화석연료 사용을 줄일 수 있지만 화학비료를 사용해야 한다

We mustn’t allow emotions to cloud our understanding of fundamental natural laws. To feed a world of 9 billion people without chemical fertilizers would irreparably damage biodiversity. Let’s reduce fertilizer overuse in China and shift that to Africa, where lack of fertilizer is a major cause of hunger.

By Prem Bindraban, Director of ISRIC (World Soil Information)

The availability of sufficient food has been a concern throughout human history. Understandably, the fear that food will be lacking evokes strong emotional reactions, especially when forecasts, often based on extrapolation of past trends, portray a bleak future. 

Emotions are a powerful driver, but they may lead to solutions based on false hopes if they ignore basic eco-physiological processes or physical, chemical and biological laws. A ban on fertilizer use as in organic agriculture, for example, would not help attain global food security; subsidizing biofuels wouldn’t reduce GHG emissions; and pleading for vegetarian diets excluding (red) meat consumption prevents exploitation of ecological opportunities, despite the honourable intentions that give rise to such strategies. 

Energy for biology
Discussion of energy use in agriculture must begin with these unavoidable processes and laws, first of all the fact that plant growth depends heavily on the availability of “reactive” nitrogen, which is commonly applied as chemical fertilizer manufactured in a process that is very energy intensive. 

Can we find a way around that? Inert nitrogen gas in the air can be converted naturally into the “reactive” nitrogen plants need by lightning or through fixation by bacteria living in symbiosis with legumes. on highly fertile soils and with sufficient water, legumes can fix 300 kg of nitrogen per year per hectare, half of which is taken up by the two consecutive crops planted thereafter. 

Such “natural” fertilizer provides a maximum yield per hectare of 2-2.5 tons of cereal equivalents, a level is comparable to average yields in Europe and North America in the year 1900. Current global cereal yield is 3.5 tons per hectare, thanks largely to chemical fertilizers. In Europe it is 6.5 tons and in the Netherlands it surpasses 9 tons. 

Yield levels for cereals in sub-Saharan Africa are between one and 1.5 tons per hectare and even so, soils are degrading because the nutrients removed with the harvest are not replenished. Africa’s yields could indeed be increased by optimizing recycling and other natural processes. But only to about two tons per hectare. Inherently poor soil conditions and erratic rainfall mean productivity is inherently low and cannot be increased without “external interventions.”

“Natural processes have limits.”

Natural processes have limits. If all farmers phased out use of chemical fertilizers and average yields fell to only 2 tons per hectare, the demand for food could only be met by expanding the acreage under cultivation. A vegetarian diet requires 1.5 kg of grain equivalents (GE)   per person per day. Thus, if we all became vegetarians, five billion tons GE would be needed for the world’s nine billion people in 2050. At yields of only two tons per hectare, agriculture would have to expand to 2.5 billion hectares –1 billion hectares more the current 1.5 billion, with dramatic implications for world biodiversity. 

“If we all became vegetarians, five billion tons Grain Equivalent would be needed for the world’s nine billion people in 2050.”

If all were to adopt a European dietary level of 4.5 kg GE, a total of over seven billion hectares would be needed, which exceeds all the available land there is. Hence the “artificial” conversion of nitrogen (N2) into reactive nitrogen is essential if everyone is to eat. 

We can limit over-use of chemical fertilizers. Yields in Europe have increased over the past two decades while fertilizer use declined. A similar process to reduce the excessive use of nitrogen in China without sacrificing yield would free up about 70 kg of fertilizer per hectare. If those 70 kg per hectare were then used in Africa it would double yields. The fact is, not using artificial fertilizers in Africa is a major cause of soil degradation, productivity loss and poverty.

“Yields in Europe have increased over the past two decades while fertilizer use declined.”

The Haber-Bosch nitrogen fertilizer-creation process will remain essential to secure world food availability while maintaining biodiversity. Making reactive nitrogen requires a lot of energy and the amount we need will increase over time. Therefore, we must seek to maximize the amount we recycle, so as to limit energy expenditure.

Energy in farmingOn farms, energy is also used in several “non-biological” practices, to enhance the productivity of labour and optimal use of external inputs and natural resources. These include the use of fossil fuels as a direct energy source for field operations such as ploughing, weeding, input application and harvesting, and as an indirect source for the production of machines and agro-chemicals like herbicides and pesticides. 

Energy is also needed in the wider food system, i.e. in transport, storage, processing and retail. Total direct and indirect energy use in agriculture in industrialized nations is about 1 percent of total energy use; and total energy use in the entire food system adds up to 10-15 percent of all energy use. Overall, energy consumption in agriculture peaked in the early 1980’s and has gradually declined in several developed economies.

Efficiency and alternatives
Several practices can be adjusted to reduce energy requirements, yet these will imply trade-offs. Because ploughing consumes so much energy, minimum or zero-tillage may reduce the use of fossil fuels by almost half. However, these tillage practices are associated with increased use of herbicides, the production of which can almost fully undo on-farm gains in energy conservation. 

Controlled traffic can reduce energy requirements by as much as 30 percent, for instance because compressed soil need not be ploughed over and over. More precise application of external inputs according to crop conditions can also raise energy efficiency. Modern greenhouses are so efficient they can even be net producers of energy. 

In principle all fossil energy can be replaced by other forms of energy. The energy needed in the food chain for stationary processes such as the production of agricultural inputs (including reactive nitrogen) and food processing may be supplied by solar or wind power.

Yet, alternative sources for traction may be difficult to obtain. There are real technological limits to solar-powered ploughing for instance. To convert the 20 litres of diesel needed to plough one hectare (200 KWh), a tractor would need to be fitted with 1800 kg (!) of fully charged Li-ion batteries, using current solar technology. Charging that in a day would require about 500 m2 of solar cells in the Netherlands and about 250 m2 near the equator. 

“There are real technological limits to solar-powered ploughing.”

An additional technological challenge is minimizing transmission losses when converting battery power to a low-speed, high-torque application such as ploughing. Other energy solutions, such as conversion of solar or wind energy to an energy-dense liquid fuel like hydrogen, or the use of biogas and biofuels, may be more suitable. 

However, biofuels production has detrimental effects on biodiversity and GHG emissions. About 25 percent of the total energy contained in rapeseed is needed for the production of the crop under optimal conditions, and about 10 percent of the energy content of biogas is needed for the production and processing into gas from corn or sugar beets. 

The fundamental point is that plants fix only 2.5 percent of the solar energy they capture, which makes the conversion of solar energy through biology highly inefficient. Biofuels put a very large claim on productive land and water resources that will ultimately compete with food production. Because of additional claims on land, loss of biodiversity is inevitable and emissions of GHG may exceed those of fossil fuels.

Productivity
Energy plays a central role in food production and that is not about to change. Over the last thousand years agricultural productivity has increased dramatically, parallel to the increased use of energy. In the Netherlands, for instance, the yields of wheat went from 800 kg per hectare in the year 1400, to 1,800 kg in 1900, and to 9,000 kg in 2000, while associated labour requirements dropped from about 600 hours per hectare in the year 1400 to 240 in 1900 and only 12 in 2000.

“Power structures, vested interests, economics and other drivers will continue to apply no matter the source of energy for farming.”

In other words, by replacing human energy with energy derived from other sources, we have dramatically increased food output from a finite amount of land. At subsistence levels today, small-scale machinery can increase the efficiency of manual labour and animal traction to about 0.5 horse power per hectare, which is considered essential to raise crop yield levels to above 2 tons per hectare.

Whether reducing use of fossil fuels in agriculture will reduce poverty and inequality remains to be seen. Differences in labour quality and income might be larger in zero-tillage production systems than in other systems for instance. In the end, power structures, vested interests, economics and other drivers will continue to apply no matter the source of energy for farming.

Download: We can reduce fossil fuel use, but we need chemical fertilizer