Nutrients
Chapter 6
Soil Analysis Ch 6 2
Plant nutrients the species that they require to obtain from outside
the plant (air, water, soil) in order to grow and survive
this chapter: the nutrients that plants gain from contact with the soil
carbon (as atmospheric carbon dioxide) and water (from the soil) not considered nutrients
without them, the plant would not grow at all
Soil Analysis Ch 6 3
Nutrients in plants move around in the plant through the course of its
growth eg corn
the major point of concentration initially is in the leaves
move towards the stalks and cobs the grain (fruit) develops last a major requirement for the nutrients mobile nutrients move from the leaves, stalks and
cobs into the fruit deficiency symptoms develop in the leaves as they
drop below the necessary nutrient content
Soil Analysis Ch 6 4
Mobility different nutrients have different abilities to move
through the plant N is very mobile, and will move easily to points where
a deficiency or need occurs when a N deficiency occurs, it will move from the
older growth to the new tissue same applies to P, K & Mg Ca & S are much less mobile when a deficiency occurs, the symptoms will appear
in the new growth classified as macro or micro on the basis of their
content in normal plants
Soil Analysis Ch 6 5
Different nutrients have different abilities to move through the plant. Nitrogen is very mobile, and will move easily to points where a deficiency or need occurs. When a nitrogen deficiency occurs, it will move from the older growth to the new tissue. The same applies to phosphorous, potassium and magnesium. Calcium and sulfur are much less mobile, and when a deficiency occurs, the symptoms will appear in the new growth.
Nutrients are classified as macro or micro on the basis of their content in normal plants: macronutrients have levels of greater than 500 mg/kg. Tables 6.1 and 6.2 lists the macro- and micronutrients and a brief description of their role in plants.
Soil Analysis Ch 6 6
Nitrogen
plants absorb all this nutrient from the soil
the most important plant nutrient
the major limitation to plant growth
Soil Analysis Ch 6 7
N cycle
N as N2(air)
N as organic N(living plant)
inorganic N(bacterial fixing)
inorganic N(soil)
fixationdenitrification
decompositionN as organic N
(dead plant) mineralisation
nitrificationimmobilisation
immobilisation
Soil Analysis Ch 6 8
Fixation nitrogen gas in the air is not soluble in water cannot be absorbed directly by any part of the plant fixation converts N2 to ammonia by an enzyme called
nitrogenase not all plants can do this those that can are known as legumes, and use a
bacteria called rhizobium it resides in the plant roots and produces absorbable
N for the plant other bacteria reside in the soil and fix nitrogen for
uptake by non-fixing plants.
Soil Analysis Ch 6 9
Mineralisation most nitrogen in the soil is in the form of organic
nitrogen, held in organic matter not available to plants bout 2% of the organic nitrogen will decompose in a
year to form inorganic (or mineralised) nitrogen as ammonia/ammonium
some plants (eg rice) are capable of absorbing ammonium ions
most prefer nitrate
Soil Analysis Ch 6 10
Nitrification ammonium converted by soil bacteria to nitrite and
then to the useful form nitrate requires oxygen; will not readily occur in compacted
or water-saturated soils pH should also be greater than 6 to encourage
nitrification nitrate is not retained on soil minerals ammonium is; provides a small reserve of nutrient in
depleted soils
Soil Analysis Ch 6 11
Immobilisation after uptake, the plant uses the N in one of the many
organic compounds that requires it most particularly protein and chlorophyll
covalently bound not be available elsewhere until the death of the plant
or the metabolism of that compound
Soil Analysis Ch 6 12
Decomposition dead plant matter becomes available as food for
organisms such as worms, and micro-organisms such as bacteria in the soil
releases nutrients such as nitrogen bound up in the plant
C:N ratio of decaying organic matter affects decomposition
dry, woody material (high ratio, eg straw) isn’t consumed by bacteria
in the absence of this source of N, will use the soil reserves
Soil Analysis Ch 6 13
Denitrification
some bacteria convert nitrate to nitrogen gas or nitrogen oxides
should be a balance between fixation and denitrification
this prevents significant runoff into groundwater adding more N (as fertiliser) creates imbalance some of this excess N ends up where it is not wanted in the waterways, producing algal blooms and
eutrophication
Soil Analysis Ch 6 14
N problems
Low levels reduced growth yellowed leaves
Excess levels rapid growth dark leaves reduced flowering/fruit
Soil Analysis Ch 6 15
Phosphorus principal source of “new” phosphorous in its cycle is
from some rock minerals it is found in the form of phosphate organic phosphorous after bacterial conversion
Soil Analysis Ch 6 16
P cycle
Organic P(in plant)
Mineral P Phosphate(soil solution)
Insoluble phosphatecompounds and
adsorbed phosphate
weathering
immobilisationdecomposition
desorptionsolubilisation
leaching
immobilisation
Organic P(in solution)
mineralisation
fixation
Soil Analysis Ch 6 17
phosphate, is not found in soil solution to any great extent formation of very insoluble compounds with Ca, Fe
& Al adsorption onto clay
processes which are slow in reverse uptake by the plant is needs to be efficient Australian native plants have adapted to soils that are
relatively low in P introduced species (grain, fruit and vegetable crops)
need addition of phosphate in the form of fertiliser temporary increase only
Soil Analysis Ch 6 18
Testing for P
can give misleading results if the purpose of test is not made clear
total phosphorus is very different to available P
various extracting solutions have been devised to simulate the availability of the element
Soil Analysis Ch 6 19
P deficiency reduced growth purpling of green leaves death of older leaves
Soil Analysis Ch 6 20
Potassium no organic form, so simpler
Mineral (bound)
Soilsolution
Plant
Mineral (adsorbed)
1
2 3
4
56solubilisation
leaching
solubilisation
immobilisation
immobilisation
decomposition
Soil Analysis Ch 6 21
In the plant
Deficiency stalks are relatively weak and break easily yellowing and death occurs around the edges of older
leaves
Excess takes up too much, at the expense of Ca & Mg
Soil Analysis Ch 6 22
Calcium & magnesium sources and cycling similar to potassium adsorption of Ca and Mg to cation sites is greater
than of K
90% of adsorbed cations in neutral or alkaline soils will be Ca & Mg
reserve supplies are likely to be good the plant’s need for these elements is much less than
that for potassium, so deficiencies are less common
Soil Analysis Ch 6 23
Sulfur similar to nitrogen except:
atmospheric source already useable by plants only one form of inorganic S
Soil Analysis Ch 6 24
Exercise 6.2
S as SO4(air)
S as organic S(living plant)
inorganic S(solution)
fallout
decompositionS as organic S
(dead plant) mineralisation
immobilisation
Natural cycle
inorganic S(adsorbed)
immobilisation
solubilisation
Soil Analysis Ch 6 25
Class Exercise 6.3 In each of the nutrient cycles in this chapter, one exit
point for the element is not included. It is one of the major reasons that extra nutrient must be added to domestic and commercial soils. What is it?
removal of plant material for human consumption