Returning Carbon to the Soil

Because under conservation farming the crop residues are left to rot down on the surface of the soil, there is a significant return of carbon to the soil. This reduces the likelihood of erosion and directly enhances yields, so creating a win-win situation. Other agricultural practices can also produce returns of carbon to the soil such as livestock rotations, the use of cover crops and composting.

Rattan Lal’s analysis of several experiments has shown that an increase of 1 ton per ha of soil carbon in degraded croplands can increase maize yields by 200 to 300 kg/ ha, wheat by 20 to 40 kg/ha, rice by 20 to 50 kg/ha, sorghum by 80 to 140 kg/ha, millet by 30 to 70 kg/ha, beans by 30 to 60 kg/ha, and soybeans by 20 to 50 kg/ha.44 The more depleted the soil, the higher the increment in yield. Figure 13.3a shows this effect for maize in Thailand, while Figure 13.3b emphasizes the point that the addition of nitrogen adds to the carbon effect.

Organic Farming

In many respects conservation and organic farming have much in common. They both emphasize the importance of returning organic matter to the soil. There are strong overlaps: some conservation tillage is certified organic, and some organic

a Effects of soil organic carbon in root zone on maize yields in Thailand

Figure 13.3a Effects of soil organic carbon in root zone on maize yields in Thailand.45

b Effects of soil organic carbon and nitrogen applications on maize on a Russian Chernozem soil

Figure 13.3b Effects of soil organic carbon and nitrogen applications on maize on a Russian Chernozem soil.46

farming makes use of minimum- or no-till practices. The difference lies in the strict exclusionary rules of organic farming: All synthetic fertilizers are banned and so are all herbicides and most insecticides and fungicides—the exceptions are various “natural or simple” chemicals (listed in Chapter 12).47 The amount and origin of manure that can be applied is also restricted. Organic farmers have to rely on organic sources of nutrients, nitrogen fixing by legumes, natural forms of pest control, and a great deal of hand weeding. Farmers who adopt these and other restrictions can obtain certification to this effect from various national bodies and can sell their products as such.

The land under certified organic production has grown steadily over the past few decades. In 2007, it is estimated there were over 30 million ha of certified organic farmland worldwide, about 1 percent of total world production.48 However, this does not include the millions of small producers who practice traditional agriculture that for one reason or another does not use inorganic fertilizers or synthetic pesticides. Such noncertified “organic” agriculture may be practiced on another 10 to 20 million ha in developing countries.49

Certified organic farming in developed countries has a well-established niche. While the costs of production relative to conventional production systems are high (largely due to the costs of the higher labor requirement), the products command premium prices. The question is whether an organic approach to farming can benefit developing countries. It certainly provides a profitable niche for the production of high-value crops for export to the developed countries. But can it, as the organic lobby maintains, provide a sustainable basis for growth, increasing incomes and helping to feed the world?

Although there is much controversy over the figures, organic agriculture produces significantly lower yields than conventional. (See the arguments between Catherine Badgley and colleagues at the University of Michigan and Keith Goulding of Rothamsted Research in the UK and colleagues.)50 Comparative studies in developing countries are not thorough enough to generate firm conclusions, but there is extensive data in the developed countries. Thus long-term wheat experiments in the United Kingdom show comparable yields are obtained only with very heavy applications of manure, well above the amounts permitted under organic farming. Figure 13.4 shows the results from the famous Broadbalk experiment. Before synthetic fertilizers became widely available, there was usually a deficiency of nutrients, and yields of crops such as wheat were small and very variable. Yields with moderate amounts of fertilizers (144 kg N/ha) were two to three times those without fertilizers or manures. Modern pesticides further increased the yields. The best yields have been

Yields of winter wheat varieties on the Broadbalk experiment at Rothamsted Research

Figure 13.4 Yields of winter wheat varieties on the Broadbalk experiment at Rothamsted Research.51 FYM=farmyard manure, NPK=nitrogen, phosphorus, potassium obtained either with 250 to 300 kg N/ha (best NPK) or from 35 tons of farmyard manure plus 96 kg N.

Careful analysis of a wide range of other experiments suggests the typical ratio of organic to conventional wheat yields is 0.65 (i.e., organic cultivation yields 30 to 40 percent less), and this seems to be the approximate ratio for other crops.52 However, the ratio could be an underestimate for developing countries. In drought-affected areas and under subsistence conditions, conversion to organic farming may well improve yields where the soils have been degraded over time.53 Moreover, performance could change if organic crop varieties were bred to be more efficient at making use of scarce resources. The challenge is to breed organic varieties that are better at photosynthesis, at taking up nutrients contained in organic matter, at synthesizing their own nitrogen, at resisting pests and diseases, and at tolerating drought conditions. Some of this can be achieved by conventional breeding, but the process would be greatly enhanced by using recombinant DNA. At present certified “organic” has to be free of genetic modification (GM), but recombinant DNA uses naturally occurring DNA and enzymes. A couple of faculty at the University of California, Davis—Pamela Ronald, who works on GM rice, and her husband Raoul Adam- chak, an organic agriculturalist—in their book Tomorrow’s Table make an excellent case for bringing genetic engineering and organic agriculture together.54

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