nutrients in soilsbmrg.org.au/files/1513/9961/5894/nutrient_dynamics_in...limit varies with soil...

Queensland Alliance for Agriculture and Food Innovation

Mike Bell QAAFI, Univ of Qld

Nutrients in soils


Nutrient dynamics in healthy soils Soils can be considered both a source (provider) and a

sink (storage area) for nutrients Those nutrients can be in either organic or inorganic

(mineral) materials, derived from local geological processes or imported from off site.

The organic materials can be inert (soil organic matter), recently deceased or alive and ‘kicking’

The inorganic materials range from ions dissolved in soil water, to native minerals with varying solubilities and synthetic bag fertilisers

Adequate nutrient supplies are a key requirement of a healthy soil


We need to understand both nutrient behaviour in soils and nutrient cycling in different land

management systems if we are to manage these nutrients.


Let’s consider natural systems

Undisturbed Groundcover Little net nutrient removal Ecosystems developed in

response to soil types and climate

We have changed these markedly, with those changes potentially impacting on soil health


Landscapes now used to produce and export food or fodder

Modification of original vegetation

More consistent product removal, taking nutrients with it

Less recycling In the case of cropping,

short intense demand periods interspersed with fallows

Nutrient depletion can be rapid


In summary….

The relative sizes of the input and output arrows are critical…especially the red ones!


A case study – Darling Downs

Cropping property with records from the mid 60’s

Productivity has increased hugely

Nutrient demand has risen as native reserves have been depleted

Fertiliser use (esp N) has increased to compensate

We need to consider removal rates to see how sustainable all this is

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1960 1970 1980 1990 2000

Yiel

d (t/

ha)

Wheat Sorghum

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1960 1970 1980 1990 2000

N f

erti

lise

r ap

pli

ed (

kg/h

a)

Wheat Sorghum


Nutrient removal in crop produce

…………. In grain (kg/t) N P K Ca Mg S Peanut (pods) 32 3 10 1 2 2 Maize 15 3 3 <0.1 1 1 Soybean 68 5 21 2 3 3 Wheat 23 4 4 1 1 2 Sorghum 17 2 3 1 1 2

Assume legume N comes from fixation (ie. free)


Nutrient removal in hay ……. Crop residues or forages (kg/t dry) N P K Ca Mg S Lucerne 30 2 24 10 3 3 Oats 13 2 17 2 1 1 Silage corn 12 2 15 2 2 1 Peanut (tops) 20 1 20 15 4 2 Soybean (trash) 5 1 10 10 5 4


Some rough calculations provide some big numbers…

Nutrient removal (kg/ha) in hays ….. N P K Ca Mg S 18 t/ha Lucerne 540 36 430 180 49 47 6 t/ha Oats 78 10 102 14 7 7 10 t/ha Silage corn 120 19 150 21 24 10 2 t/ha Peanut hay 40 3 40 30 8 4 2 t/ha Soybean trash 10 1 20 20 10 8


Putting this in $ terms…

Nutrient to be replaced ($/ha) ….. N P K Ca Mg S 18 t/ha Lucerne - $700/ha 6 t/ha Oats $230/ha 10 t/ha Silage corn $360/ha 2 t/ha Peanut hay - $70/ha 2 t/ha Soybean trash - $45/ha


We are still mining nutrients!. Budgets were calculated by subtracting nutrients removed

in grain from fertilizers added on farms across the region. Budgets were derived for individual crops and for ‘typical’

crop rotations over a number of years Data are shown for N, P and K

Comparisons over a rotation sequence of 5 crops

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-50

0

50

CentralHighlands

DawsonCallide

EasternDowns

Goondiwindi-Moonie

LiverpoolPlains

Moree-Narrabri North Star

WesternDowns

Def

icit

or s

urpl

us (k

g/ha

)

N P K

Comparisons over a rotation sequence of 5 crops

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-250

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-150

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0

50

CentralHighlands

DawsonCallide

EasternDowns

Goondiwindi-Moonie

LiverpoolPlains

Moree-Narrabri North Star

WesternDowns

Def

icit

or s

urpl

us (k

g/ha

)

N P K


Where have these nutrients come from??

Forms – depends on the nutrient. Most N and S has come from organic matter,

decomposed by microbes (see next slides on how release happens).

Most other nutrients have come from soil minerals (eg. P, K, Ca, Mg, Zn etc). Most soils have (had?) some minerals that can dissolve to help meet plant demand.

In both cases, a good analogy is a credit card. The credit limit varies with soil type, but eventually deposits (manures, fertilisers, legumes) have to balance withdrawals.


Where have these nutrients come from??

Position – depends on the nutrient. Most N and S are in the topsoils with organic matter. However, once ‘mineralised’ by microbes, N and S can move with water into deeper soil layers

Most other nutrients are much less mobile. They stay near where they are put (or dissolved). Most net depletion is in the subsoil.

Roots deplete subsoils; residues and fertilisers replenish topsoils, with only some nutrients able to get back into subsoils quickly


Breakdown of plant material by microbes

Microarthropods

(Collembola, Mites)

Mesofauna Macrofauna

Macroarthropods

Earthworms Enchytraeids

Shredders Shredders & buriers

Fungi Bacteria

Microflora

Protozoa Mycophagous

Amoebae

Bacteriovorus Nematodes

Fungal-feeding Nematodes

Microfauna

Predators, parasites etc

All groups ‘leak’ nutrients (death or excretion)


Eg. N release from fungal or bacterial decomposition of residues

PLANT RESIDUE

Bacteria (C/N = 4)

Fungi (C/N = 8)

Bacterial-feeding nematodes (C/N = 6)

Fungal-feeding nematodes (C/N = 9)

Nematodes excrete excess N in mineral form for use by plants


Key points about the position of nutrients in the profile

Remember nutrients behave differently in soil

N and S are mobile in water, so can move into deeper soil layers

P and K are not mobile, so stay where they are put

All have to be where plant roots can get at them to grow a good crop

N and S – Mineralised in topsoil, then move in water

P and K – Depleted in subsoil (native fertility); Topsoils replenished by fertiliser and stubble


What does this mean for our soils?

Much more of our soil nutrients are now in the topsoil layers

When those layers are dry, crops can’t get at them and there is less underneath.

When we have an erosion event, we can lose both organic matter and immobile nutrients.

In essence, our soils have lost some of their natural resilience


Key points about plant roots

Top 10cm – highest root density if wet, but can be dry for long periods

Next 20-30cm – Wetter for longer, high root density. NB for supply of nutrients during dry periods

Subsoil (30-90cm) – Variable depth of wetting, much lower root density. NB for water and NO3-N during dry periods


What are our management options?

Balance our nutrient removal with inputs (fertilisers, manures, use of legumes)

Put those nutrients where they will do the most good (deep for immobile nutrients)

Try to rebuild soil organic matter (a nutrient store and driver of soil biology)


The way we place immobile nutrients is critical for crop access – especially in subsoils

If we put immobile nutrients in bands, we have to get lots of roots in and around those bands

Alternately we need to mix the nutrients better - lots of bands or lots of tillage

The nutrient mix in the band can encourage roots to exploit those nutrients. N and P help root activity

Band with root proliferation

Dilute nutrient through a large soil volume

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Maize Cotton

K up

take

(g)

Control 80 kg/ha KCl, 20 kg/ha RbCl80 kg/ha KCl, 20 kg/ha RbCl + 30 kg/ha TSP 80 kg/ha KCl, 20 kg/ha RbCl + 48 kg/ha MAP


But be realistic about changing soil organic matter….

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0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Bulk density (g/cm3)

Am

ount

of c

arbo

n in

the

0-10

cm

laye

r (M

g C

/ ha)

1% SOC 2% SOC 3% SOC 4% SOC 5% SOC

10

20

Amount of C required: 10 t C/ha 24 t Dry Matter (DM) This is in addition to what is returned already Remember also that only a fraction of C added is actually retained


You will also tie up nutrients if you increase soil organic matter

Assumptions: C/N =10 and C/P=120)

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0.0 0.2 0.4 0.6 0.8 1.0 Change in soil carbon

(% of soil mass)

Am

ount

of N

(kg

/ha)

BD = 1.0

BD = 1.6

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140

0.0 0.2 0.4 0.6 0.8 1.0 Change in soil carbon

(% of soil mass)

Am

ount

of P

(kg

/ha)

BD = 1.0

BD = 1.6

Increasing soil organic C by 1% will require 900-1500 kg N/ha and 70-120 kg P/ha


Maintaining soil fertility is an ongoing challenge

Business as usual won’t keep us producing food long term

Economics are against us in the short term…it is expensive to add more nutrients!

We have to get smarter… - changing our rotations to include more legumes (N) - use the lack of N fertiliser needed for legumes to free

up $ to invest in other nutrients - deep place the immobile nutrients periodically - use available water to grow things to contribute to

rebuilding soil organic matter


Specific issues arising from recent floods


Nutrient issues…

Two main processes - erosion (loss of nutrient

rich topsoil); and - loss of mobile nutrients

like N and S through leaching (both) or denitrification (N).

Floods ain’t floods! Very different implications

and management responses


Erosion of topsoil…

Impacts Loss of organic matter,

especially in top10cm – effects on N and S as well as soil biota

Removal of immobile nutrients (P, K, Zn etc)

Exposure of less hospitable subsoils (sodic, poor structure, hard setting…)

Response Know what the new topsoil

is like – soil test and apply ameliorants like gypsum if needed

Use amendments like manures to restore some OM and microbial activity

Get a crop in the ground asap. Green manure if possible

Recognize the new nutrient reality and fertilise accordingly


Example from Mundubbera

Classic case of erosive loss of (at least) approx 10cm of topsoil

Changes in pH, organic C, P, K and Zn

The ‘new’ topsoil needs some help before it will be productive again

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C (%

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No wash Wash

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DTPA

Zn

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No wash Wash


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wel

l P (m

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)No wash Wash

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No wash Wash

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pH

No wash Wash


Prolonged heavy rainfall, water logging…

Loss pathways will vary with nutrient and soil type

Effects can be more transient – nutrients leached to deeper layers can be retrieved by deep rooted crops.

If lost as a gas (denitrification) it is gone forever

You have to measure the losses to know what you are dealing with.

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0-30cm 30-60cm 60-90cm

Nitr

ate

N (m

g/kg

)

Nov-10 Mar-11

Loss = 93 kg N/ha


Leaching or denitrification…

Impacts Leached but not gone -

delayed crop access to reserves of plant available N and S

Leached beyond root zone – more likely N than S, and effectively gone forever

Denitrified – only N, but definitely gone forever

Can leach undesirable salts like chloride too!

Response Know what the new soil

profile is like – soil test to see where the mobile nutrients have gone to

If just leached into deeper layers, perhaps change your starter fertiliser to add some S and get enough N on pre-planting

If gone forever, increase N rates or plant legumes in the short term


Example from Boobyjan

Little evidence of wash/erosion

Sites with contrasting periods of inundation

Very little mineral N (all lost??), so hard to use that as an indicator of denitrification or leaching

However chloride and sulphate-S data tell a story of leaching of mobile nutrients with extended inundation

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Chlo

ride

(mg/

kg)

Short inundation Long inundation

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Chlo

ride

(mg/

kg)

Short inundation Long inundation


In some cases, flooding can possibly bring benefits (deposition)

Whole civilizations have subsisted on ‘fertility’ deposited on large river deltas in the form of ‘silt’ (someone else's topsoil!)

A possible example from a sandier soil near Mundubbera

Did we gain some silt here, or were the paddocks different to start with?? I suspect the latter.

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Not flooded Flooded

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Queensland Alliance for Agriculture and Food Innovation 3

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Not flooded Flooded

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Not flooded Flooded


In summary..

Recovering post-flooding can be a long term process Soil testing is the key, to work out what has changed, and

by how much Once you have that information you can design a strategy

to return the land to productivity The cost and effectiveness of any strategy relies on

knowing what you are trying to achieve, and the key success factors


Questions??

nutrients in soilsbmrg.org.au/files/1513/9961/5894/nutrient_dynamics_in...limit varies with soil...

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