nutrients chapter 6. soil analysis ch 62 plant nutrients the species that they require to obtain...

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