1. Nutrients
Chapter 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
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)
(soil)
fixation
denitrification
decomposition
(dead plant)
mineralisation
nitrification
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. compounds and adsorbed phosphate
P cycle
Organic P
(in plant)
Mineral P
Phosphate
(soil solution)
Insoluble phosphate
compounds and adsorbed phosphate
weathering
immobilisation
decomposition
desorption
solubilisation
leaching
(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 Plant decomposition3 2
decomposition
immobilisation 4
Mineral (bound)
immobilisation
Mineral (adsorbed)
Soil
solution 1
solubilisation 5
solubilisation 6
leaching
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
decomposition
(dead plant)
mineralisation
immobilisation
Natural cycle
(adsorbed)
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
Soil Analysis Ch 6