The Green Revolution boosted crop yields in developing nations by introducing dwarf genotypes of wheat and rice capable of responding to fertilisation without lodging. We now need a second Green Revolution, to improve the yield of crops grown in infertile soils by farmers with little access to fertiliser, who represent the majority of third-world farmers. Just as the Green Revolution was based on crops responsive to high soil fertility, the second Green Revolution will be based on crops tolerant of low soil fertility. Substantial genetic variation in the productivity of crops in infertile soil has been known for over a century. In recent years we have developed a better understanding of the traits responsible for this variation. Root architecture is critically important by determining soil exploration and therefore nutrient acquisition. Architectural traits under genetic control include basal-root gravitropism, adventitious-root formation and lateral branching. Architectural traits that enhance topsoil foraging are important for acquisition of phosphorus from infertile soils. Genetic 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. Genetic variation in rhizosphere modification through the efflux of protons, organic acids and enzymes is important for the mobilisation of nutrients such as phosphorus and transition metals, and the avoidance of aluminum toxicity. Manipulation of ion transporters may be useful for improving the acquisition of nitrate and for enhancing salt tolerance. With the noteworthy exceptions of rhizosphere modification and ion transporters, most of these traits are under complex genetic control. Genetic variation in these traits is associated with substantial yield gains in low-fertility soils, as illustrated by the case of phosphorus efficiency in bean and soybean. In breeding crops for low-fertility soils, selection for specific root traits through direct phenotypic evaluation or molecular markers is likely to be more productive than conventional field screening. Crop genotypes with greater yield in infertile soils will substantially improve the productivity and sustainability of low-input agroecosystems, and in high-input agroecosystems will reduce the environmental impacts of intensive fertilisation. Although the development of crops with reduced fertiliser requirements has been successful in the few cases it has been attempted, the global scientific effort devoted to this enterprise is small, especially considering the magnitude of the humanitarian, environmental and economic benefits being forgone. Population growth, ongoing soil degradation and increasing costs of chemical fertiliser will make the second Green Revolution a priority for plant biology in the 21st century.

Roots of the Second Green Revolution

The United Nations declared 2016 as the International Year of Pulses (grain legumes) under the banner ‘nutritious seeds for a sustainable future’. A second green revolution is required to ensure food and nutritional security in the face of global climate change. Grain legumes provide an unparalleled solution to this problem because of their inherent capacity for symbiotic atmospheric nitrogen fixation, which provides economically sustainable advantages for farming. In addition, a legume-rich diet has health benefits for humans and livestock alike. However, grain legumes form only a minor part of most current human diets, and legume crops are greatly under-used. Food security and soil fertility could be significantly improved by greater grain legume usage and increased improvement of a range of grain legumes. The current lack of coordinated focus on grain legumes has compromised human health, nutritional security and sustainable food production.

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Neglecting legumes has compromised human health and sustainable food production

Food security, lowering the risk of climate change and meeting the increasing demand for energy will increasingly be critical challenges in the years to come. Producing sustainably is therefore becoming central in agriculture and food systems. Legume crops could play an important role in this context by delivering multiple services in line with sustainability principles. In addition to serving as fundamental, worldwide source of high-quality food and feed, legumes contribute to reduce the emission of greenhouse gases, as they release 5–7 times less GHG per unit area compared with other crops; allow the sequestration of carbon in soils with values estimated from 7.21 g kg−1 DM, 23.6 versus 21.8 g C kg−1 year; and induce a saving of fossil energy inputs in the system thanks to N fertilizer reduction, corresponding to 277 kg ha−1 of CO2 per year. Legumes could also be competitive crops and, due to their environmental and socioeconomic benefits, could be introduced in modern cropping systems to increase crop diversity and reduce use of external inputs. They also perform well in conservation systems, intercropping systems, which are very important in developing countries as well as in low-input and low-yield farming systems. Legumes fix the atmospheric nitrogen, release in the soil high-quality organic matter and facilitate soil nutrients’ circulation and water retention. Based on these multiple functions, legume crops have high potential for conservation agriculture, being functional either as growing crop or as crop residue.

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Multiple benefits of legumes for agriculture sustainability: an overview

Increasing food demands by a growing human population require substantial increases in crop productivity. In rain-fed arid and semiarid areas where the water supply is limited, an increase in the precipitation use efficiency (PUE) is the key to reach this goal. This chapter examines the scientific basis of a ridge-furrow mulching system (RF system) for increasing PUE, and summarizes the effects of this system on crop performance, microclimates, soil attributes, and environmental sustainability. Studies have shown that using crop straw, plastic film, or gravel–sand materials to mulch the soil surface significantly reduces the evaporation of soil moisture, increases water availability to crop plants, and decreases soil erosion caused by wind and water. Plastic mulching increases topsoil temperature during cool spring, promoting plant growth; during hot summer, straw mulching can moderate soil temperature, preventing the topsoil from reaching temperatures that inhibit plant growth. Ridge furrows with plastic mulching on the ridges and crop straw covering the furrows channel water to the furrows, and enhance soil water infiltration and water availability to the crop. Microclimates under mulched ridges and furrows favor soil microbial activity, increase soil biodiversity, and improve environmental benefits. The effectiveness of ridge-furrow systems is reflected in increased crop yields (20–180%) compared with that of the conventional-flat planting. Although more research is required to document physiochemical strengths, technique details and potential drawbacks, and more importantly to define long-term sustainability, we strongly suggest that RF systems are an innovative approach for increasing crop water availability, improving soil productivity, and enhancing food security for arid and semiarid rain-fed areas.

Ridge-Furrow Mulching Systems—An Innovative Technique for Boosting Crop Productivity in Semiarid Rain-Fed Environments

Plants acquire mineral nutrients from their native soil environments. Most crop plants are glycophytes and have evolved under conditions of low soil salinity. Consequently, they have developed mechanisms for absorbing, transporting, and utilizing mineral nutrients in nonsaline soils. Under saline conditions, which are characterized by low nutrient-ion activities and extreme ratios of Na / Ca , Na /K , Mg /Ca , and Cl /NO3 , nutritional disorders can develop and crop growth and quality may be reduced. This is not surprising, since under saline conditions, Na and/or Cl often exceed macronutrient concentrations by one or two orders of magnitude and even more in the case of micronutrients. Halophytes, native to saline environments, may also develop nutrient disorders despite their remarkable ability to absorb nutrients selectively from soil solutions dominated by Na and Cl . Nutrient imbalance can develop in salt-stressed plants in different ways. It may result from the effect of salinity on nutrient availability, uptake, transport, or partitioning within the plant or may be caused by physiological inactivation of a given nutrient resulting in an increase in the plant’s internal requirement for that essential element. It is likely that salinity may affect one or more of these processes at the same time, but whether or not this imbalance results in loss of crop yield or quality depends on its severity, which is influenced by a number of environmental factors. Nutrient availability and uptake by plants grown in saline environments is related to (a) the activity of the nutrient ion in the soil solution which depends on pH, pE (the negative log of

Mineral Nutrient Acquisition and Response by Plants Grown in Saline Environments

Conservation agriculture for small holder rainfed farming: Opportunities and constraints of new mechanized seeding systems

Although there is increasing awareness of the importance of food legumes in human, animal and soil health, adoption of improved production technologies for food legume crops is not proceeding at the same pace as for cereal crops. Over the previous decade, the only food legumes to have shown significant production increases have been chickpea, lentil and faba bean in North America, chickpea in Australia, and faba bean in Europe. In smallholder farming in developing countries, production trends have mostly been static or have declined over the past decade despite the existence of technology that should permit higher and more stable yields. Ability to reverse negative trends is jeopardized by climate change as food legumes are mostly grown rainfed and are being exposed to increasingly variable and extreme weather. This review examines recent innovations in cultivation technology for the major food legumes—chickpea, lentil, dry pea, faba bean, lupin, common bean, mung bean, black gram, cowpea, and pigeonpea—and explores constraints to their adoption, particularly by resource-poor smallholder farmers. Conservation agriculture, involving minimum soil disturbance, maximum soil cover, and diverse rotations, has contributed to sustainable cropping system production in large-scale commercial farming systems in the Americas, Europe, Australia, and Turkey. Temperate food legumes have been incorporated into such systems. Adoption of conservation agriculture is only just beginning for smallholder farming in Asia and Africa, catalyzed by the development of low-cost implements suitable for minimum tillage. Water use efficiency improves with conservation agriculture as it allows for earlier planting, reduced soil evaporation, better weed management, and increased access to nutrients. Ecosystem-based approaches to plant nutrition are evolving which place more reliance on accessing organic and mineral reservoirs than in replenishing the immediately available pool with chemical fertilizers, leading to enhanced nutrient use efficiency of cropping systems. Ecosystem-based approaches are also being applied to management of weeds, diseases, and insect pests of food legumes, again with decreased reliance on synthetic chemicals. In achieving sustainable agricultural production systems, there is increasing realization of the need to move towards the tenets of organic agriculture, as exemplified in conservation agriculture and ecosystem-based approaches to plant nutrition and pest management. This does not necessarily imply a desire to qualify for organic product certification but more a realization of the need for sustainable agriculture. The movement towards conservation and organic agriculture encourages greater inclusion of food legumes, and legumes generally, in cropping systems. Unfortunately, however, technology transfer to resource-poor farming situations, where most food legumes are produced, remains a major bottleneck to meeting global demand. More participatory approaches to technology development, testing, and dissemination are required than hitherto practiced. It is suggested that this process could be enhanced by better focusing on major constraints within the value addition chain for food legumes.

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Innovations in agronomy for food legumes. A review

Chickpea (Cicer arietinum) is a promising post-wet-season crop to follow rainfed rice (Oryza sativa) in the High Barind Tract of northwestern Bangladesh. Yields in farmers' fields, however, remain low (<1 t ha-1) primarily due to factors such as poor crop establishment, late sowing, and terminal drought and heat stress. Having been shown to improve plant stand and yield of chickpea in other environments, seed priming - soaking the seed overnight before surface drying and sowing the next day - of chickpea cv. Barichola-2 was tested for its efficacy under Barind conditions. In the 1998-99 season, 30 trials, grown entirely on residual soil moisture were conducted in farmers' fields in districts of Rajshahi and Nawabganj. A statistically significant mean yield response to priming of 47% was obtained. In 1999-2000, 105 on-farm trials (in Rajshahi, Nawabganj, Noagaon and Niamatpur) and 15 demonstrations (in Godagari) comparing presence and absence of priming were conducted. The crop received rain during this growing season. In a randomly chosen subset of 35 trials, scientists recorded a mean yield increase due to priming of 20%; it was 22% (from 1.02 to 1.25 t ha-1) in the remaining 64 trials (6 of the 105 trials were abandoned) where farmers recorded yields. Using a different cultivar (Barichola-5), the mean yield response to priming in 15 demonstrations was 17% (from 1.25 to 1.46 t ha-1). The priming response was attributed mainly to rapid seedling establishment, with higher plant stand and earlier crop maturity allowing escape from end-of-season stresses. Priming also reduced the incidence of stem and root diseases, and increased nodulation by native rhizobia. This simple technology can substantially increase chickpea yields to remunerative levels for the resource-poor farmers in this difficult environment. Furthermore, it is suggested that this technology can act as a catalyst for the introduction of further technologies that will permit reliable and profitable cultivation of post-rainy-season crops and thus improve the livelihoods of the rural population

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Short duration chickpea to replace fallow after aman rice: the role of on-farm seed priming in the high barind tract of bangladesh

The problems and challenges of rapidly increasing world population, global climate change, shortages of water suitable for irrigation and degradation of agricultural land are increasing the demand to improve grain production from rainfed arable lands. Specific challenges include estimating the size and thus the value of the yield gap, identifying the factors limiting current average production and designing profitable remedial strategies for a range of agro-ecological regions. This review of the rainfall-limited potential yields and the gap between actual or average yields of cereal and legume crops and the rainfall-limited potential indicates that there is still substantial room to increase the average yield of crops in rainfed systems in both developed and developing regions. The review has indicated that (1) the size of the gap between average and potential yields varies according to the agro-ecological zone and the available technologies from about 0.5 to over 5 t/ha, leaving considerable scope for future yield improvement; (2) there is relatively less information applicable at the farm or field scale that assesses the spatial and temporal variability of the yield gap, the reasons for the gap and the possible methods to close the gap; (3) there is also limited information on the feasibility and profitability of applying various approaches to close the gap, including tactical and strategic management practices and plant breeding; (4) the evidence of the impact of the components of conservation agriculture on crop yields in a wide range of agro-ecological regions supports the adoption of zero tillage and crop rotation but is less clear in support of residue retention; (5) objective identification and testing of factors that limit production can lead to a rational sequence of amelioration that is specific to each agro-ecological or field situation and can close the yield gap in winter-dominant rainfall environments; and (6) farmer-participatory varietal selection, including breeding for specific adaptation can make a substantial contribution to closing the gap in a range of environments. A common observation from the reports reviewed here is that sustainable yield improvement will need to employ a range of methods that are appropriate to specific agro-ecological conditions—previous approaches based on single inputs, practices or genotypes can only be partial solutions.

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Addressing the yield gap in rainfed crops: a review

In on-farm field trials at 30 locations in the Ataher, Amnura and Nachole soil series area of the High Barind Tract, Cicer arietinum cv. Barichola-2 seeds were primed (soaked in water overnight and then surface dried) or not primed before sowing at 50 kg seed/ha. Seed priming increased seedling emergence, plant height, number of pods/m², 1000-seed weight, grain yield and residue yield by 21, 22, 39, 6, 47 and 31%, respectively, and decreased the number of diseased plants/m², pod borer (Helicoverpa armigera) damaged pods/m² and number of unfilled pods/m² by 45, 13 and 21%, respectively.

Chris Johansen

Papers

Conservation agriculture for small holder rainfed farming: Opportunities and constraints of new mechanized seeding systems

Innovations in agronomy for food legumes. A review

Short duration chickpea to replace fallow after aman rice: the role of on-farm seed priming in the high barind tract of bangladesh

Addressing the yield gap in rainfed crops: a review

Response of Chickpea to Seed Priming in the High Barind Tract of Bangladesh