Zero-carbon fertilizer
To achieve high crop yields, large amounts of nitrogen are necessary, far beyond what is naturally available. Nitrogen is primarily obtained by plants through ammonia produced by microorganisms in the soil. Adding synthetic fertilizers is a common way to boost nitrogen levels, as plants will continue to grow as long as they have enough nitrogen. However, synthetic fertilizers have downsides. First, the microorganisms in the soil that make nitrogen sense the added nitrogen and stop producing it naturally. Second, producing synthetic fertilizers requires the burning of natural gas, which releases greenhouse gases, and their transport to farms is done using gasoline-powered trucks. Third, much of the nitrogen applied is never absorbed by plants, leading to pollution in ground and surface waters and the release of nitrous oxide into the atmosphere, a potent greenhouse gas. Fertilizers were responsible for roughly 1.3 billion tons of greenhouse gas emissions in 2010 and are expected to contribute 1.7 billion tons by mid-century.
There is currently no practical zero-carbon alternative for fertilizers, although using clean electricity to synthesize ammonia would reduce emissions. However, this process is expensive and would increase the cost of fertilizer considerably. Furthermore, there is no way to capture the greenhouse gases that result from applying fertilizer, making a complete Green Premium calculation impossible. However, there are potential solutions being developed, such as genetic modification of crops that can recruit bacteria to fix nitrogen and the development of genetically modified microbes that produce nitrogen even when it's already present. While these solutions hold promise, they are still in development and need significant innovation to become practical. In the meantime, some farmers are using additives to help plants take up more nitrogen, but these have limited effectiveness and are not widely used.
Gates, Bill. How to Avoid a Climate Disaster (p. 123-125). Penguin Books Ltd. Kindle Edition.