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market-farming - RE: Nitrate Acc (was greenhouse ques)

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  • From: "David Inglis" <mhcsa AT bcn.net>
  • To: "market farming" <market-farming AT franklin.oit.unc.edu>
  • Subject: RE: Nitrate Acc (was greenhouse ques)
  • Date: Thu, 1 Feb 2001 16:34:18 -0500

Last weekend  I attended a workshop on out of season production with 70+ others and there was a whole section on Nitrate accumulation.
A lot of the data on nitrate comes out of Holland which has a serious greenhouse industry, both conventional and organic and it should be remembered that they are a lot further north than most of us and therefore have much lower light levels.
Nitrate build up becomes a problem when the ability of the plant to absorb nitrate develops faster than the ability of the plant to metabolize the nitrate into protein. The conditions when this occurs are influenced by many factors, most of which are under our control in the greenhouse.
Low levels are achieved by:
  • harvesting before maturity
  • low light level [only one of the presenters used supplemental lighting and then only at 1/3 of the recommended strength ]
  • harvesting late in the day
  • use mature vegetable based compost
  • avoid 'butterhead type varieties
  • High p.h.
  • balanced fertility
  • adequate moisture
In other words non forced low stress conditions we need for quality production anyway.
 
European Community Nitrate limits are 675-1015 mg/kg in lettuce and 450-790 mg/kg in other leafy greens.
Nitrate meters for measuring leaf levels of nitrate are expensive but can easily be shared and considering the claimed income /sq.ft  for winter production should be considered an essential tool. Peaceful Valley farm supply has them I think.
Also, one presenter hung 1lt. bottles filled with water from the cross ties of his house by the thousand as a thermal flywheel.
This was a three day workshop so the above info is of course only a small part of the story.
Yours
David Inglis
Mahaiwe Harvest C.S.A.
Massachusetts.



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