Thursday, March 24, 2011

Energy and agriculture

We live from one four-hundredth of the sun's energy
(another posting about energy and food will come soon)
Approximately 130 Joule[1] per cm2 reaches the earth as solar radiation. Some of it is absorbed by the atmosphere directly or is reflected. 91 Joule reaches the surface of the planet, of this 18 Joule is reflected again; 31 Joule is radiating as heat; 36 Joule is used for the evaporation of water; 6 Joule heats the soil and the plants tie up some 1 Joule as chemical energy (Bayliss-Smith 1982). It is this little part, less than a percent of the sunlight that reaches the surface that is the plants' share of the solar energy. And it is this tiny fraction we use for food, feed, fibre or biofuels. Or rather, it is only a part of this tiny fraction as we rarely use the whole plant. If one calculates backwards from the crops actually harvested, we find that in 1993 harvested products represented only 0.4 percent of the solar energy reaching the fields. Of these 0.4 percent we actually only used 61 percent, i.e. our real use was only around 0.25 % of the solar energy that reach the ground (Uhlin 1997). There is a theoretical maximum for the efficiency of the photosynthesis on around 3-4 percent, but in reality, it is most likely hard to go above 1 percent for a total farming system. Of the solar energy not absorbed into plants, we use a small fraction in the form of hydroelectricity and wind power. And then we use biomass from forests and some of the solar energy that reaches the ocean in the form of fish, sea food and sea weed.

The simple agriculture equation has always been that one has to get substantially more energy out of the food than one put into the production of food. As long as in-energy is human labour it is an iron law that can only be skipped for shorter periods. Obviously we also need other things than energy from food, we need proteins, vitamins etc. but without a positive energy balance at the core, these others are not important as the farmers, or their families at least, will fade away. The agriculture worker should not only feed herself or himself but also other family members who are to young, too old or sick to work, as well as some few others that supply services. Finally, in almost all society there have been rulers that has taken a great share of the production. In pure agrarian societies, around 80 percent of the population is engaged in farming. The other 20 percent are living from the farmers.

One can roughly divide the agriculture systems into three groups based on their energy use. This division is also reflected in the tools used, the degree of specialization and market orientation etc.

- pre-industrial farming, where the external energy is less than ten percent of the total energy (i.e. human labour represents the totally dominating energy source);

- semi-industrial agriculture systems where external energy is 10-95% of the total energy, and

- industrial systems where internal energy supplies constitute less than 5% down to completely negligible proportions.

Looking at these system one can see the following pattern

- the total energy harvested per hectare can increase with increased use of ancillary energy, perhaps with a factor of five, i.e. one can increase yield per hectare fivefold with the use of more energy. This energy can be in the form of better (and more timely) soil preparation, irrigation, fertilizers etc.

- The ratio between energy out and energy in, i.e. efficiency in use of energy, seem to be fairly constant to a certain level after which it rapidly deteriorates. In industrial farming system we have since long passed the optimal use level

- Harvested energy per labour unit increases dramatically with increased input of energy with a factor of between ten and hundred, allowing the most advanced agriculture systems to have one farmer per hundred persons. Admittedly a lot of support work is needed to that farmer and the whole modern food industry employs a lot of people so the actual net efficiency increase is less, but still very substantial. (Bayliss-Smith 1982)

Table 6 energy efficiency in seven agriculture systems

Energy use per hectare (MJ)

Energy harvest per hectare and year (MJ)


fossil fuel



productivity per person day (MJ/person day)

Energy ratio

(harvested energy/used energy)


New Guinea[2]






England 1826













India 1955






India 1975







Soviet union






England 1971






source: Bayliss-Smith 1982

A barrel of oil for a ton of maize

According to FAO, 6,000 MJ of fossil energy (corresponding to a barrel of oil) is used to product one ton of maize in industrial farming, while for the production of maize with traditional methods in Mexico only 180 MJ (corresponding to 4.8 litre oil) is used. This calculation claims to include energy for synthetic fertilizers, irrigation and machinery, but not "shadow energy", i.e. energy used for making machinery, transporting products to and fro the farm, and for construction of farm buildings (FAO 2000). The energy ratio is negative (below 1) for modern rice farming and just above one for modern maize farming, while traditional production of rice and maize give a return of 60 to 70 times on energy used. Schneider and Smith (2008) land on an energy ratio that is a lot more depressive reading. Their figures state that almost 150 times more energy put in than taken out from the food system (more than just farming). Their calculations include livestock and are drawn from macro level data and not cases. Their figures appear to be implausible if put in relation to the total energy use of modern society. Johansson and others (2010) say that total agriculture energy output content is 19,900 TWh, of which 17, 560 TWh was considered edible. This corresponds to four fifth of all transport energy in the world or roughly one fourth of all energy consumption. If that is the case to output/input ratio can't clearly not be much lower than 0.5-1 as there are many other uses for energy than farming. In any case, the ratio is bad, and there is no doubt that it is worse in high income countries than in low income countries; in industrial farming system than in traditional farming systems (with the exception of swiddening farming[3]).

FAO has also compiled average data for energy yields for developed and developing countries respectively (see table). It shows that developed countries use more than double the amount of energy to produce a ton of grain, and three times as much per hectare (the reason for it being more per hectare is that yields are a bit higher in developed countries). FAO notes that “productivity is higher” when more energy is used, and with that they mean in particular productivity per labour unit. One could of course put it the other way round and say that the productivity measured on used energy is very low. When we discuss bio-energy this discussion is suddenly very relevant.

Table 7 Energy use and efficiency in Developed and Developing Countries

Energy (MJ) per hectare

Energy (MJ) per ton grain

Energy (MJ) per farm worker

Developing countries

4 019

2 009

4 144

Developed countries

13 062

4 856


Source: FAO 2000

Different kinds of agriculture production and different food also have different energy ratios. The energy ratio is very low for deep sea fishing; for meat production from feed lots and from vegetables in heated green houses[4]. A big share (often above 50%) of the energy use in farming is for the production of synthetic fertilizers, in particular nitrogen fertilizers, and pesticides. This also means that the contentious debate about organic versus conventional (non-organic) farming has a strong element of energy dependency debate. If improved energy ratio is a primary goal for farming, skipping, or at least dramatically reducing, nitrogen fertilizers, is one of the best ways to get there.

Figure Energy use per kg for selected foods

source: Carley and Spapens 1998

Why the petrol price and the grain price follow each other

Farming uses energy in many different forms: diesel for tractors and pumps; electricity for pumps; fans and in-door machinery such as milking machines. Fertilizers represent a big energy use. Energy represents 90 percent of the production costs for nitrogen fertilizers, 30 percent for phosphorus fertilizers and 15 percent for potassium fertilizers. For production in the USA energy costs represented between 22% and 27% of the production costs for wheat, maize and cotton and 14% of the production costs for soy beans[12] (US CRS 2004). These figures do not include embedded costs in buildings; machinery etc. so the actual share of the costs is substantially higher. In Argentina energy costs were calculated to 43% of production costs in 2006 (Baltzer et al 2008). In a situation with rising energy prices, agriculture prices will follow suit. This could also be seen in the food price – and oil price hike 2007-2008[13]. Energy prices influence food prices in four different ways:

- by making the production more expensive

- by making biofuel more interesting to produce and therefore reduce the production of food

- increased transport costs which directly reflect on food prices

- reduced competition in the food sector (increased transport costs means that the pressure of global competition is reduced)

[1] An energy unit, 1 Joule is 1 Ws

[2] The example from New Guinea is of a very primitive farming, based on swiddening. Energy in the swiddening itself doesn’t seem to be included. As can be seen the energy ratio appears to be maintained initially to be radically lower when external energy is becoming dominating.

[3] I have not found any figures for energy use for swiddening, but if we assume that there is some 100 qubik meters of wood per hectare which is burnt and that the land is used for farming three years, that would mean that some 35 kubik meters of wood is "used" per year. That would correspond to the energy of some 3 kubik meters if oil, which would mean an appalling energy efficiency, even worse than most industrial systems.

[4] This is hardly surprising as they don’t contain a lot of energy and heated greenhouses in the Northern hemisphere need a lot of energy for heating and even artificial light.

[12] Who can be grown without nitrogen fertilizers as they have natural nitrogen fixation.

[13] There were also other factors driving this, but increased oil price doubtless was one of, if not the main driver.

Extract from Garden Earth

More energy posts.

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