4973.2 Soil Management
Soil is the thin layer of fertile land that nourishes life on earth directly or indirectly.
Plants depend on soil for physical support as well as water and nutrients. Under
natural conditions, soil is formed through the activities of soil biota but is dependent
on soil type, climatic conditions and vegetation type. Agricultural soils need con-
tinuous care and management to maintain soil health for sustainable production.
Dryland soils are not as well developed as soils in humid regions. Most dryland soils
are shallow and poor in organic matter. Organic carbon plays a key role in nutrient
dynamics, water relations and the maintenance of biological and physical health of
the soil and is thus directly correlated to soil productivity (Srinivasarao et al. 2013 ).
Further, soils in dryland areas remain uncropped for long periods during the year
leading to loss of nutrients. Therefore, the maintenance of soil fertility is a real chal-
lenge in dryland areas.
Soil structure, i.e. the arrangement of the solid material and the pores in between
the solid material, is the key parameter to overall soil function. Pores constitute
about 35–60 % of the soil volume. Tiny pores (capillaries) are normally filled with
water while larger pores circulate air and supply oxygen to plant roots including soil
biota. Thus, a stable and well-formed soil structure, particularly in an agricultural
setting, is important for water and air movement, biological activity, root penetra-
tion, access to nutrients, and seedling emergence. A well-structured soil with stable
aggregates not only prevents the collapse of pore spaces between aggregates during
heavy, saturating rains but also reduces compaction. Furthermore, improved soil
structure reduces soil erosion and surface runoff by absorbing substantial amounts
of rain.
Soil microorganisms play an important role in the formation and maintenance of
soil structure by producing exopolysaccharides and other beneficial organic acids
and enzymes. Fungal hyphae, through the spatial extension of filaments, help in the
formation of soil macro-aggregates and improve aggregate stability by producing
extracellular polysaccharides. Mycorrhizal hyphae form channels between plant
roots and soil for the acquisition of nutrients. The recalcitrant glycoprotein, gloma-
lin, produced by mycorrhizal fungi, plays an important role in stabilizing aggregates
(Wright and Upadhyaya 1996 ; Rillig et al. 2002 ). Extracellular polysaccharides
form a protective capsule layer around the bacterial colonies. The clay particles
along with mineral and decomposed organic matter are deposited on the polysac-
charide layer which acts as glue to form soil aggregates. Soil microorganisms effi-
ciently break down organic matter into stable humus, a key component of aggregate
formation. Soil aggregates provide the surface area for nutrient adsorption thus
increasing nutrient availability for plant uptake.
An active population of soil microbes promotes soil fertility and structure. Soils
rich in organic matter have better structure than soils with low organic matter or
those treated with conventional synthetic fertilizers. Most dryland soils are low in
organic carbon due to the rapid oxidation process in dry regions. The important fac-
tors leading to a decline in soil fertility in tropical regions, especially dryland areas,
Application of Microbiology in Dryland Agriculture