Agriculture Reference
In-Depth Information
deficiency symptoms in the plant system, respectively. Chemical fertilization is a
fast way of providing necessary macro- and micronutrients to the plants.
4.3 Nutrient Availability to Plants
The mineral nutrients present in the soil must be in bioavailable form, so that the
plant takes them up easily (Barber
1995
). Availability of nutrients to the plants
depends on their amount, nature and their association with other nutrients in the
solid phase. It can also be explained as the capacity of soil-plant system to supply/
absorb nutrients, which includes release of nutrients from solid phase to solution,
their movement and absorption by the plant (Comerford
2005
).
The concentration of mineral nutrients in soil solution varies and depends on a
number of factors like soil moisture, soil depth, pH, cation exchange capacity,
redox potential, quantity of organic matter, microbial activity, etc. (Marschner
1995
). Presence of excess minerals in the soil can also hinder the plant growth by
limiting the water availability and accumulation of heavy metals in the soil that can
cause severe toxicity.
Soil and root structure are the major factors that affect availability of nutrients to
the plant. Even in well-structured soils, the contact of root with soil varies and
depends on many factors. For instance, maintenance of root respiration and soil
bulk density for nutrient uptake are affected by soil aeration and fertility
(Marschner
1995
). Soil pH affects not only the nutrient availability from the soil
but also the growth of plant roots which are involved in nutrient uptake. Weathering
of rocks is favored by acidic pH which results in release of various ions such as K
+
,
Mg
2+
,Ca
2+
, and Mn
2+
and increases the solubility of carbonates, sulfonates, and
phosphates, thereby facilitating their availability to the roots. Rainfall and decom-
position of organic matter are the major factors that lower the soil pH (Taiz and
Zeiger
2010
).
The proper growth and development of plant roots is an important factor that
affects nutrient absorption. The uptake of nutrients through the root surface from
soil takes place by either diffusion or mass (bulk) flow. Diffusion refers to the
movement of nutrients down the concentration gradient and occurs due to the
movement of individual molecules. Short-distance flow (lateral flow) of fluids in
plants, i.e., cell to cell, or in the roots from soil, occurs through diffusion. With due
course of time, depletion zones near the roots develop, and their shape primarily
depends on the balance between different factors like uptake of nutrients by roots,
their replenishment and the mobility of ions by diffusion. Diffusion coefficient is
the measure of mobility of ions (Marschner
1995
). Mass flow refers to movement of
molecules together due to the pressure gradient. Long-distance flow (mediated by
xylem and phloem) employs mass flow which depends on transpiration rates and
amount of nutrients present in soil (Mengel and Kirkby
2001
). The relative contri-
bution of mass flow varies with factors like plant species, age of plant, and time of
the day (Marschner
1995
).
