"Tim Lamb" > wrote in message
...
> In message >, Jim Webster
> > writes
>>
>
> I usually avoid mega-threads:-)
>
> Somewhere, way back up this one, is the assumption that all acres of land
> are equal and could produce average yields of Soya, Wheat beef etc.
>
> There is also the assumption that cereals and legumes can be grown without
> necessary rotation.

There is a strong underlying lack of knowledge about the practicality. I
know that there has been work done now with organic systems of rotation
which will get yields up to about the same as conventional, continuous
cereals, but only for two or three yields a decade when you have the cereal
crop, in the other years you tend to be using livestock to build up the
fertility.
Also as you say there are problems of climate and land type. Anyone in the
UK dependent on soya as their protein source is going to be importing most
of their protein, althrough of course they could make do with broad beans
and peas.

>
> Taking the top end figures for each case does not make a strong argument:
> ranched beef may well take 4 years to finish but not on land that would
> support continuous Wheat. Soya may well produce high yields of usable
> protein but I doubt it can be grown in all parts of the US. Continuous
> cropping usually leads to reduced yields and higher chemical inputs.
>
> regards

this is true but one of the advantages of GM varieties is that it helps
limit this and allow continuous cropping to go on longer without depleting
soil moisture too much

Jim Webster