🚀 go-pugleaf

In article <1990Nov19.130439.9168@newcastle.ac.uk> you write:
>In the past crop rotation without the use of artificial fertilizers
>was used to maintain the productivity of fields.
>
>How did this work?

Nature does not practice monoculture.  When man does, some crops can
remove nutrients faster than they are provided by weathering of
parent material or accumulate in organic matter.  By rotating crops
time allows  the effects of a crop which might be a heavy feeder to
be offset by season(s) of crops which do not feed as heavily, or
actually add nutrient(s).  Maize for example, feeds heavily on
nitrogen.  The object of growing maize is to feed man or beast.  
Nutrients are removed from the landscape when the cobs and/or stalks
are harvested.  Alfalfa planted with rhyzobium innoculant (nitrogen-
fixing bacteria) adds nitrogen back to the soil when the alfalfa is
plowed under.  The nutrients besides nitrogen which have been removed
from the landscape as grain or hay must be relaced by weathering of
parent material or they will be limitting factors just as if only
one crop was grown.

"Old" agriculture where crops were rotated, had soils with nutrient
levels which could support long term average yields in the range of
30-40 bushels of corn and 2-3 tons of alfalfa.  Those were open
pollinated varieties.  They could respond to added "artificial 
fertilizers", but not to the degree modern hybrids do.
THERE IS NO FREE LUNCH.  There is danger in trying to justify the 
idea that productivity is maintained just by rotating crops.  
Economics dictate the "old" yields of 30-40 bushels of corn and 
2-3 tons of hay per acre probably aren't profitable.  

>
>The soil requires various elements - nitrogen, sulphur, phosphorus. 
>Now the nitrogen can be obtained from the atmosphere by growing certain
>plants in the field(eg clover), but where do the other elements come
>from to replace those amounts of them used in the previous seasons?

Soil does not require anything.  It is a recording of abiotic and biotic
activity as parent material is weathered and organic mater accumulates
to an equilibrium between gain and loss.

The real advantage of crop rotation is that on sloping land, the years
in which row crops are not planted, top soil is not being eroded at as
fast a rate.  This means different things to different soils.  In general
though erosion of top soil permanently reduces theoretical productivity
of the soil.

It is not wise to erode soils as one strives to achieve ever higher 
yields.  Also needing consideration is the amount of energy required
to make, store, transport, and apply commercial fertilizer.

Organic farming types tend to claim otherwise, but in reality, a plant
can take nitrogen up in 2 forms ammonium or nitrate.  It does not
know whether it is from an organic source or applied "chemical fertilizer".
Make no mistake about it concentrated commericial fertilizers can and do
adversely affect soil in the immediate vicinity to granuals or liquid.
This affect is short lived in the presence of adequate moisture.  The
fertlizers move in solution from areas of high concentration to low
concentration.  The other nutrients also have no superior organic source.


Jeff Frank	

USENET Email: {rutgers, harvard, ames, uunet}!cs.utexas.edu!uwm!grce!jeff