Farmers Review Africa Sept/Oct 2018 FRA - September - October 2018 digital 5 | Page 30

FEATURE
C
Mineral nutrition for fruit trees
rops require adequate nutrition
for the production of food,
fibre and fuel. However, soil
conditions often limit the ability of
crops to acquire mineral nutrients.
To address this, mineral nutrients
can be applied as inorganic or
organic fertilizers to the soil or as
liquid fertilizers to foliage. However,
production and use of fertilizers can
have negative environmental impacts.
Tree-fruit crops vary greatly in size
from year to year. In some years trees
and shrubs are laden with seeds and
fruits of various kinds, but in other
years bear almost none. Fruiting
depends partly on the natural rhythm
of the trees themselves and partly on
the weather. Trees of most species
require over a year to accumulate the
nutrient reserves necessary to produce
a fruit crop. In addition, for a good
crop, the weather must also be fine
and warm in the preceding autumn
when the fruit buds form, and again in
the spring when the flowers set.
Plant nutrients
Other than water, sunlight, and carbon
dioxide from the air, plants require 13
mineral nutrients that are found in the
soil. The macronutrients nitrogen (N),
phosphorus (P), potassium (K) are
needed by plants in relatively large
amounts and often have to be added
to the soil. Intermediate amounts of
secondary nutrients magnesium (Mg),
calcium (Ca), and sulfur (S) are needed
by plants. Trace or micronutrients
[boron (B), chlorine (Cl), copper
(Cu), iron (Fe), manganese (Mn),
molybdenum (Mo) and zinc (Zn)] are
needed in small amounts
These nutrients perform a variety of
functions in plants ranging from being
structural components of cell walls
and membranes to activating enzyme
systems. About 95% of the dry
weight of a typical plant is made up
of carbon, oxygen and hydrogen. The
soil-supplied minerals make up only
5% of a plants total dry weight.
28 |September - October 2018
Fruit growers have three main tools to
use in evaluating the mineral nutrition
status of their plantings; examine
visual symptoms exhibited by leaves,
stems, and fruit; analyzing leaf tissue
and testing the soil. When used
together properly, these are powerful
tools that can be used to prevent
nutrient deficiencies or toxicities as
well as to assess current fertility
management practices.
In the soil all mineral elements
move by a combination of mass
flow and diffusion. As such, all
strategies designed to improve mineral
acquisition by crops affect one or
both of these processes Diffusion
of mineral elements is determined
by the concentration gradients
between the root surface and the soil
solution which are often dictated by
interactions with soil mineral surfaces.
It operates over short distances,
and is especially important for the
macronutrients P and K that often limit
crop production.
If essential mineral elements are not
present in the soil, then they must be
supplied as fertilizers to enable crop
production and ensure produce quality.
Many agronomic strategies can be
adopted to increase the efficiency with
which inorganic and organic fertilizers
are used to provide the essential
mineral elements required by crops in
principle, these optimize the chemistry,
quantity, placement, and timing of
fertilizer applications.
The chemical form and quantity
of a mineral element required for
crop production in a particular
location depends critically on soil
characteristics. Many fertilizer
recommendations are based on
estimating the phytoavailability of
mineral elements in the soil
Improvement of crop quality
In order to improve the ability of
crops to acquire mineral elements,
a number of strategies are available.
These include better monitoring of
nutrient status, improved formulations
of fertilizers, access to alternative
sources of nutrients, better agronomy
and improved genotypes for nutrient
use and better understanding of the
physiology of uptake of nutrients
through both roots and shoots.
However, the success of any one
intervention is dependent on how
these strategies interact with the
environment in which they are
deployed and the suitability of the
management system for the specific intervention.