The work output of a machine is less than the work input what factor cause this

Total work energy on the input side is WE = Fs; where F is a force acting on a mass to push it s distance. This is the so-called work function. Let fs = we, which is the work energy (useful energy) attained as output when WE is input. 

From the conservation of energy WE = Fs = fs - kNs = Total Output energy. Net force f = F - kN where kN is friction force acting against the pushing (input) force F. In the real world, there is always friction at some level. That is kN > 0 always. 

Thus Fs = (F - kN)s; kNs = the energy lost to friction where k is the friction coefficient and N is the normal force on the surface(s) where the friction is generated. By definition, efficiency = fs/Fs = useful work/work input. Clearly fs = Fs - kN < Fs . Thus efficiency = fs/Fs < 1.00, which means output fs < Fs the input whenever kN > 0, which in the real world it always is. 

The short answer is...output is always less than input because of friction and, sometimes, other losses like wind drag (which is a form of friction anyway).

What even is this. where is the question

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.