Checks and Balances

Chapters:

  1. Fertilizer

Tables:

Figures:


1. Fertilizer

Humans convince plants to grow and grow more in places that have never seen the plant grow naturally. With the aid of two contrived components, water and fertilizer, we can plant a seed and calmly, reasonably expect the plant to provide nurture. Usually its many more seeds than what where originally planted. 

The International Fertilizer Industry Association (IFA) tabulates a usage of 140 megatonnes (Mt) in the year 2002. The following table show the relative amount.

Table 1: Fertilizer Components

Component Amount (Mt)
Nitrogen (N)  84.1
Phosphate (P2O5) 33.5
Potash 23.2

There is no doubt that adding these components increases the crop yield. But, there's a maximum benefit (see [1]). And there's a cost to making or gathering the fertilizer and depositing it on the ground.

The European Fertilizer Manufacturers Association (EFMA) paints a rosy picture. They indicate an energy production cost for the fabrication of Nitrogen as in the following (see[2]).

Table 2: Nitrogen Production Cost

Year Method Rate (GJ/t)
1910 Birkeland-Eyde Electric Arc 400
1915 Cyanamid 200
1930 Haber-Bosh Synthesis 100
1975 Steam Reforming Natural Gas 50

In their example, they use wheat as the seed. By experimentation, they determined an optimum application of 170 kg N per hectare. This yielded 8.2 tonnes per hectare (having absorbed 126 GJ of solar energy). Without Nitrogen, the yield was 3.5 tonnes per hectare (with 71 GJ of solar energy captured).

Next, they wish that all 16.8 million hectares used for growing wheat used this optimum application of Nitrogen. They believe that the result is a yield of 252 GJ worth of biomass with 126 GJ of grain (ie seed) and 126 GJ of straw.

Also, by their estimate, the energy cost for the production, transportation and spreading of Nitrogen is 40, 1, and, 3 GJ per tonne respectively. If 8.2 tonnes are applied to one hectare, the total cost is 360.8 GJ/ha. This results in an increase of 55 GJ of solar energy or an energy benefit of 0.15. Presumably this doesn't take into account the cost to harvest the wheat, deliver the wheat to manufacturers, mill the wheat into flour and make the flour into food. But, it is a good indicator.

[1] Yara, Agricultural Energy Balance

[2] EFMA, 'Harvesting Energy with Fertilizers', accessed 2009 Feb.

by Mark Foster Mortimer