Hello

Thanks for the guided tour to the rooftop garden demonstration site. I’m glad I finally got to see what I have been hearing about: quite impressive!

I looked up the problem you had pointed out on some of the lettuce plants, chlorosis on the margins of the lower leaves. I’m not 100% sure but it might be a mild potassium deficiency; if the symptoms progress and show distinct necrotic areas (dried-up brown or tan areas) on the lower leaves then it would be definitely a potassium deficiency. Mind you, this could occur not because there is necessarily a lack of potassium in the fertilizer but because there is an excess of certain other minerals, especially cations like ammonium, sodium, etc.

There has been a little bit of research on using organic fertilizers in hydroponic systems (see for example an article in HortTechnology 14(2):235-239 or an on-line report at http://www.ces.ncsu.edu/depts/hort/greenhouse_veg/pdf/lettuceorgfert.pdf ). The results are variable; one of the issues seems to be the need to ensure adequate aeration of the fertilizer solution so that organic compounds will be decomposed to release nutrients into readily available forms. This is particularly true for nitrogen as plants will only absorb nitrogen in the nitrate or ammonium forms. My guess is that ideally the fertilizer solution should be aerated before being applied AND the plants have to be grown in a medium with lots of air spaces (e.g. perlite or vermiculite).

Biomer (a.k.a. BioSea) organic fertilizer (a blend of fish emulsion and seaweed) is available from Plant Prod. and Aero-kure; Aquaterre has some similar products. This on-line document has an extensive list of organic fertilizers and suppliers in Canada (careful: it’s a big file): http://www.agrireseau.qc.ca/agriculturebiologique/Documents/MIB.pdf

I wish you a great growing season. Perhaps I’ll be able to drop by again later in the summer.

Regards,

David Wees

P.S. thanks for the invitation to today’s garden party but I’m feeling a little under the weather so I don’t think I’ll be able to make it.

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David Wees, agr.
Macdonald Campus of McGill University
Sainte-Anne de Bellevue, Quebec
tel: (514) 398-7756
fax: (514) 398-7955
david.wees@mcgill.ca
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