100 713 Land Resources and Environment in Sustainable Agriculture

Soil fertility

Patma Vityakon Rambo, PhD

Soil fertility studies

Soil fertility is the study of soils as a source of plant nutrients. The objectives of studying soil fertility is to learn about ‘behaviour’ of plant nutrients in soils, and how soils supply nutrients to plants. To achieve the above objectives, two major concepts are employed.

Concepts employed for soil fertility study:

• Soil is a colloidal system.

• The availability concept: Available forms of nutrients and nutrient availability.

Figure1. Profile of soil colloidal system as related to soil nutrients and their absorption by roots

Soil solution

Nutrient ions in soil solution Soil

colloid

Nutrient ions adsorbed to soil colloid

Nutrient absorption by root

Nutrient ion being transported to root

Ro

O

O

T

The availability conceptThe words used most commonly to describe the supply the plant nutrients in soil are available and availability.Available means susceptible to absorption by plants.Availability means effective quantity (Black, 1993)

Available forms of nutrients

Nutrients exist in soils in different forms, such as water soluble, adsorbed, and fixed forms. These nutrients become available to plants to different degrees but with time it can be said that all will become available. However, the most readily available form is those in soil solution (water soluble form).

Availability factors

• The ability of soils to supply nutrients to plants is affected by the following factors:

- Intensity- Quantity- Buffering capacity- Mobility

Further reading on the availability concept

• Black, C.A. 1993. Soil fertility evaluationand control. Lewis Publishers. 746 p.

Nitrogen in soils

Table 1. Distribution of N in atmosphere, geosphere and biosphere.

--------------------------------------------------------Pools of N Total mass % total N mass

(x 1019)--------------------------------------------------------------------------Atmosphere 390 1.9

(N gas)Hydrosphere 2.3 0.01

(soluble N)Lithosphere 19,085 >98

Igneous rock 19,000Biosphere 0.2

0.001Terrestrial ecosys 0.0682(N in soil, plants)

Marine ecosystem 0.1314---------------------------------------------------------------------------Adapted from Haynes (1986a), Stevenson (1986)

Global Fluxes of Nitrogen into and out of the Terrestrial BiosphereProces

s rate(Tg N yr-1 )Inp

utsWet and dry deposition (NH3)/NH4+ )Wet and dry deposition (NOx )Wet and dry deposition (organic N )Atmospheric fixation (lightning )Biological fixationIndustrial fixation (fertilizers)

90-20010-1000.5-300100-2006

0

30-80

Outputs

Process rate(Tg N yr-1 )

Ammonia volatilizationDenitrification(N2 + N2O)Biogenic NOx productionFossil fuel burning (NOx)Fires (NOx)Leaching and runoff (inorganic)Leaching and runoff (organic)

36-25040-3501-1510-2010-205-205-20

Source: Haynes (1986)

Table 2. Distribution of N in a nutrient-poor Amazonian rain forest, an oak-hickory forest, a short grass prairie, and a wet meadow tundra ecosystem.

Amazonianfore

st

Oak-hickoryfore

st

Short grassprairie

Meadowtun

draNitrogen pool

kg N ha-1

% kg N ha-1

kg N ha-1

kg N ha-1

% % %117

9

336

843

132

785

(75 cm)

209

56.2

16.2

40.0

6.3

37.7

492

357

135

274

5080

(60 cm)

584

8.4

6.1

2.3

4.7

86.9

66

15

51

76

3374

(30 cm)

351

0.92

0.24

0.68

0.10

91

(20 cm)

92

1.9

0.4

1.5

2.1

96.0

1.0

0.3

0.7

0.1

98.9

Total vegetationAbove groundRoots

Litter

(sampling depth in cm)Tota

l

Soil

Source: Haynes (1986)

Figure 2. The Nitrogen cycle

Table 3. Global fluxes of nitrogen into and out of the terrestrial biosphere. Process

rate(Tg N yr-1

)Inputs Wet and dry deposition (NH3 )/NH4+

)90-200

Wet and dry deposition (NOx ) 30-80Wet and dry deposition (NOx ) 10-

100Atmospheric fixation (lightning )0.5-300Biological fixation 100-200Industrial fixation (fertilizers) 6

0OutputsAmmonia volatilizationDenitrification (N2 + N2O)Biogenic NOx productionFossil fuel burning (NOx)Fires (NOx)Leaching and runoff (inorganic)Leaching and runoff (organic)

36-25040-350

1-1510-20

10-205-205-20

Nitrogen cycling in different ecosystems

• Natural ecosystems, e.g. forest

• Agricultural ecosystems

Plant tops

and

roots

Soil

reserves

Fertilizer112 NIrrigatio

n 10 N

Leaching

15 N

Runoff

16 N

Gaseous losses

15 N

Return of plantresidues 41 N

Plant uptake 126 N

Product removal

85 N

Figure 3. N cycle in a corn crop in USA (values in kg N/ha) (Haynes 1986)

Forms of N in soils• Organic nitrogen: More than 90% of total

N in soils is organic N.

• Inorganic nitrogen: Major forms:

NH4+ (soluble, exchangeable, non-exchangeable)

NO3 - (soluble, and exchangeable)

Figure1. Profile of soil colloidal system as related to soil nutrients and their absorption by roots

Soil solution

Nutrient ions in soil solution Soil

colloid

Nutrient ions adsorbed to soil colloid

Nutrient absorption by root

Nutrient ion being transported to root

Ro

O

O

T

Soil N pools

Quantity (% soil N)Acid-insoluble or

non-hydrolysableAcid hydrolysable

-NH3-N- Amino acid- Amino sugar- Hydrolysable unknown-N or HUNSource: Stevenson

(1982)

20-3520-3530-455-1010-20

Table 4. Pools of soil organic N as chemically separated by acid hydrolysis.

Fig Hypothetical structure of humic acid. Nitrogen is incorporating into the humic acid in three ways: (a) as a bridge unit, (b) in the form of N – phenylamino acid, (c) in the

Soil processes which transform organic N to inorganic N

• N mineralization : organic N is transformed to inorganic N.

• N immobilization: inorganic N is transformed to organic N.

Both are microbially mediated.

Mineralization Immobilization

Nitrogen mineralization and immobilization occurs simultaneously in soils but in opposite directions. Higher N mineralization relative to N immobilization results in net N mineralization and vice versa. This is affected by the relationships between energy and nutrient nitrogen in decomposing organic materials.

Carbon to nitrogen ratio

The C/N ratio is used as an indicator of the likelihood of net N mineralization or net N immobilization taking place upon decomposition of organic materials or soil organic matter in question.

The C/N ratio estimates the energy/N ratio of organic materials.

Figure 4. Schematic diagram of the different forms of cations associated with a 2:1 clay mineral.

Loss of nitrogen from soils

This can occur through different processes:

• Ammonia volatilization (Sherlock, 1986)• Denitrification • Leaching (especially of nitrate) (Cameron

and Haynes, 1986)

Adsorbed NH4+

NH4+ (aq) in soil solution

NH3+ (aq) in soil solution

NH3 (g) gas in soil

NH3 (g) gas in atmosphere

(2)

(1)

(3)

(4)

Fig. 5. The various equilibria that govern ammonia loss from soils

Fig 2. Hierachical class of soil pore space accoding to Elliott and Coleman(1988).

1= macropores, 2= pore between macroaggregates

Nitrogen requirement of plants

Figure 6. Accumulation and distribution of abovegroud biomass and N in different parts of corn: A. seed, B. cobs, C. tassels, D. leaf sheath, and E. leaves (Goh and Haynes, 1986)

Evaluation of nitrogen availability

2 types of soil analysis for this purpose:

• Evaluation of residual nitrogen in soils,

• Evaluation of nitrogen mineralization potential

(Goh and Haynes, 1986)

Table2.10 Chemical extraction methods to determine soil available NExtraction

Temp.

(hr)

NMild extraction methodsWater0.01 MCaCl20.01 MCaCl20.01 M NaHCO31 M KCl2 M KCl0.01 M CaCl20.1 MBa(OH)2Source: Goh and

100°C100°C121°CRoom100°C80°C121°CRoom

16410.25

0.5

16201

Total NTotal or NH+4-NNH+

4-NTotal N or UV absorbanceNH+4 and NO3NH+4and NO3-NSoluble carbohydrate

Soluble carbohydrate

Extraction

Temp.

(hr)

N

Intermediate intensity extractionAlkaline KMnO4Na2CrO4+H3PO4 Neutral 0.5 N Na4P2O7 1 M Na OH1 M Na OH1 M H Cl 1 M H2SO4

100°C

Room

NH+

4-N

RoomRoom

100°C100°C

100°C

NH+

4-N

NH+

4-N

NH+

4-NNH+

4-NNH+

4-NNH+

4-N

0.25260.54.2261

Source: Goh and Haynes (1986)

Extraction

Temp.

(hr)

N

Intensive extraction NH+4

-NNH+4-NNH+4-N

28

Room4.5 M Na

OHK2Cr2O7+H2SO4

6 NH2SO4 Na OH

distillationWalkley-black oxidn

Source: Goh and Haynes(1986)

Further reading on soil nitrogen

• Haynes, R.J. (ed.). 1986. Mineral nitrogen in plant-soil system. Academic Press. New York, London.

• Stevenson, F.J. (ed.). 1982. Nitrogen in agricultural soils. American Society of Agronomy, Madison, Wisconsin, USA.

• Stevenson, F.J. 1986. Cycles of soil: Carbon, nitrogen, phosphorus, sulfur, micronutrients. Wiley, New York, 380 p.

Soil Organic Matter (SOM)

Table 3.1 Classification of SOM into livin non-living components.Compon

ents ofSOMComponentsof SOM

Living/non-living

Quantity%

of living

component

% total C SOM

non-living

living Ro

otsMacro-organismsMicro-organismsParticulate OMHumus

1004

5-1015-3060-80

1-398

10-3070-90

Table1. Pools of SOM and nutrients, generalized turnover rates, and hypothesized primary controls of pool sizePool tim

e

Turnover

Pool sizecontrolsUnprotec

ted(microbial biomass)

BIOLAB(labile)ProtectedCOM(colloidal protection)POM(structural protection)

2.50.25205

1000

yr+, temperateyr+, humidtropicsyr+, temperateyr+, humid tropicsyr+Depends onPhysical disturbance

Substrate availabilityResidue inputs, climateSoil mineralogy, textureTillage and aggregatedisruptio

n,soilparticle

sizedistribution

100 713 Land Resources and � Environment in � Sustainable AgricultureSoil fertility studies��Soil fertility is the study of soils as a source of plant nutrients. The objectives of studying soil fertility is to learn about ‘behaviour’ of plant nutrients in soils, and how soils supply nutrients to plants. To achieve the above objectives, two major concepts are employed.Concepts employed for soil fertility study:Slide Number 4The availability concept�The words used most commonly to describe the supply the plant nutrients in soil are available and availability.�Available means susceptible to absorption by plants.�Availability means effective quantity (Black, 1993)Available forms of nutrients��Nutrients exist in soils in different forms, such as water soluble, adsorbed, and fixed forms. These nutrients become available to plants to different degrees but with time it can be said that all will become available. However, the most readily available form is those in soil solution (water soluble form).�Availability factorsFurther reading on the availability conceptNitrogen in soils�Table 1. Distribution of N in atmosphere, geosphere and biosphere.�--------------------------------------------------------�Pools of N Total mass % total N mass� (x 1019)�--------------------------------------------------------------------------�Atmosphere 390 1.9� (N gas)�Hydrosphere 2.30.01 � (soluble N)�Lithosphere 19,085>98� Igneous rock 19,000�Biosphere 0.20.001� Terrestrial ecosys 0.0682� (N in soil, plants)� Marine ecosystem 0.1314�---------------------------------------------------------------------------Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Nitrogen cycling in different ecosystemsSlide Number 17Forms of N in soilsSlide Number 19Slide Number 20Slide Number 21Soil processes which transform organic N to inorganic NNitrogen mineralization and immobilization occurs simultaneously in soils but in opposite directions. Higher N mineralization relative to N immobilization results in net N mineralization and vice versa. This is affected by the relationships between energy and nutrient nitrogen in decomposing organic materials.Carbon to nitrogen ratio��The C/N ratio is used as an indicator of the likelihood of net N mineralization or net N immobilization taking place upon decomposition of organic materials or soil organic matter in question.�The C/N ratio estimates the energy/N ratio of organic materials.Slide Number 25Loss of nitrogen from soils��This can occur through different processes:Slide Number 27Slide Number 28Nitrogen requirement of plantsSlide Number 30Evaluation of nitrogen availability��2 types of soil analysis for this purpose:Slide Number 32Slide Number 33Slide Number 34Further reading on soil nitrogen Soil Organic Matter (SOM)Slide Number 37Slide Number 38