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4. Melachas for loss prevention:
It is permitted to perform all Rabbinical Melachas for the sake of preventing loss or damage to ones crops or garden. This is called “Leikumeiy”. It is however forbidden to do so simply for the sake of its extended growth and nurture. This is called “Leivruyei”. It is hard to define which Melachos are defined as Leikumeiy and which are defined as Leivruyei. Nevertheless certain Melachas are clearly defined as Leikumeiy and permitted while others are clearly defined as Leivruyei and are forbidden.
 See Rambam 1/8 regarding watering the land