Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 86

an optimum temperature range which is typical for the respective species. C 4 -
plants are thus characterised by a higher temperature optimum (over 30 °C) than
C 3 -plants (approximately 20 °C).
The bottom margin of the photosynthetic activity, the temperature minimum, is
a few degrees below zero for plants in cold and moderate climate zones. With an
increase of the annual mean temperature (up to approximately 30 °C) the biomass
yield potential of a site also increases if the water supply is sufficient (Fig. 2.50,
on the right).
The upper temperature limit for various plants varies between 38 and 60 °C, as
proteins are destroyed above this temperature, leading to a reduced enzyme activ-
ity and damaging of membranes. This stops the metabolic processes.
Soil and nutrients. Soil is produced by weathering of the earth's crust under the
influence of microorganisms (biosphere). It consists of minerals of different kinds
and sizes and humus produced from organic substances. Furthermore, it contains
water, air and various living organisms. The soil offers the plants space for their
roots, anchoring, and the supply with water, nutrients and oxygen.
Growth and development respectively the yield of plants are strongly influ-
enced by the physical, biological, and chemical properties of the soil. Physical
attributes are the thickness of the soil, i.e. the depth of the top layer that can be
reached by the roots. Other physical properties are the texture or granulation size,
the proportion of air-conducting pores and the capacity of the soil to store water
and to store and release heat. A sufficiently large root space is important for an
optimum plant growth in order to tap nutrients and water. Among the chemical
properties are i.e. the nutrient content of the soil and its pH-value. Biological
properties of the soil are determined by the presence and activity of micro-
organisms in the soil. To a large extent these microorganisms live from organic
matter released into the ground from dead plants. Microbial activity releases nu-
trients that are absorbed by plants through their roots.
The non-mineral nutrient elements carbon (C) and oxygen (O 2 ) are absorbed
from the air by the leaves of the plant. In contrast to oxygen which is largely
available, carbon dioxide (CO 2 ), with only 0.03 Vol.-%, is only available in a very
low concentration in the air. If irradiation is stronger, the CO 2 -supply of the
chloroplasts can limit the production rate of a plant body.
The main mineral nutrients nitrogen, phosphorus, potassium, magnesium, and
sulphur plus the micronutrients iron, manganese, zinc, copper, molybdenum, chlo-
rine, and boron mainly have to be absorbed from the soil by the plants through
their roots.
The larger the root surface, i.e. the better the root system can develop the more
nutrients and water can be absorbed by the plant. The rooting decreases with an
increasing density of the soil components and the occurrence of compression
zones in the soil, which can occur when soil cultivation is performed on wet soil
or by too heavy machinery.