Teaching Organic Farming and Gardening

Soil Chemistry and Fertility

22 | Unit 2.2
Students’ Lecture Outline

phosphorus deficiency symptoms include:

• Slow growth, stunted plants

• Purplish coloration on foliage of some plants

• Dark green coloration with tips of leaves dying

• Delayed maturity

• Poor grain, fruit or seed development

c) the phosphorus cycle (see Figure 4)

phosphorus is not easily leached from the soil as it remains bound to soil particles. Consequently,
the main losses of p from the system are either by export of crops or by soil erosion. most
phosphate pollution of lakes and streams is from sediment that is high in p.

d) phosphorus in soils and factors affecting its availability

phosphorus is taken up as h 2 pO 4 - or hpO 4 2- or pO 4 3- depending on ph. h 2 pO 4 - is more available in
very acid conditions while pO 4 3- is more available in very alkaline conditions. most of the total p
in the soil is tied up in compounds with limited solubility. In neutral to alkaline soils, it is usually
bound as calcium phosphate, while in acid soils it forms iron and aluminum phosphates. maximum
availability occurs between ph 6.5 and 7.2.

Some soils are capable of fixing (binding nearly irreversibly) phosphorus when the p gets bound
with reactive forms of iron, aluminum, and manganese. the fixing capacity of a soil is largely
dependent on the amount and types of clay present in the soil. Clays made up of iron, aluminum,
and manganese oxides or allophane have high p fixing ability. these clays are commonly found in
warm humid climates (oxisols and ultisols) and areas affected by volcanic ash (Andisols). Usually
these conditions are dealt with by adding enough p to the soil to satisfy its p fixing ability.

figUre 3. the nItROGen CyCLe (ADApteD FROm mIChIGAn StAte UnIVeRSIty extenSIOn BULLetIn e-2646)

Livestock

Animal

manure

Urea

fe rtilizers

Ammonium

and nitrate

fe rtilizers

Ammonia

(NH 3 )

NO

N O 2

N 2

Ammonium

(NH +

4 )

SOM (organic nitrogen)

and Microbial biomass

Nitrate

(NO 3 - )

MI

NE

RA

LI

ZATIO

N

IMMOBILIZ

AT

ION

PL

AN

T U

PTAK

E

NITR

IFICA

TION

LEA

CHIN

G

DE

NI

TR

IFI

CA

TI

ON

PLA

NTUP

TAK

E

BIOLOGICAL

NITROGEN

FIXATION

RUNOFF,

EROSION

PRECIPITA TION

HARVEST

AMMONIA

VOLITALIZATION

Root

Aboveground

residue

residue

Cathy Genetti Reinhard