If Substance A has an LD50 of 20 mg/kg of body mass, and Substance B has an LD50 of 10 mg/kg of body mass, which substance is considered more deadly? Substance A is deadlier than Substance B. Substance B is deadlier than Substance A. Both substances are equally deadly enough to half a population. Neither of them are deadly because they do not reach at least 50 mg/kg.

desert perennials often survive by remaining dormant during dry periods of the year, then springing to life when water becomes available. most annual desert plants germinate only after heavy seasonal rain, then complete their reproductive cycle very quickly.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.