil organisms,
soils graciously support all forms of higher plant life. Moreover, the destiny of dead
plants or their parts is intimately linked to numerous, diverse processes occurring
within soils and to the existence of soils.
The birth of soils continued as the earliest of plants were caught at the initial
appearance of fi ne cracks in solid rocks on the surface of the Earth - indeed, their
thin and fragile roots had just gained a major victory against the massive rocks. As
these nets of cracks or fi ssures increased, plant numbers and vegetative mass
increased. Their seeds and dead bodies contributed to the extension of growing
plants. If the seeds fell on a real soil and not merely into a crack, they had an easier
responsibility to transform themselves. On the other hand, the dead, decaying plant
parts had in principle a closely related future: their own transformation that we call
humifi cation . Due to their transformation, the top horizon of a soil often has a dark
color in spite of the relatively low concentration of organic matter found in the great
majority of soils. Even in fertile soils, the amount of decayed and humifi ed rem-
nants of plants is an order of magnitude smaller than the amount of mineral matter.
Speaking about percentages, the total content of organic matter in the top horizon of
many soils is about 3 % by weight and ranges from less than 1 % in desert soils to
about 5 % and in prairie soils. If it is higher but still below 50 %, we speak about
peaty soils and adequate terms according to the majority of taxonomic systems, and
if the content is above 50 %, then the soil is called peat.
The transformed organic matter in soil is humus. The term is derived from the
Latin humus meaning earth or ground. Humus is also a gourmet spread that origi-
nated in the Middle East (in Arabic hoummous meaning chickpea) and has a variety
of spellings. In popular speech of many languages, humus is used in still another
completely different sense; it means a great mess. During my (MK) travels over the
world, I once had an accommodation in a bungalow for a couple of months. As my
Yugoslav colleague who was a mathematician, not a soil scientist, pointed his fore-
fi nger to the fi lthy stove smeared by decades of unwashed fat, he exclaimed: you