The Problem of Soil and Land Degradation

P.C. Wall

CIMMYT®

Two major types of degradation:

• Soil erosion• Fertility decline

These are linked as erosion generally removes the most fertile soil, and the processes which lead to highly erodible soils are caused largely by soil fertility decline.

Effect of Tillage on Soil Erosion

Brazil (Sidiras)

Paraguay (Venialgo)

Conventional tillage 68.2 22.9Chisel plough 55

Soil Loss (t/ha/yr)

Zero tillage 6.9 0.5

Soil fertility degradation

What is a fertile soil?

A fertile soil allows the crop to produce close to the maximum dictated by the environment (moisture and radiation) … provided other

management factors are optimal.

Wheat Yield Over Time

Years of continuous agriculture

Yie

ld (t

/ha)

0-0-0

120-60-0

Argentinian Pampa

Chemical Fertility

Biological Fertility

Physical Fertility

Biological Fertility

The Triangle of Soil Fertility

The forgotten parts of soil fertility

Profitable

Not Profitable

Years of Continuous Agriculture

Rel

ativ

e Pr

ofita

bilit

y (%

)

100

0

Decline of Crop Yields and Profitability over Time using Conventional Agriculture

Cost of production

Problems Caused by Soil Physical Degradation

• Weak aggregates• Lower soil porosity• Soil crusting and surface sealing• More soil compaction

Copied from Elwell, 1989

Chemical Fertility

Physical Fertility

Biological Fertility

S.O.M.

The Triangle of Soil Fertility

Soil Organic Matter (soil carbon) is linked to all aspects of soil fertility and Soil Quality

Place Soil Virgin or Pasture

Cultivated Years of cultivation

Itapúa, Paraguay Alfisol 4.5 1.5 Pampa, Argentina Argiudol 4.4 2.8 Paraná, Brasil Oxisol 5.0 2.0 Santa Cruz, Bolivia Inceptisol 3.1 1.4 2

Effect of Conventional Agriculture on

Soil Organic Matter (%)

Time

Raindrop effects (from Elwell, 1989)

Effect of Conventional Agriculture on

Water Infiltration Rates (mm/h)

Place Soil Virgin or Pasture

Cultivated Years of cultivation

Brasil (animal traction) Oxisol 136 31.3 7 Brasil Oxisol 136 0.2 20 Brasil (4 yrs. ZT) Oxisol 136 7.5 24

The surface crust impedes both water infiltration and gas interchange – especially oxygen.

And so this woman farmer knows she will get benefits from breaking the crust – but that involves a lot of work!

More rapid crop development

High Temperatures

Faster oxidation of organic matter

High intensity rainfall

Surface sealing

Surface crusting

Reduced plant populations

Reduction in Soil Structural Stability

Reduced water infiltration

Water stress

More crops per year

More tillage

Less root exploration

Soil compaction

Waterlogging

Soil erosion

High Temperatures

Faster oxidation of organic matter

High intensity rainfall

Surface sealing

Surface crusting

Reduced plant populations

Reduction in Soil Structural Stability

Reduced water infiltration

Water stress

More tillage

Less root exploration

Soil compaction

Waterlogging

Soil erosion

High Temperatures

Faster oxidation of organic matter

High intensity rainfall

Surface sealing

Surface crusting

Reduced plant populations

Reduction in Soil Structural Stability

Reduced water infiltration

Water stress

More tillage

Less root exploration

Soil compaction

Waterlogging

Soil erosionThe Vicious Circle

Chemical Fertility

Biological Fertility

Physical Fertility

Biological Fertility

The Triangle of Soil Fertility

Carbon lost as CO2 contributes to Global Warming

TILLAGE-INDUCED CO2 "FLUSH" AND CURRENT CROP RESIDUE19 days after tillage

249

129.5106.6 99.8

49.9

MP

MP+DH2 DH CP NT

TILLAGE TYPE

0

50

100

150

200

250

300

TOTA

L C

LO

ST IN

19

DA

YS (g

C/m

)2

185 g C/m from residueof previous wheat crop

2

1079.2

2686.1

0

500

1000

1500

2000

2500

3000

3500

NT MP

Cum

ulat

ive

CO

2 Flu

x ( g

CO

2 m-2)

Long Term Study July 14 - October 9, 1998Cumulative loss after 87 days (avg of 4 reps)

" A nation that destroys its soil destroys itself."

-- Franklin D. Roosevelt, 1937