Soil Fertility Management

Test Study

What are the four macronutrients needed by plants which impact production the most?

N,K,S,P

Of the four major macronutrients, which is needed in the least quantities?

Phosphorous

Which two macronutrients have the greatest impact on rapid plant production regardless of the quantities needed?

N and P

In which soils is P particularly problematic?

Strongly weathered tropical soils Ultisols and Oxisols)

In what form is Phosphorous always present?

As the oxyanion Orthophosphate or its polymers

In combination with which organic group is Phosphorous most important in the biosphere and why?

Adenosine for the energetics and productivity of the biosphere

How is Phosphorous used for energetics?

The energy associated with forming and breaking the bond between phosphate groups is universally used to store energy in the short term and then release it for life reactions

When P supply is low what impact does that have on the solar energy captured by plants?

It is limited

True or false, Phosphate grups and their bonds are important to structural components of DNA and RNA

True

What represents the only significant reserve of P for plant growth?

The soil

Where was P in soil originally derived from?

Minerals formed within igneous rocks

What is the average concentration of P in soil solution?

10^-5 M

In which soil fraction is P contained in temperate regions?

The sand fraction

What is the general Phosphorous content of soils in temperate regions?

0.02-0.5%, topsoil average 0.05% or 100kg P/ha

Where are primary P minerals found?

Igneous rock

On what timescale is P weathered from sand fraction in soil?

Geological time

When the sand fraction P weathers, what does it produce?

Soluble P and then secondary P minerals

What are the most common primary P minerals?

Apatites

What is Ca10(PO4)6F2?

Flourapatite, present in igneous deposits and the enriched phase of the secondary mineral carbonatites

Is Flourapatite reactive?

No

How is Flourapatite used commercially?

It’s mined and acidulated to make P fertiliser

What is Ca10[PO4]6-x(CO3)x(F,OH)2-x?

Francolite, a carbonate substituted apatite formed in sedimentary rocks in which CO3²- replaces some PO4³-

What is Francolite used for commercially?

It is used as a reactive phosphate rock for direct application to pastures and in fertiliser manufacture

What is RPR?

Reactive Phosphate Rock

What makes organic P?

Vegetation

Why might apatite not be present in topsoils?

It has been weathered out over thousands of years, and now vegetation is producing organic P

Why is apatite weathering more complete in the top soil?

Because there is more H+ generation and more organic acids and chelates, and a greater sink for Calcium cations therefore increased CEC and plant uptake. Ca10(PO4)6F2+12H+=6H2PO4- + 10jCA²+ +2F-

Why is Francolite used as an RPR but Flourapatite and hydroxyapatites are not?

Flourapatite and hydroxyapatites are unreactive and dissolve too slowly in most soils with a pH of 5-6 and therefore don’t provide sufficient plant available P for rapid plant growth

At what pH can Francolites begin to dissolve?

5.5

What are the parent materials for reactive iron and aluminium oxides?

ferromagnesium minerals and micas

How does Phosphorous interact with iron and aluminium oxides and in what form?

Phosphate from apatites will react strongly with oxides to form secondary phosphate minerals

What is non occluded Phosphorous?

Phosphorous which is adsorbed onto exchange site surfaces which can be moved back into solution when concentrations drop

What is occluded Phosphorous?

Phosphorous which has been absorbed by hydrous oxides or precipitated

What is the problem with occluded Phosphorous?

It cannot be released into soil solution unless a chemical reaction occurs.

What is mainly responsible for P sorption in most soils and lake sediments?

Fe and Al hydrous oxides

What is a ligand reaction?

An ion or molecule attached to a metal atom by donating a pair of electrons to the central metal atom or ion to form a coordination complex

What is a ligand reaction?

A chemical reaction in which a ligand in a compound is replaced by another

Which occurs first, surface adsorption of P or absorption of P?

Surface adsorption

How are ligands incorporated into surface metal ions?

They form part of the coordination shell

Which anions of interest to plant growth do not react through ligand exchange in the same way as Phosphorous?

Cl- and NO3-

Ligands are attached directly to __________ and held __________

Soil surface, very strongly (specific adsorption)

Are ligands part of the diffusion layer?

No, they are not involved in non-specific adsorption

How is P retention measured in the soil?

A P retention test AKA the Anion Storage Capacity

What does the ASC capacity tell us?

The relative ability of a soil to adsorb P

What determines the ability of a soil to immediately replenish solution P which has been taken up by plant roots?

The equilibrium conditions between the amount of P adsorbed on a soils surface and the amount in the solution

Which soils have high P retention?

Allophanic soils

What kind of fertiliser P does an allophanic soil need?

High amounts of a soluble P

Which soils have low P retention?

Pallic soils

What is the most common test used in NZ for plant available P?

The Olsen P test

What roles does P play in the plant other than DNA and RNA structure?

Cell division, root growth, flowering, fruiting, and seed formation

What % of DM does P occupy in plants?

0.1-0.5%

At what levels is a pasture deficient in P?

Ryegrass or clover: <3%

Cereals: <0.2%

What are some symptoms of P deficiency?

Stunting, restricted root systems, narrow stems, reduced fruit and seed formation

In what form is P taken into the roots?

H2PO4-

Below pH7, what is the main phosphate anion?

H2PO4-

How is P lost in the soil?

Runoff or soil erosion, recycled into organic matter, or strongly absorbed/occluded

In developed soils, what does total soil P consist of?

~50% inorganic P and ~50% organic P

Where is most inorganic P stored in the soil?

Absorbed to clay minerals, unavailable to plants

What does non-labile mean?

not easily broken down

How is most of the P returned by grazing animals?

POOP

How is P taken up by the roots?

Diffusion along a concentration gradient

What are the important factors influencing P uptake by plant roots?

Soil solution P concentration

Soil water content

Anion Storage Capacity of the soil

Soil pH

Root development and other plant adaptations

Is occluded P able to be extracted by an Olsen P test?

Nope

What is the P retention % of an Allophanic soil?

95%

What is the P retention % of a Brown Soil?

60%

What is the P retention % of a Pallic soil?

20%

How much P fertiliser is needed per ha of an Allophanic Soil to rais Olsen P by 1mg/L?

18 kg P/ha

How much P fertiliser is needed for a Pallic soil to raise Olsen P by 1mg/L?

4 kg P/ha

What are three ways that plants have adapted to low P conditions?

1. Root architecture changes: primary root is reduced, lateral root formation and growth is reduced, root hair production and length are increased

2. Release of acids from roots: some plants release acid from their roots to increase the availability of P in the soil close to the roots

3. Symbiotic mycorrhiza: increases the zone of access to find more P

What does a Total P (TP) Soil test measure?

It’s a multiple step P fractionation test set which can be used to determine the proportion of both labile and non-labile forms of P in the soil

TP concentrations are high in what layer of pasture soil?

The top few centimetres

What are the units used to display results in an Olsen P test?

Mg P/L soil or mg P/kg soil

What is an Olsen P test and what does it measure?

It uses a 0.5 M Sodium Bicarbonate at a pH of 8.4 as an exacting solution to provide an indication of plant available P

What P does an Olsen P test not measure?

non-labile P

How is P lost in the soil?

Immobilisation into Soil Organic Matter and absorption onto clay minerals rich in Fe and Al hydrous oxides

Animal transfer

Product loss

Soil erosion/slips

Loss to water via surface runoff

True or False: P is easily leached down the soil profile because it has a net negative charge

False, leaching is minimal for P

How much P is lost per ha from surface runoff on average?

less than 2kg P/ha

Why is surface runoff of P a problem?

Loss of P, but also it is a significant contribution to surface freshwater P concentrations and declining water quality

What is the River Standard for Dissolved Reactive P (DRP)?

0.006-0.015 mg/L (ppm)

What is the typical concentration of DRP in surface runoff water from dairy pastures?

0.5-3 mg P/L

What is the concentration of DRP in drainage water from dairy pastures on average?

0.2 mg P/L

What factors increase the rate of P loss from surface runoff?

Higher rainfall intensity, soils with poor drainage, steeper slopes, lower soil ASC, higher soil P levels (Olsen P), runoff occurring soon after grazing (1-2 days)

What is N important for in plants?

Amino acid formation

Chlorophyll and photosynthesis

Root formation and function

Plant enzymes, growth and development

How does N deficiency present differently from P deficiency in plants?

Chlorosis is general not at the edges of leaves

What is the atmospheric half life of nitrous oxide?

over 100 years

What is the half life of methane?

12 years

Which nutrient is needed in the greatest quantity?

Nitrogen

When is ammonia volitilisation more of a problem?

Hotter dryer summer

When would applying AGROTAIN be less effective?

Cool, wet winter

What is AGROTAIN?

A urease inhibitor

What enzyme is involved in ammonia volatilisation?

Urease

What are the key factors affecting ammonia volatilisation?

Urea concentration

Soil moisture

Temperature

Soil CEC

pH

pH buffering capacity

Will a soil with higher or lower CEC have a better pH buffer capacity?

higher CEC

True or false: ammonia volatilisation happens more in cooler temperatures

False

Why is ammonia volatilisation worse in dry soils?

If the soil is dry the urea won’t dilute quickly and there are areas of high concentration which volatilise.

When are N losses through leaching highest?

In the winter when there’s less evapotranspiration

What crop type runs the higher risk of N leaching?

Winter crops

What is one way to reduce winter crop N leaching?

Split applications of N fertiliser

True or False: dairy farmers only have to worry about winter grazing to manage N leaching

False, research has found that the leadup from late summer through winter is important for N leaching management

What are the key transformative processes in the Nitrogen cycle?

1. mineralisation

2. ammonia volatilisation

3. nitrification

4. denitrification

5. plant uptake

6. immobilisation