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The role of conservation agriculture in sustainable agriculture
Peter R Hobbs, Ken Sayre and Raj Gupta
Phil. Trans. R. Soc. B 2008 363, doi: 10.1098/rstb.2007.2169, published 12 February 2008
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Phil. Trans. R. Soc. B (2008) 363, 543–555
doi:10.1098/rstb.2007.2169
Published online 24 July 2007
The role of conservation agriculture in
sustainable agriculture
Peter R. Hobbs1,*, Ken Sayre2 and Raj Gupta3
1
Department of Crops and Soil Science, Cornell University, Ithaca, NY 14853, USA
2
CIMMYT Apdo, Postal 6-641, 06600 Mexico DF, Mexico
3
ICARDA-CAC office, P.O. Box 4564, Tashkent 700000, Uzbekistan
The paper focuses on conservation agriculture (CA), defined as minimal soil disturbance (no-till,
NT) and permanent soil cover (mulch) combined with rotations, as a more sustainable cultivation
system for the future. Cultivation and tillage play an important role in agriculture. The benefits of
tillage in agriculture are explored before introducing conservation tillage (CT), a practice that was
borne out of the American dust bowl of the 1930s. The paper then describes the benefits of CA, a
suggested improvement on CT, where NT, mulch and rotations significantly improve soil properties
and other biotic factors. The paper concludes that CA is a more sustainable and environmentally
friendly management system for cultivating crops. Case studies from the rice–wheat areas of the
Indo-Gangetic Plains of South Asia and the irrigated maize–wheat systems of Northwest Mexico are
used to describe how CA practices have been used in these two environments to raise production
sustainably and profitably. Benefits in terms of greenhouse gas emissions and their effect on global
warming are also discussed. The paper concludes that agriculture in the next decade will have to
sustainably produce more food from less land through more efficient use of natural resources and
with minimal impact on the environment in order to meet growing population demands. Promoting
and adopting CA management systems can help meet this goal.
Keywords: conservation agriculture; direct seeding; zero-tillage; rice–wheat systems; bed planting;
mulching
1. INTRODUCTION
Conservation agriculture (CA) defined (see FAO CA
web site: http://www.fao.org/ag/ca/1a.html ) as minimal
soil disturbance (no-till, NT) and permanent soil cover
(mulch) combined with rotations, is a recent agricultural management system that is gaining popularity in
many parts of the world. Cultivation is defined by the
Oxford English dictionary as ‘the tilling of land’, ‘the
raising of a crop by tillage’ or ‘to loosen or break up
soil’. Other terms used in this dictionary include
‘improvement or increase in (soil) fertility’. All these
definitions indicate that cultivation is synonymous with
tillage or ploughing.
The other important definition that has been
debated and defined in many papers is the word
‘sustainable’. The Oxford English dictionary defines
this term as ‘capable of being borne or endured,
upheld, defended, maintainable’. Something that is
sustained is ‘kept up without intermission or flagging,
maintained over a long period’. This is an important
concept in today’s agriculture, since the human race
will not want to compromise the ability of its future
offspring to produce their food needs by damaging the
natural resources used to feed the population today.
This paper will introduce and promote CA as a
modern agricultural practice that can enable farmers in
many parts of the world to achieve the goal of
sustainable agricultural production. But first, the
paper discusses some issues related to tillage.
2. CULTIVATION TECHNIQUES OR TILLAGE
The history of tillage dates back many millennia when
humans changed from hunting and gathering to more
sedentary and settled agriculture mostly in the Tigris,
Euphrates, Nile, Yangste and Indus river valleys (Hillel
1991). Reference to ploughing or tillage is found from
3000 BC in Mesopotamia (Hillel 1998). Lal (2001)
explained the historical development of agriculture
with tillage being a major component of management
practices. With the advent of the industrial revolution
in the nineteenth century, mechanical power and
tractors became available to undertake tillage
operations; today, an array of equipment is available
for tillage and agricultural production. The following
summarizes the reasons for using tillage.
(i) Tillage was used to soften the soil and prepare a
seedbed that allowed seed to be placed easily at a
suitable depth into moist soil using seed drills or
manual equipment. This results in good
uniform seed germination.
(ii) Wherever crops grow, weeds also grow and
compete for light, water and nutrients. Every
gram of resource used by the weed is one less
gram for the crop. By tilling their fields, farmers
were able to shift the advantage from the weed to
the crop and allow the crop to grow without
* Author for correspondence (ph14@cornell.edu).
One contribution of 16 to a Theme Issue ‘Sustainable agriculture I’.
543
This journal is q 2007 The Royal Society
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544
P. R. Hobbs et al.
(iii)
(iv)
(v)
(vi)
(vii)
Role of CA in sustainable agriculture
competition early in its growth cycle with
resulting higher yield.
Tillage helped release soil nutrients needed
for crop growth through mineralization and
oxidation after exposure of soil organic matter
to air.
Previous crop residues were incorporated along
with any soil amendments (fertilizers, organic or
inorganic) into the soil. Crop residues,
especially loose residues, create problems for
seeding equipment by raking and clogging.
Many soil amendments and their nutrients are
more available to roots if they are incorporated
into the soil; some nitrogenous fertilizers are
also lost to the atmosphere if not incorporated.
Tillage gave temporary relief from compaction
using implements that could shatter belowground compaction layers formed in the soil.
Tillage was determined to be a critical management practice for controlling soil-borne diseases
and some insects.
There is no doubt that this list of tillage benefits was
beneficial to the farmer, but at a cost to him and the
environment, and the natural resource base on which
farming depended. The utility of ploughing was first
questioned by a forward-looking agronomist in the
1930s, Edward H. Faulkner, in a manuscript called
‘Ploughman’s Folly’ (Faulkner 1943). In a foreword to
a book entitled ‘Ploughman’s folly and a second look’
by EH Faulkner, Paul Sears notes that:
Faulkner’s genius was to question the very basis of
agriculture itself—the plough. He began to see that the
curved moldboard of the modern plough, rather than
allowing organic matter to be worked into the soil by
worms and other burrowing animals, instead buries this
valuable material under the subsoil where it remains
like a wad of undigested food from a heavy meal in the
human stomach.
( Faulkner 1987, p. x)
The tragic dust storm in the mid-western United
States in the 1930s was a wake-up call to how human
interventions in soil management and ploughing led to
unsustainable agricultural systems. In the 1930s, it was
estimated that 91 Mha of land was degraded by severe
soil erosion (Utz et al. 1938); this area has been
dramatically reduced today.
3. CONSERVATION TILLAGE AND CONSERVATION AGRICULTURE
Since the 1930s, during the following 75 years,
members of the farming community have been
advocating a move to reduced tillage systems that
use less fossil fuel, reduce run-off and erosion of soils
and reverse the loss of soil organic matter. The first
50 years was the start of the conservation tillage (CT)
movement and, today, a large percentage of agricultural land is cropped using these principles. However,
in the book ‘No-tillage seeding’, Baker et al. (2002; a
second edition of this excellent book was published in
2006) explained ‘As soon as the modern concept of
reduced tillage was recognized, everyone, it seems,
Phil. Trans. R. Soc. B (2008)
invented a new name to describe the process’. The
book goes on to list 14 different names for reduced
tillage along with rationales for using these names.
The book is also an excellent review of the
mechanization and equipment needs of no-tillage
technologies. Baker et al. (2002) defined CT as:
the collective umbrella term commonly given to
no-tillage, direct-drilling, minimum-tillage and/or
ridge-tillage, to denote that the specific practice has a
conservation goal of some nature. Usually, the
retention of 30% surface cover by residues characterizes the lower limit of classification for conservationtillage, but other conservation objectives for the
practice include conservation of time, fuel, earthworms, soil water, soil structure and nutrients. Thus
residue levels alone do not adequately describe all
conservation tillage practices.
(Baker et al. 2002, p. 3)
This has led to confusion among the agricultural
scientists and, more importantly, the farming community. To add to the confusion, the term ‘conservation
agriculture’ has recently been introduced by the Food
and Agriculture Organization (see FAO web site), and
others, and its goals defined by FAO as follows:
Conservation agriculture (CA) aims to conserve,
improve and make more efficient use of natural
resources through integrated management of available
soil, water and biological resources combined with
external inputs. It contributes to environmental
conservation as well as to enhanced and sustained
agricultural production. It can also be referred to as
resource efficient or resource effective agriculture.
(FAO)
This obviously encompasses the ‘sustainable agricultural production’ need that all mankind obviously
wishes to achieve. But this term is often not
distinguished from CT. The FAO mentions in its CA
website that:
Conservation tillage is a set of practices that leave crop
residues on the surface which increases water infiltration and reduces erosion. It is a practice used in
conventional agriculture to reduce the effects of tillage
on soil erosion. However, it still depends on tillage as
the structure forming element in the soil. Never the
less, conservation tillage practices such as zero tillage
practices can be transition steps towards Conservation
Agriculture.
In other words, CT uses some of the principles of
CA but has more soil disturbance.
4. CONSERVATION AGRICULTURE DEFINED
The FAO has characterized CA as follows:
Conservation Agriculture maintains a permanent or
semi-permanent organic soil cover. This can be a
growing crop or dead mulch. Its function is to protect
the soil physically from sun, rain and wind and to feed
soil biota. The soil micro-organisms and soil fauna take
over the tillage function and soil nutrient balancing.
Mechanical tillage disturbs this process. Therefore,
zero or minimum tillage and direct seeding are
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Role of CA in sustainable agriculture
Table 1. Extent of no-tillage adoption worldwide. (adapted
from Derpsch 2005; includes area in India, Pakistan,
Bangladesh and Nepal in South Asia.)
country
area under no-tillage (Mha)
2004/2005
USA
Brazil
Argentina
Canada
Australia
Paraguay
Indo-Gangetic Plains ()
Bolivia
South Africa
Spain
Venezuela
Uruguay
France
Chile
Colombia
China
others (estimate)
total
25.30
23.60
18.27
12.52
9.00
1.70
1.90
0.55
0.30
0.30
0.30
0.26
0.15
0.12
0.10
0.10
1.00
95.48
important elements of CA. A varied crop rotation is
also important to avoid disease and pest problems.
(see FAO web site)
Data reported by Derpsch (2005) indicate that the
extent of no-tillage adoption worldwide is just over
95 Mha. This figure is used as a proxy for CA although
not all of this land is permanently no-tilled or has
permanent ground cover. Table 1 details the extent of
no-tillage by country worldwide. Six countries have
more than 1 Mha. South America has the highest
adoption rates and has more permanent NT and
permanent soil cover. Both Argentina and Brazil had
significant lag periods to reach 1 Mha in the early 1990s
and then expanded rapidly to the present-day figures of
18.3 and 23.6 Mha, respectively, for these countries.
By adopting the no-tillage system, Derpsch (2005)
estimated that Brazil increased its grain production by
67.2 million tons in 15 years with additional revenue of
10 billion dollars. Derpsch also estimated that at an
average rate of 0.51 t haK1 yrK1 Brazil sequestered 12
million tons of carbon on 23.6 Mha of no-tillage land.
Tractor use is also significantly reduced saving millions
of litres of diesel.
The three key principles of CA are permanent
residue soil cover, minimal soil disturbance and crop
rotations. The FAO recently added controlled traffic to
this list. Each of these will be briefly dealt with before
providing some case studies. Table 2 shows a comparison of CA with CT and traditional tillage (TT).
(a) Permanent or semi-permanent organic
soil cover
Unger et al. (1988) reviewed the role of surface residues
on water conservation and indicates that this association between surface residues, enhanced water
infiltration and evaporation led to the adoption of CT
after the 1930s dust bowl problem. Research since that
time has documented beyond doubt the importance of
surface residues on soil water conservation and
Phil. Trans. R. Soc. B (2008)
P. R. Hobbs et al.
545
reduction in wind and water erosion (Van Doran &
Allmaras 1978; Unger et al. 1988). Bissett & O’Leary
(1996) showed that infiltration of water under longterm (8–10 years) conservation tillage (zero and subsurface tillage with residue retention) was higher
compared to conventional tillage (frequent plowing
plus no residue retention) on a grey cracking clay and a
sandy loam soil in south-eastern Australia. Allmaras
et al. (1991) reviewed much of the literature on CT up
to that time and goes on to describe a whole array of
CT-planting systems operating in the US, their
adoption and benefits.
This paper will not go into detail about other soilconserving practices that are used throughout the
world and over time, like terracing or contour bunds
that are essentially designed to prevent soil erosion on
sloping lands. Lal (2001) described some of these
systems and notes that the effectiveness of these
systems depends on proper construction and regular
maintenance; if not done properly degradation can be
catastrophic.
Kumar & Goh (2000) reviewed the effect of crop
residues and management practices on soil quality, soil
nitrogen dynamics and recovery and crop yield. The
review concluded that crop residues of cultivated crops
are a significant factor for crop production through
their effects on soil physical, chemical and biological
functions as well as water and soil quality. They can
have both positive and negative effects, and the role of
agricultural scientists is to enhance the positive effects.
This paper will restrict the discussion of crop residues
to their benefits when used as mulch. Crop residues
result when a previous crop is left anchored or loose after
harvest or when a cover crop (legume or non-legume) is
grown and killed or cut to provide mulch. Externally
applied mulch in the form of composts and manures can
also be applied, although the economics of transport of
this bulky material to the field may restrict its use to
higher-value crops like vegetables.
The energy of raindrops falling on a bare soil result in
destruction of soil aggregates, clogging of soil pores and
rapid reduction in water infiltration with resulting runoff and soil erosion. Mulch intercepts this energy and
protects the surface soil from soil aggregate destruction,
enhances the infiltration of water and reduces the loss of
soil by erosion (Freebairn & Boughton 1985; McGregor
et al. 1990; Dormaar & Carefoot 1996). Topsoil losses of
46.5 t haK1 have been recorded with TTon sloping land
after heavy rain in Paraguay compared with 0.1 t haK1
under NT cultivation (Derpsch & Moriya 1999). NT
plus mulch reduces surface soil crusting, increases water
infiltration, reduces run-off and gives higher yield than
tilled soils (Cassel et al. 1995; Thierfelder et al. 2005).
Similarly, the surface residue, anchored or loose,
protects the soil from wind erosion (Michels et al.
1995). The dust bowl is a useful reminder of the impacts
of wind and water erosion when soils are left bare.
Surface mulch helps reduce water losses from the
soil by evaporation and also helps moderate soil
temperature. This promotes biological activity and
enhances nitrogen mineralization, especially in the
surface layers (Dao 1993; Hatfield & Pruegar 1996).
This is a very important factor in tropical and subtropical environments but has been shown to be a
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P. R. Hobbs et al.
Role of CA in sustainable agriculture
Table 2. A comparison of tillage, conservation tillage (CT) and conservation agriculture (CA) for various issues.
issues
traditional tillage (TT)
conservation tillage (CT)
conservation agriculture (CA)
practice
disturbs the soil and leaves
a bare surface
minimal soil disturbance and soil surface
permanently covered
erosion
wind and soil erosion:
maximum
the lowest of the three
used to reduce compaction
and can also induce it by
destroying biological pores
the lowest of the three owing
to frequent disturbance
lowest after soil pores clogged
oxidizes soil organic matter
and causes its loss
reduces the soil disturbance
in TT and keeps the soil
covered
wind and soil erosion:
reduced significantly
significantly improved
reduced tillage is used to
reduce compaction
soil physical health
compaction
soil biological health
water infiltration
soil organic
matter
weeds
controls weeds and also
causes more weed seeds
to germinate
soil temperature
surface soil temperature:
more variable
diesel use: high
highest costs
operations can be delayed
diesel use and costs
production costs
timeliness
yield
can be lower where planting
delayed
moderately better soil
biological health
good water infiltration
soil organic build-up
possible in the surface
layers
reduced tillage controls
weeds and also exposes
other weed seeds for
germination
surface soil temperature:
intermediate in variability
diesel use: intermediate
intermediate costs
intermediate timeliness of
operations
yields same as TT
hindrance in temperate climates due to delays in soil
warming in the spring and delayed germination
(Schneider & Gupta 1985; Kaspar et al. 1990; Burgess
et al. 1996; Swanson & Wilhelm 1996). Fabrizzi et al.
(2005) showed that NT had lower soil temperatures in
the spring in Argentina, but TT had higher maximum
temperatures in the summer, and that average
temperatures during the season were similar.
Karlen et al. (1994) showed that normal rates of
residue combined with zero-tillage resulted in better
soil surface aggregation, and that this could be
increased by adding more residues. Recent papers
confirm this observation; Madari et al. (2005) showed
that NT with residue cover had higher aggregate
stability, higher aggregate size values and total organic
carbon in soil aggregates than TT in Brazil; Roldan
et al. (2003) showed that after 5 years of NT maize in
Mexico, soil wet aggregate stability had increased over
conventional tillage (TT) as had soil enzymes, soil
organic carban (SOC) and microbial biomass (MBM).
They conclude that NT is a sustainable technology.
A cover crop and the resulting mulch or previous
crop residue help reduce weed infestation through
competition and not allowing weed seeds the light
often needed for germination. There is also evidence of
allelopathic properties of cereal residues in respect to
inhibiting surface weed seed germination (Steinsiek
et al. 1982; Lodhi & Malik 1987; Jung et al. 2004).
Weeds will be controlled when the cover crop is cut,
rolled flat or killed. Farming practice that maintains
soil micro-organisms and microbial activity can also
lead to weed suppression by the biological agents
(Kennedy 1999).
Phil. Trans. R. Soc. B (2008)
wind and soil erosion: the least of the
three
the best practice of the three
compaction can be a problem but use of
mulch and promotion of biological
tillage helps reduce this problem
more diverse and healthy biological
properties and populations
best water infiltration
soil organic build-up in the surface layers
even better than CT
weeds are a problem especially in the
early stages of adoption, but problems
are reduced with time and residues
can help suppress weed growth
surface soil temperature: moderated the
most
diesel use: much reduced
lowest costs
timeliness of operations more optimal
yields same as TT but can be higher if
planting done more timely
Cover crops contribute to the accumulation of
organic matter in the surface soil horizon (Roldan et
al. 2003; Alvear et al. 2005; Diekow et al. 2005;
Madari et al. 2005; Riley et al. 2005), and this effect is
increased when combined with NT. Mulch also helps
with recycling of nutrients, especially when legume
cover crops are used, through the association with
below-ground biological agents and by providing
food for microbial populations. Greater carbon and
nitrogen were reported under no-tillage and CT
compared with ploughing ( TT; Campbell et al. 1995,
1996a,b). Others have shown that this is restricted to
the surface horizons, and that the reverse occurs at
greater depths in humid soils of eastern Canada
(Angers et al. 1997). Schultz (1988) showed that C
and N declined by 6% with burning but increased by
1% with stubble retention. Vagen et al. (2005)
concluded that the largest potential for increasing
SOC is through the establishment of natural or
improved fallow systems (agroforestry) with attainable
C accumulation rates from 0.1 to 5.3 Mg C haK1 yrK1.
They continue to say that in cropland, addition of
crop residues or manure in combination with NT can
yield attainable C accumulation rates up to
0.36 Mg C haK1 yrK1. SOC is a key indicator of soil
quality and Lal (2005) calculated that increasing SOC
by 1 Mg haK1 yrK1 can increase food grain production by 32 million Mg yrK1 in developing countries.
Heenan et al. (2004) in Australia showed that changes
in SOC at the surface ranged from a loss of 8.2 t haK1
for continuous tilled cereals and residues burnt to a
gain of 3.8 t haK1 where stubble was retained and soil
no-tilled. Nitrogen content followed similar trends. If
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Role of CA in sustainable agriculture
the rotation included a legume, SOC accumulation
was the highest.
Soil microbial biomass (SMB) has commonly been
used to assess below-ground microbial activity and is a
sink and source for plant nutrients. Amendments such
as residues and manures promote while burning and
removal of residues decrease SMB (Doran 1980;
Collins et al. 1992; Angers et al. 1993a,b; Heenan
et al. 2004; Alvear et al. 2005). Balota et al. (2004) in
Brazil in a 20-year experiment showed that residue
retention and NT increased total C by 45% and SMB
by 83% at 0–50 cm depth compared with TT. Soon &
Arshad (2005) showed that SMB was greater with NT
than TT by 7–36%; frequent tillage resulted in a
decrease in both total and active MBC. Increased SMB
occurs rapidly in a few years following conversion to
reduced tillage (Ananyeva et al. 1999; Alvarez &
Alvarez 2000). Increased MBM increased soil aggregate formation, increased nutrient cycling through slow
release of organically stored nutrients and also assisted
in pathogen control (Carpenter-Boggs et al. 2003).
Cover crops help promote biological soil tillage
through their rooting; the surface mulch provides food,
nutrients and energy for earthworms, arthropods and
micro-organisms below ground that also biologically
till soils. Use of deep-rooted cover crops and biological
agents (earthworms, etc.) can also help to relieve
compaction under zero-tillage systems. There is a lot of
literature that looks at the effects of burning, incorporation and removal of crop residues on soil properties,
and much less where mulch is left on the surface. An
early paper by McCalla (1958) showed that bacteria,
actinomycetes, fungi, earthworms and nematodes were
higher in residue-mulched fields than those where the
residues were incorporated. More recent studies also
show more soil fauna in no-tillage, residue-retained
management treatments compared with tillage plots
(Kemper & Derpsch 1981; Nuutinen 1992; Hartley
et al. 1994; Karlen et al. 1994; Buckerfield & Webster
1996; Clapperton 2003; Birkas et al. 2004; Riley et al.
2005). Tillage disrupts and impairs soil pore networks
including those of mycorrhizal hyphae, an important
component for phosphorus availability in some soils
(Evans & Miller 1990; McGonigle & Miller 1996).
Zero-tillage thus results in a better balance of microbes
and other organisms and a healthier soil.
Ground cover promotes an increase in biological
diversity not only below ground but also above ground;
the number of beneficial insects was higher where there
was ground cover and mulch (Kendall et al. 1995; Jaipal
et al. 2002), and these help keep insect pests in check.
Interactions between root systems and rhizobacteria
affect crop health, yield and soil quality. Release of
exudates by plants activate and sustain specific
rhizobacterial communities that enhance nutrient
cycling, nitrogen fixing, biocontrol of plant pathogens,
plant disease resistance and plant growth stimulation.
Sturz & Christie (2003) gave a review of this topic.
Ground cover would be expected to increase biological
diversity and increase these beneficial effects.
(b) Minimal soil disturbance
Many of the benefits of minimal soil disturbance were
mentioned in the above section on permanent soil cover,
Phil. Trans. R. Soc. B (2008)
P. R. Hobbs et al.
547
and, in fact, combining these two practices is important
for obtaining the best results. The following comparisons
between tillage and zero-tillage systems are made to
highlight some other benefits not mentioned above.
Tractors consume large quantities of fossil fuels that
add to costs while also emitting greenhouse gases (mostly
CO2) and contributing to global warming when used for
ploughing (Grace et al. 2003). Animal-based tillage
systems are also expensive since farmers have to maintain
and feed a pair of animals for a year for this purpose.
Animals also emit methane, a greenhouse gas 21 times
more potent for global warming than carbon dioxide
(Grace et al. 2003). Zero-tillage reduces these costs and
emissions. Farmer surveys in Pakistan and India show
that zero-till of wheat after rice reduces costs of
production by US$60 per hectare mostly due to less
fuel (60–80 l haK1) and labour (Hobbs & Gupta 2004).
Tillage takes valuable time that could be used for
other useful farming activities or employment. Zerotillage minimizes time for establishing a crop. The time
required for tillage can also delay timely planting of
crops, with subsequent reductions in yield potential
(Hobbs & Gupta 2003). By reducing turnaround time
to a minimum, zero-tillage can get crops planted on
time, and thus increase yields without greater input
cost. Turnaround time in this rice–wheat system from
rice to wheat varies from 2 to 45 days, since 2–12 passes
of a plough are used by farmers to get a good seedbed
(Hobbs & Gupta 2003). With zero-till wheat this time
is reduced to just 1 day.
Tillage and current agricultural practices result in
the decline of soil organic matter due to increased
oxidation over time, leading to soil degradation, loss of
soil biological fertility and resilience (Lal 1994).
Although this SOM mineralization liberates nitrogen
and can lead to improved yields over the short term,
there is always some mineralization of nutrients and
loss by leaching into deeper soil layers. This is
particularly significant in the tropics where organic
matter reduction is processed more quickly, with low
soil carbon levels resulting only after one or two
decades of intensive soil tillage. Zero-tillage, on the
other hand, combined with permanent soil cover, has
been shown to result in a build-up of organic carbon in
the surface layers (Campbell et al. 1996a; Lal 2005).
No-tillage minimizes SOM losses and is a promising
strategy to maintain or even increase soil C and N
stocks (Bayer et al. 2000).
Although tillage does afford some relief from
compaction, it is itself a major cause of compaction,
especially when repeated passes of a tractor are made to
prepare the seedbed or to maintain a clean fallow. Zerotillage reduces dramatically the number of passes over
the land and thus compaction. However, natural
compaction mechanisms and the one pass of a
tractor-mounted zero-till drill will also result in
compaction. The FAO CA web site now includes
‘controlling in-field traffic’ as a component of CA; this
is accomplished by having field traffic follow permanent
tracks. This can also be accomplished using a ridge-till
or permanent bed planting system rather than planting
on the flat (Sayre & Hobbs 2004). Some farmers feel
that sub-soiling or chiselling may be needed to resolve
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below-ground compaction layers before embarking on
a NT strategy, especially in drier areas.
Higher bulk densities and penetration resistance
have been reported under zero-tillage compared with
tillage (Gantzer & Blake 1978) and are described as
natural for zero-tillage. This problem is greater in soils
with low-stability soil aggregates (Ehlers et al. 1983).
Bautista et al. (1996) working in a semi-arid ecosystem
found that zero-tillage plus mulch reduced bulk density
(BD). The use of zero-till using a permanent residue
cover, even when BD was higher, resulted in higher
infiltration of water in NT systems (Shaver et al. 2002;
Sayre & Hobbs 2004). Scientists hypothesized that
continued use of reduced, shallow and zero-tillage
would require a shift to short-term TT to correct soil
problems. However, Logsdon & Karlen (2004) showed
that BD is not a useful indicator and confirm that
farmers need not worry about increased compaction
when changing from TT to NT on deep loess soils in
USA. Fabrizzi et al. (2005) also showed higher BD and
penetration resistance in NT experiments in Argentina,
but the values were below thresholds that could affect
crop growth; wheat yields were the same as in the tilled
treatments. This experiment left residues on the
surface in NT. The authors concluded that the
experiment had a short time frame and more time
was needed to assess the effect on BD.
The role of tillage on soil diseases is discussed by Leake
(2003) with examples of the various diseases affected by
tillage. He concluded that the role of tillage on diseases is
unclear and acknowledges that a healthy soil with high
microbial diversity does play a role by being antagonistic
to soil pathogens. He also suggested that NT farmers
need to adjust management to control diseases through
sowing date, rotation and resistant cultivars to help shift
the advantage from the disease to the crop. A list of the
impacts of minimum tillage on specific crops and their
associated pathogens can be found in Sturz et al. (1997).
An added economic consideration is that tillage
results in more wear and tear on machinery and higher
maintenance costs for tractors than under zero-tillage
systems.
(c) Rotations
Crop rotation is an agricultural management tool with
ancient origins. Howard (1996) reviewed the cultural
control of plant diseases from an historical view and
included examples of disease control through rotation.
The rotation of different crops with different rooting
patterns combined with minimal soil disturbance in
zero-till systems promotes a more extensive network of
root channels and macropores in the soil. This helps in
water infiltration to deeper depths. Because rotations
increase microbial diversity, the risk of pests and
disease outbreaks from pathogenic organisms is
reduced, since the biological diversity helps keep
pathogenic organisms in check (Leake 2003). The
discussion of the benefits of rotations will be handled in
other chapters of this publication.
Integrated pest management (IPM) should also be
added to the CA set of recommendations, since if one
of the requirements is to promote soil biological
activity, minimal use of toxic pesticides and use of
alternative pest control methods that do not affect
Phil. Trans. R. Soc. B (2008)
these critical soil organisms are needed. A review of
IPM in CA can be found in Leake (2003).
5. EQUIPMENT FOR CONSERVATION
AGRICULTURE
Before going on to describe a couple of case studies from
Asia and Mexico, there is a need to discuss the critical
importance of equipment for success with CA; zero-till
and CA are bound to fail if suitable equipment is not
available to drill seed into residues at the proper depth
for good germination. It is urgent that CA equipment is
perfected, available and adopted for this new farming
system. Iqbal et al. (2005) studied NT under dryland
conditions in Pakistan and showed that NT gave lower
yields than TT, but the experiment was planted with
improper equipment and with no mention of residue
management; the results are therefore suspect.
There are some excellent reviews of the equipment
needs for zero-tillage systems. Baker et al. (2002, 2006)
devoted an entire book to this topic. A book on CA
(‘Environment, farmer experiences, innovations and
socio-economic policy’) edited by Garcia-Torres et al.
(2003) has several papers on equipment for small- and
large-scale farmers. The main requirements of equipment in a CA system are a way to handle loose straw
(cutting or moving aside), seed and fertilizer placement, furrow closing and seed/soil compaction. There
is also a need for small-scale farmers to adapt directdrill seeding equipment to manual, animal or small
tractor power sources (reduced weight and draft
requirements) and reduce costs, so equipment is
affordable by farmers, although use of rental and
service providers allows small-scale farmers to use
this system even if they do not own a tractor or a seeder.
A simple three-row small grain seeder has been
developed for small-scale animal-powered farmers in
Bolivia (Wall et al. 2003). This equipment uses a shovel
rather than a disc opener to save weight. It has straw
wheels attached to the coulter to help move residues
aside and reduce clogging. It also has the benefit that it
can be used in ploughed or unploughed soil. The main
benefit farmers mentioned about this drill was savings in
time; it takes 10 h to plant a hectare with this machinery
and 12 days for the TT and seeding method. The
conventional system also required the farmer to walk
100 km haK1 to undertake all the tillage and seeding
with his animals. The stand with the new drill was 246G
37 plants mK2 compared with 166G39 for the conventional system. The cost of the drill was only $330–390 in
Bolivia. Similar information was provided by Ribeiro
(2003) for planters in Brazil. These can be manually
applied jabber planters to animal-drawn planters. In
both countries, the participation of farmers, local
manufacturers and extensionists was vital for success.
Saxton & Morrison (2003) looked at equipment
needs for large-scale farmers where tractor horsepower,
weight and draft are less important. Earlier machines
were developed for clean tilled farm fields, whereas new
NT machines provide precision seed placement through
consistent soil penetration and depth and also supply
fertilizer in bands which is crucial for minimizing
nutrient losses in zero-till systems. This paper discusses
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the use of disc openers versus hoe and chisel openers and
the use of additional straw and chaff spreading devices.
P. R. Hobbs et al.
3.0
2430
Phil. Trans. R. Soc. B (2008)
millions of hectares
2.5
6. CASE STUDY FROM THE RICE–WHEAT
SYSTEMS OF SOUTH ASIA
(a) Case study from Asia
The first case study looks at the 13.5 Mha of the rice–
wheat systems of the Indo-Gangetic Plains for South Asia
(RWC web site: http://www.rwc.cgiar.org/RWC-Crop.
asp). The traditional cultivation technique used for
growing rice in this system, and also in much of the rice
growing regions of Asia, is wet ploughing of soils in the
main rice field (puddling), followed by transplantation of
rice seedlings grown in separate seedbeds. Interestingly,
this system of rice cultivation is often cited as being used
for centuries without declining productivity. However, it
was at relatively low subsistence rice yield levels. There
are several excellent reviews of a number of Asian longterm experiments using modern varieties on this issue,
some rice–wheat and others rice–rice, with some showing
yield declines while others do not (Cassman et al. 1995;
Abrol et al. 1997; Dawe et al. 2000).
Puddling was done by farmers over the centuries for
very specific reasons, the most important being to help
control weeds; farmers found that keeping soils
anaerobic and flooded reduced weed problems and
also hand weeding was easier with these softened soils.
The puddling essentially reduced water percolation
and infiltration and ponded the water on the surface.
Less is written on the puddling effect on soil biological
properties although some work is available from
research done at IRRI in the 1990s (Reichardt et al.
1997, 2001). The authors conclude that SMB plays a
significant role as a passive nutrient pool and suggests
that its reduction, found in puddled soils in the second
half of the cropping season, could be a mechanism that
contributes to declining productivity in continuous rice
cropping systems. Little has been published on soil
microbes in rice–wheat systems.
When modern varieties of wheat and rice were
introduced to South Asia in the 1960s, farmers in
Northwest India and Pakistan introduced rice into their
wheat systems and farmers in the eastern side of South
Asia introduced wheat into their rice systems; wheat
was grown in the cool dry season and rice in the warm
wet monsoon months. This intensified the system that
has grown to 13.5 Mha since the 1960s. It is now one of
the most important cropping systems for food security
in South Asia along with rice–rice systems. One of the
main issues that confronted farmers when this new
system was introduced and found feasible and profitable was the soil physical properties left after harvest of
a puddled transplanted rice crop. The effect of
puddling reduced soil structure, especially stable soil
aggregates, and led to formation of compaction layers
(Hobbs et al. 1993). Soil cracking was higher under
intensive puddling (Mohanty et al. 2004). Unpuddled
direct-seeded rice maintained the soil in a better
physical condition, although yields were lower where
weeds were not controlled. Farmers ploughed their
fields many times to obtain a suitable seedbed for
planting wheat (Hobbs & Gupta 2003, table 7.1). This
ploughing takes time and often results in late planting
549
1910
2.0
1.5
1156
1.0
561
0.5
0
0.022 0.255 2.1 12.8 71
209
1996– 1997– 1998– 1999– 2000– 2001– 2002– 2003– 2004– 2005–
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
years
Figure 1. Estimated area growth of no-till wheat in the IndoGangetic Plains for the past 10 years (adapted from Rice–
Wheat Consortium (RWC) for the Indo-Gangetic Plains
(2006). http://www.rwc.cgiar.org).
and decline in wheat yield potential, plus many other
negative effects (Hobbs & Gupta 2003, 2004).
The solution for late planting and problems of
delayed turnaround from rice harvest to wheat planting
came from the introduction of zero-tilled wheat into
rice stubbles that started in the region in the mid1980s. Efforts to adapt and promote resource conserving technologies (RCTs that include NT) in the IndoGangetic Plains (IGP) have been underway for nearly
three decades, but it is only in the past 4–5 years that
the technologies are finding accelerated acceptance by
the farmers (figure 1). The spread of NT is taking place
in the irrigated RW regions but is yet to be rooted in the
rainfed agro-ecoregions. In the last 2004–2005 wheat
season, it was estimated that nearly 2 Mha of zero-till
wheat was being grown by 425 000 farmers in the four
South Asian countries (RWC Highlights 2005: http://
www.rwc.cgiar.org/Pub_Info.asp?IDZ152); both
large- and small-scale farmers adopted this technology
with small-scale farmers renting zero-till drill services
from service providers. The key to this rapid adoption
in the last 5 years was the use of farmer participatory
approaches to allow farmers to experiment with the
technology in their own fields and promotion of the
local machinery manufacturers in the region to be
partners in the programme; cheap, affordable, effective
drills are available based on the use of the inverted T
coulter technology that was introduced into India and
Pakistan from New Zealand.
One major need of this system is the development
and availability of equipment that will allow good
germination of rice and wheat while, at the same time,
minimizing soil disturbance and sowing the seed and
banded fertilizer into loose and anchored stubbles. The
RWC members are working vigorously in partnership
with local manufacturers and farmers to make new
equipment available for experimentation at an affordable price, with provisions for after-sales service and
supply of needed spare parts to make this system
successful. Recently, multicrop zero-till ferti-seed drills
fitted with inverted T openers, disc planters, punch
planters, trash movers or roto-disc openers have been
developed for seeding into loose residues (RWC
Highlights 2004–2005; figure 2).
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550
P. R. Hobbs et al.
Role of CA in sustainable agriculture
Figure 2. Various equipments for planting wheat no-till in
RWC. (a) inverted T coulter, (b) Indian no-till drill using
inverted T, (c) disc-type planter, (d ) star-wheel punch
planter, (e) ‘happy planter’ where straw picked up and
blown behind seeder and ( f ) disc planter with trash mover.
technology intercrops direct-seeded rice with a green
manure (Sesbania aculeata). After a month, the crop is
sprayed with 2,4-D herbicide to knock down the green
manure and kill any germinating broadleaf weeds. The
dying weeds and GM provide nutrients to the rice crops
as they decompose, but new weeds are suppressed by
the ground cover and allelopathic properties of the
mulch. The results look good and this research will be
reported soon. In addition to zero-till rice and wheat,
attempts are also being made to diversify the cropping
systems by introducing other crops into new rotations
that help break disease and insect cycles and provide
more income and diversity for farmers. This may help
with some other problems that have surfaced when
rice is shifted from anaerobic to aerobic systems.
Widespread infestations of the root knot nematode
(Meloidyogyne graminicola) on rice were found when
direct-seeded rice was grown instead of puddled rice in
Bangladesh (Padgham et al. 2004).
Many advantages have been mentioned and characterized for this innovation including US$145 million
savings in fuel costs (2004 costs) and the benefits of less
greenhouse gas emissions, less weeds, more beneficial
insect activity, improved water use efficiency, and also
important higher yields at less cost, improved incomes
from wheat and savings in time that can be used for
other productive uses (Hobbs & Gupta 2004). It is
anticipated that as machinery manufacturers keep pace
with demand for drills, as the message about the
benefits of zero-till wheat reaches more farmers, and as
farmer mindsets about the need for tillage are overcome that the acreage for this innovation will cover the
bulk of the wheat planted after rice in South Asia.
However, the story does not end here. In order for
the benefits of zero-tilled wheat to be seen in the entire
system and for soil physical and biological health to
improve and the rice–wheat system to become more
sustainable, rice practices will also have to change. A
move is already afoot in the rice–wheat consortium
partners (national and international programmes) to
research, experiment and promote a move to zero-till
direct-seeded rice: a more aerobic rice system that does
away with puddling of soils. These aerobic systems are
based on direct-seeded rice systems on either the flat or
raised beds, with and without tillage and with or
without transplanting (RWC DSR paper: http://www.
rwc.cgiar.org/Pub_Info.asp?IDZ123). The ultimate
system would allow farmers to grow zero-till rice
followed by zero-till wheat. Wheat yields are best
after direct-seeded rice without puddling (DSNP;
Hobbs et al. 2002; Tripathi et al. 2005).
Various national and international research and
breeding agencies are now exploring aerobic rice
(Bouman et al. 2002). The main issue to resolve relates
to effective weed control in a non-puddled rice system.
Various innovative integrated ways are being sought to
handle this problem, including use of cover crops and
mulches, more competitive rice varieties against weeds
and use of selective herbicides. Availability of roundupready transgenic rice and/or development of cultivars
suited to direct seeding with zero-till drills, having early
vigour and competitive with weeds, would go a long
way to help resolve this issue. One encouraging
(b) Case study from Mexico
Maize–wheat cropping patterns are common in the
irrigated northwest areas of Mexico and the rainfed
areas of the altiplano areas of central Mexico. In both
situations, the major limiting factor is moisture. This
case study introduces the concept of permanent bed
systems for addressing efficient use of water. In bed
systems, soil is raised in a ridge-and-furrow configuration. These bed systems involved tillage to prepare
the soil before making the beds. However, many
traditional bed planting systems did not receive tillage;
the chinampas of pre-Colombian Mexico and the waru
warus of Peru and Bolivia used crop residue mulching
or only superficial tillage ( Thurston 1992). Bed
systems reduce compaction in the rooting zone by
confining wheel traffic to the furrow bottoms. The case
study described here from Mexico looks at the results of
using a permanent bed planting maize–wheat system
where soil disturbance is minimized, crop residues are
retained on the surface from previous crops and
reshaping of beds is done only as needed between
crop cycles (Limon-Ortega et al. 2002).
The experiment used for this paper was undertaken
by the International Maize and Wheat Improvement
Center (CIMMYT) in the State of Sonora in Northwest Mexico. Farmers have 225 000 hectares of
irrigated land in this area with maize, sorghum,
soybean, safflower, dry beans, cotton and wheat, the
major crops. Ninety-five per cent of the farmers now
grow crops on beds with farmers changing from the
conventional planting on the flat with basin irrigation in
the last 20 years (Aquino 1998). This change was a
result of water shortages from the water storage
reservoir system; farmers had to find more efficient
water use systems in order to expand acreage. Results
suggest that bed planted systems need 29% less
water than flat planting systems for an 8% higher yield
(Sayre & Hobbs 2004). Most of the farmers still use TT
to remake the beds for each new crop, but results that are
reported below suggest that use of permanent bed
systems where tillage is minimized and crop residues
are left on the surface will be more sustainable.
The treatments in this long-term trial were as
follows.
(a)
(b)
(c)
(d)
(e)
(f)
Phil. Trans. R. Soc. B (2008)
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Role of CA in sustainable agriculture
(i) CT with formation of new beds for each crop
and with all the crop residues incorporated.
(ii) Permanent beds with all the residues burned.
(iii) Permanent beds with 30% of the residues
retained on the surface and the rest baled for
fodder.
(iv) Permanent beds with the maize residue baled for
fodder and the wheat residue retained.
(v) Permanent beds with all the residues retained.
A detailed account of this experiment can be found
in Sayre & Hobbs (2004). Yield differences were small
in the first 5 years supporting the idea that some
transitional years are needed before changes occur in
soil properties with changes in management. Changes
started to appear between treatments in the sixth and
subsequent years with the permanent bed treatment
with all the residues retained the best and highest
yielding plot, and the permanent bed treatment with
residues burnt the worst and lowest yielding plot. The
treatment with CT with residues incorporated was
statistically at par with the best permanent bed system
but incurred higher costs for land preparation.
Organic matter, nitrogen levels, surface soil aggregate size and SMB were higher in the permanent beds
with residue retention. One valuable insight from this
experiment was the lower soil strength/compaction in
all treatments, except the one where residues were
burned. The addition of the residue plays a significant
role in reducing compaction at the soil surface and
increasing water infiltration in minimal tilled plots.
Similar data confirmed this finding in a similar maize–
wheat long-term experiment under rainfed conditions
in the altiplano areas near Mexico City (Govaerts et al.
2005). In this rainfed experiment, zero-tilled plots with
residue retention resulted in higher and more stable
yields than conventionally tilled plots with residues
incorporated. Zero-tilled plots without residue retention had much reduced yields. In the same experiment,
permanent raised beds combined with rotation and
residue retention yielded the same as zero-tilled plots
with residue retention. The bed system gave farmers an
added advantage of being able to use more varied weed
and fertilizer practices.
Larger-scale demonstrations have been planted
on farmer’s fields. The permanent beds averaged
7.2 t haK1 compared with 6.2 t haK1 for the conventionally made beds. The data also show that average
returns over variable costs increased by 75% for the
permanent bed system with residue retention
compared with the conventional tilled treatment. The
importance of suitable equipment that will allow seeding
of crops into permanent beds with residue retention
cannot be overemphasized. Systems based on discs,
punch planters and strip tillage are being experimented
with in Mexico and South Asia (Sayre & Hobbs 2004).
7. CLIMATE CHANGE AND CONSERVATION
AGRICULTURE
Lal (2005) suggested that by adopting improved
management practices on agricultural land (use of
NT and crop residues), food security would not only be
enhanced but also offset fossil fuel emissions at the rate
Phil. Trans. R. Soc. B (2008)
P. R. Hobbs et al.
551
of 0.5 Pg C yrK1. Climate change is likely to strongly
affect rice–wheat, rice–rice and maize-based cropping
systems that, today, account for more than 80% of the
total cereals grown on more than 100 Mha of
agricultural lands in South Asia. Global warming may
be beneficial in some regions, but harmful in those
regions where optimal temperatures already exist; an
example would be the rice–wheat mega-environments
in the IGP that account for 15% of global wheat
production. Agronomic and crop management practices have to aim at reducing CO2 and other greenhouse gas emissions by reducing tillage and residue
burning and improving nitrogen use efficiency. In the
IGP, resource-conserving technologies continue to
expand in the rice–wheat cropping systems and save
50–60 l of diesel haK1 plus labour, and significantly
reduce release of CO2 to the environment. Methane
emissions that have a warming potential 21 times that
of CO2 are common and significant in puddled
anaerobic paddy fields and also when residues are
burnt. This GHG emission can be mitigated by shifting
to an aerobic, direct seeded or NT rice system. A review
of the other benefits of direct seeding and NT in RW
areas of South Asia can be found in Grace et al. (2003).
Nitrous oxide has 310 times the warming potential of
carbon dioxide, and its emissions are affected by poor
nitrogen management. Sensor-based technologies for
measuring normalized differential vegetative index and
moisture index have been used in Mexico and South
Asia to help improve the efficiency of applied nitrogen
and reduce nitrous oxide emissions.
8. CONCLUSIONS
Crop production in the next decade will have to
produce more food from less land by making more
efficient use of natural resources and with minimal
impact on the environment. Only by doing this will
food production keep pace with demand and the
productivity of land be preserved for future generations. This will be a tall order for agricultural
scientists, extension personnel and farmers. Use of
productive but more sustainable management practices
described in this paper can help resolve this problem.
Crop and soil management systems that help improve
soil health parameters (physical, biological and
chemical) and reduce farmer costs are essential.
Development of appropriate equipment to allow these
systems to be successfully adopted by farmers is a prerequisite for success. Overcoming traditional mindsets
about tillage by promoting farmer experimentation
with this technology in a participatory way will help
accelerate adoption. Encouraging donors to support
this long-term applied research with sustainable
funding is also an urgent requirement.
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