The Royal Society Report - Push-Pull

system and the benefits of P acquisition. Variation in the
length and density of root hairs is important for the
acquisition of immobile nutrients such as phosphorus and
potassium. Genetic variation in root cortical aerenchyma
formation and secondary development (‘root etiolation’)
are important in reducing the metabolic costs of root
growth and soil exploration (Lynch 2007).
Lynch has argued that genetic variation in these traits is
associated with substantial yield gains in low-fertility soils
and that crop genotypes with greater yield in infertile soils
will substantially improve the productivity and sustainability
of low-input agroecosystems (Lynch 2007). In high-input
agroecosystems, these traits will reduce the environmental
impacts of intensive fertilisation.
Engineering of nitrogen fixation into non-legume crops
has been a long-standing target of biotechnologists.
Three approaches have been envisioned. The first involves
modification of crop plants so that they support symbiosis
with a nitrogen-fixing bacterium or blue-green alga. The
second approach involves transfer of bacterial
nitrogenase genes into the chloroplasts of crop plants.
These approaches are both still long term, there is little
research activity in this area and it is unlikely that they
could be harnessed to develop a nitrogen fixing crop
within the next 15 years. A third approach is to move the
plant genes required for production of a symbiotic
nitrogen-fixing nodule from leguminous plants to others
that cannot currently support such a symbiosis. As the
plant genes required for nodule development become
better understood, this prospect now appears less
fanciful, but is still at least 10 years away (Markmann &
Parniske 2009).
3.3.5 Soils
Intensive cultivation of soils damages soil structure and
leads to overuse of groundwater resources. Soils become
cracked, and seedbed preparation increasingly requires
frequent ploughing. This damage both increases costs and
reduces yield. Zero-till systems of production have been
developed to address these problems. This requires a new
generation of cheap and affordable machinery. Zero-till
sites have reported increased yield, as well as evidence of
reduced greenhouse gas emissions, fewer weeds, more
beneficial insects and improved water use efficiency
(Hobbs et al. 2008; see also Case study 3.6).
Double digging is a method of deep soil preparation which
can be used to improve soil fertility and structure. The idea
of double dug beds is being widely promoted by local
NGOs in Kenya. Double dug beds are combined with
composts and animal manures to improve the soil. A
considerable initial investment in labour is required, but the
better water-holding capacity and higher organic matter
mean that they are able to sustain vegetable growth long
into the dry season. Once the investment is made, little
more has to be done for the next two to three years. Many
vegetable and fruit crops can be cultivated, including kales,
onions, tomatoes, cabbage, passion fruit, pigeon peas,
spinach, peppers, green beans and soya. The use of double
dug beds in Kenya has improved food security. In
particular, the health of children has improved through
increased vegetable consumption and longer periods of
available food (Pretty et al. 2003).
Biochar (charcoal) addition to soils is an ancient practice
which has recently begun to assume wider significance.
As a by-product of the pyrolysis of plant-derived biomass
(for energy generation without releasing carbon),
incorporation of biochar represents a means of
sequestering carbon (due to its long half-life in soil) and
there is increasing evidence that it can also reduce
nutrient leaching and impact on the slow release of
nutrients to enhance crop yields (Marris 2006).
Case study 3.6. Conservation agriculture in Burkina Faso, West Africa
The predominant ecosystem type in southwest Burkina Faso is moist savannah with tropical grassland and widely
spaced trees. This region is sometimes referred to as a potential breadbasket for Africa due to its high crop and
livestock productivity potential. However, productivity is currently low across much of the region due to poor soil
nutrient fertility, variable rainfall and inadequate biomass availability. Farmers usually grow a range of subsistence
crops—mainly maize, pearl millet, sorghum, groundnut and cowpea.
Over 20 million ha of savannah land (with similar agroecology to Burkina Faso) have been sustainably intensified and
diversified in Brazil using conservation agriculture principles. From 2002 to 2007, an FAO conservation agriculture pilot
study was carried out in five communities in Burkina Faso, with the following aims:
• to expand crop choices to increase production of livestock feed;
• to improve soil-crop-water management for sustainable production intensification; and
• to diversify and expand the range of food, feed and tree crops and their integration with livestock into the existing
cotton- and maize-based systems.
What is conservation agriculture
Conservation agriculture is resource-saving agricultural crop production that aims to deliver high and sustained
production levels while conserving the environment. Interventions such as mechanical soil tillage are minimised (or
34 I October 2009 I Reaping the Benefits The Royal Society