AgBioView - http://www.agbioworld.org, http://agbioview.listbot.com
The limitation on organic farming comes from nitrogen fixation rates in the soil. That is to get a decent yield
you have to alternate with legumes as the best possibility otherwise you don't get enough N in the soil.
Legumes fix about four times the natural N fixing organisms. If you use pasture instead and apply manure
to your crop some of the N is wasted by growing cattle, other is wasted by loss of N from ammonia in the
manure unless inserted immediately in the soil. Then you have problems with microbial contamination of
food. Composting loses volatile N in ammonia and NO. The standard reference to look at is Smil. Global
population and the Nitrogen Cycle. Scientific American. July 1997. He gives a figure of 4 billion that can be
fed through organic processes. However I disagree with his calculations and I would mark that down to 3
billion for many reasons.
There is variation in available agricultural land between countries with different levels of population i.e.
China with 7% of the arable land surface and nearly one quarter of the world's population. Thus they have
about 100 million ha and a population density of about 10-15/ha. Most food is grown and eaten still in the
country of origin. There are of course different climates and other factors ameliorating factors below.
However the primary reason for ignoring such calls about organic food is why bother for the sake of
ideology. We use knowledge to improve our lot on this planet not to obey some form of agriculture set in
stone by people who didn't understand much about it. Organic food is one of those new kinds of religion
that are adhered to by people with little biological knowledge of the world in which they live. We know
synthetic pesticide residues are irrelevant in conventional food (one ten thousandth of total carcinogen
intake; Ames 1990) and the Haber cycle for N reduction is perfectly sustainable. We also use land for crops
other than food crops; e.g. wood, flowers, oils, pharmaceuticals, spices or cotton that occupy land but we
do not eat or that provide little in the way of calories.
I think Smil's estimate of four billion is based on N fixation by Alfafa since he quotes N fixation as about
200Kg/ha. N fixation by peanut or bean is one quarter to one fifth of alfafa (Chrispeels and Sadava page
454) . The land currently under the plough is about 1.34 billion acres according to Lloyd Evans. The
calculation goes like this. Recommended levels of N /person/day for human consumption are 60 gm which
turns into 22Kg of protein/ year. If you grow beans on a field and plough it in then you get about 50 Kg of
N /ha. No yield from this field that year because you need all the N for your cereal which would turn into 50
Kg of plant protein on the following year. But of course nothing is 100% ( 2nd law of thermodynamics of
first law I can't remember which) and losses are to be expected but I cannot find suitable figures for green
manure. Since two years are required to produce this yield of say wheat then only 0.67 billion ha are really
available for food. For agriculture run this way the world could feed only 1.4 billion. The numbers could be
hypothetically increased by using red clover which is about double the N fixed for bean and four times this
if you use alfafa. Alfafa grows in semitropical conditions not temperate climates.
Those who have tried the organic method out, e.g. CWS farming systems in the UK and compared it with
integrated crop management find that a legume crop ploughed in has to survive a winter leaching which
reduces N available when it is really needed i.e. during canopy expansion. Furthermore wheat unlike corn
does not respond well to manure (Chrispeels and Sadava ) and as said above manure unless infiltrated into
soil loses a lot of N through ammonia and NO. Studies here in the UK showed that manure breaks down
into soluble N throughout the year and it continually leaches from organic fields into water courses often
exceeding recommended levels throughout the year. So the time when N is really needed, when the canopy
is forming the N is there but it is also there when you don't need it and you lose it. So organic is wasteful of
this particular product as well. Also the quantity of N in manure is variable as is K and P. Many organic
farmers must rely on magnesium and sulphur recycling or rain water because they don't use other minerals.
Furthermore manure is used for other purposes in tropical and semitropical parts of the world such as fuel
and building huts. The interview with Conway some months back showed that he recommended Mugabe
of Zimbabwe to use minerals because there are no minerals in the soil, crop quality is poor and manure is
used for other purposes i.e fuel. That along with other non-edible crops grown on land and the mal-
distribution of people with available land led me to estimate last year that three billion are the maximum on
present arable land and with present technology for organic farming. Lloyd Evans quotes the same figures
based on an average of one ton/ha from traditional methods. Of course if one wants to plough up more
wilderness then more is possible! But average yields with added N are about three times what are routinely
gained by organic farmers although this depends on the inputs and how it is done and where it is done.
Average corn yields in the USA are about 7 tons/ ha and records about 20 tons/ha. Record yields I have
seen for organic corn are about 7 tons/ha.
Most benefits so far as I can see claimed for organic agriculture are simply provided by crop rotation. Of
course you should realise that none of this will convince organic zealots just as all the predictions about
the end of the world at 2000 haven't changed the attitude of those who believe such nonsense; just the
date. Fortunately governments usually believe in pragmatism and economic forces and requirements for
food lead to more sensible attitudes.
Anthony Trewavas FRS
Institute of Cell and Molecular Biology
University of Edinburgh
Edinburgh EH9 3JH
Phone 44 (0)1316505328
Fax 44 (0)1316505392
web site http://www.ed.ac.uk/~gidi/main.html
To view the web site simply click on the address