ferti -fre- gismu

x₁ is fertile/conducive for supporting the growth/development of x₂; x₁ is fruitful/prolific.

Also fecund (note that the Lojban covers both potential and actual/realized fertility). See also vanbi, sidju, rorci, farvi, banro, cange.

In notes:

banro: x₁ grows/expands [an increasing development] to size/into form x₂ from x₃.
cange: x₁ is a farm/ranch at x₂, farmed by x₃, raising/producing x₄; (adjective:) x₁ is agrarian.
farvi: x₁ develops/evolves towards/into x₂ from x₃ through stages x₄.
rorci: x₁ engenders/procreates/begets x₂ with coparent x₃.
sidju: x₁ helps/assists/aids object/person x₂ do/achieve/maintain event/activity x₃.
vanbi: x₁ (ind./mass) is part of an environment/surroundings/context/ambience of x₂.
tolferti: x₁ is sterile/non-fertile/unable to procreate x₂; x₁ is unable to support the growth/development of x₂
fertis: x₁ is Virgo [constellation/astrological sign].