Re: Soil Quality Attributes

I have been observing the discussion on soil quality and would like 
to add a query.  An issue which has been taken up with fervor by my 
colleagues in soil science has been the notion that potential for 
nitrate contamination of groundwater is **reduced** by using chemical 
rather than organic sources of N.  

The logic presented is that organic matter mineralizes in 
ways/rates/timing which a) is not controllable, and b) is not easily 
predictable, and hence, c) has the potential to continue to pump out 
labile N long after the crop has been harvested.  On this basis, they 
discount the merit of long-held organic farming wisdom, namely, "feed 
the soil" as a source of immobilized and slowly released mineralized 
nutrients to feed the crop.  The next dimension to this logic is that 
perennial forages, which enhance soil OM, are hazardous to the 
environment because when they are plowed under, they mineralize and 
release copious amounts of N - with again, the potential to pollute.

In response, I can only agree with their logic **if** one continues 
to crop in conventional ways - with the land bare or under dead 
stubble for 7- 10 months of the year.  However, a great deal of the 
rationale behind organic field crop agriculture is to keep the land 
covered with a live - and predominantly, growing - crop year-around.  
This practice, in addition to supplying a sink for labile nutrients, 
also keeps the soil colder longer and enhances the synchrony between 
such T-sensitive processes as microbially-based mineralization and 
plant-based uptake.

I would be interested in the comments of SANETers on these issues, 
particularly as they pertain to the definition of soil quality, and 
ultimately, the attributes of the "ideal" soil that we should be 
shooting for.  Ann

Dr. E. Ann Clark
Associate Professor
Crop Science
University of Guelph
Guelph, ON  N1G 2W1
Phone:  519-824-4120 Ext. 2508
FAX:  519 763-8933