Fertilizer use and energy production

Photosynthesis is a process that is unique to plants. Photosynthesis allows a plant to convert the energy from the sun into sugars. Sugars are the building blocks for any growth of biomass.

Agriculture produces biomass in the form of vegetables, cereals and feed, which are the energy sources for humans and animals. Biomass and the processed food (e.g. bread made from wheat grain) have a defined energy content, which is measured in joules or calories. Today, questions are often asked about the energy efficiency of modern agriculture. This is mainly because of the high consumption of energy in the production of farm inputs, in particular mineral nitrogen fertilizers, and the high degree of mechanization on the farms.

Generally, energy inputs into agricultural systems result in a considerable energy output from crop production. Crops produced convert so much solar energy into energy stored in plant biomass that the energy input is more than offset, that means that the energy balance of biomass production in agriculture is very positive. The amount of energy obtained depends primarily on the crop yield and is therefore influenced by the growing conditions, the kind of crops grown and the level of mineral nitrogen fertilizer application to the crop.

Crop yields increase as more mineral nitrogen fertilizer is applied, but there is an economic optimum nitrogen fertilizer rate which gives the farmer the highest profit per hectare. This is shown in Fig.1 in an example for winter wheat production. In these winter wheat trials the economic optimum nitrogen application rate was 170 kilograms of nitrogen per hectare. At this nitrogen fertilizer rate, wheat yields were approximately 8.2 tonnes a hectare, compared with 4.7 tonnes a hectare without nitrogen fertilizer. All crop yields can be converted into energy, as one ton of winter wheat grain contains about 15 GJ of energy.

Fig. 1: Grain yield increases with increasing nitrogen fertilizer rates (170 kg N/ha = economic optimum nitrogen fertilizer rate)

Fig. 2 shows the biomass data from Fig.1 converted into energy outputs per ha. The energy output increases with increasing N fertilizer rates. Without N fertilizer an energy input of 7.5 GJ/ha is required to produce the wheat. However, the extra energy captured when using N fertilizer at economic optimum rate is much higher then the additional energy input required to produce, transport and spread the nitrogen fertilizer. For winter wheat the extra 55 GJ captured when using nitrogen fertilizer at optimum rate is more then 6 times the 8 GJ used for the fertilizer input.

Fig. 2: The extra energy captured through fertilizer input at optimum N rate is more than 6 times the energy consumed to produce, transport and spread the nitrogen fertilizer

Crops differ in their energy output per ha of land. Fig. 3 shows the energy output for winter wheat and sugar beet at optimum nitrogen fertilizer rates of 170 and 154 kg per hectare, respectively. The energy output in form of one hectare of beet is much higher than the energy output for winter wheat. At first sight this is surprising, because one tonne of beet contains much more water and therefore less energy than one ton of wheat grain (3.7 GJ/t compared to 15.4 GJ/t). The differences in the dry matter content between both crops, however, are more than compensated by the differences in the total amount of biomass that is produced on one hectare of land. In this study the sugar beet produced 58 tons of biomass per hectare compared to only 8.2 t wheat grains per hectare.

Figure 3: At optimum nitrogen fertilizer rates different crops produce different biomass yield, which in turn results in different energy outputs per hectare

Many energy balances on agricultural production that have been carried out confirm the results as shown above. The energy balance (energy output minus energy input) is positive for all crops, although the amount of energy produced per hectare of land may differ from crop to crop. Mineral N fertilizer use at optimum rate helps to enhance this positive energy balance further.