You could say 'peasant farmer' but 'peasant' does have rather negative connotations once you get past the middle ages in historical terms. :-)

ew. part-time worker/part-rime farmer

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Note added at 20 mins (2006-06-08 09:41:14 GMT)
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wg słownika rolniczego

e.g. :

Campaigns & Issues... 2600 (25%) farm workers are considered temporary or seasonal, ... Official statistics indicate farm workers earn the following average wages, ...
www.afl.org/campaigns-issues/farmworkers/background.cfm

I'd say "temporary" is a proper translation, but "seasonal" may be good in an appropriate context.

"farm worker" is way more common both in normal usage and in legal usage, while "agricultural worker" is rarely used albeit correct.

See also:
FINAL BILL REPORT ESSB 5599 C 374 L 99 Synopsis as Enacted Brief ...In 1996 the Legislature exempted goods and services used to construct, repair or improve housing for temporary farm workers from sales and use tax. ...
www.leg.wa.gov/pub/billinfo/1999-00/ senate/5575-5599/5599-s_fbr_05181999.txt

College Assistance Migrant Program (CAMP)Seasonal farmworker: a person employed in farmwork for at least seventy-five days on a temporary or seasonal (not year-round) basis. ...
www.equity.psu.edu/camp/proginfo/prog_info.asp

and countless others.

Also

Definition & synonyms + related words here :
http://www.thefreedictionary.com/farm worker

You can search Google for "farm worker legal definitions" and you'll see :-)


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Note added at 2 hrs (2006-06-08 11:47:50 GMT)
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P.S. I based my translation on definition of "Chłoporobotnik" as per Polish Wikipedia :

http://pl.wikipedia.org/wiki/Chłoporobotnik

Chłoporobotnicy kategoria społeczno-zawodowa, mająca znamiona warstwy społecznej oznaczająca osoby zatrudnione na stanowiskach pracowniczych, zazwyczaj jako niebieskie kołnierzyki, które jednocześnie prowadzą gospodarstwa rolnicze. Kategoria ta występowała w Polsce okresie PRL-u. Po 1989 roku wiele osób z tej warstwy stawało się bezrobotnymi ze względu na to, iż wobec konieczności zmniejszania zatrudniania w zakładach pracy, tego typu osoby posiadały dodatkowe środki utrzymania.