( This ( ( is ( why ( ( ( the toxicity ) ( of ( these herbicides ) ) ) ( is ( very low ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (SBAR (WHADVP (WRB why)) (S (NP (NP (DT the) (NN toxicity)) (PP (IN of) (NP (DT these) (NNS herbicides)))) (VP (VBZ is) (ADJP (RB very) (JJ low)))))) (. .)))
This is why the toxicity of these herbicides is very low.
(ROOT (S (NP (NP (DT The) (JJ main) (NN advantage)) (PP (IN of) (NP (NN urea)))) (VP (VBZ is) (NP (PRP$ its) (ADJP (RB very) (JJ low)) (NN toxicity))) (. .)))