5.2.8—Synthetic fertilizers

Synthetic fertilizers supply soluble nutrients rapidly and can sustain high intensive-system productivity, but fertility also depends on organic matter, soil structure, water retention and biological cycling.

Approach	Main fertility mechanism	Boundary or trade-off
Synthetic fertilizer	rapidly supplies selected mineral nutrients	manufacture can depend on fossil fuels; excess may leach or run off
Fallowing	allows vegetation and nutrient cycles to recover	requires time and land
Manure, humanure or compost	returns nutrients and organic matter	must be managed to limit pathogens and nutrient loss
Herbal mixed leys or mycorrhizae	supports roots, soil organisms and nutrient access	benefits depend on crop and soil conditions
Continuous-cover forestry or agroforestry	retains cover and cycles biomass through roots and litter	may complicate mechanization or compete for light and water

If a field is nutrient-poor, compacted and low in organic matter, soluble nitrogen may give a short response while compost, cover and biological methods address slower structural and cycling problems.

Match the input to the limiting process and judge nutrient supply, retention, timing and external losses together.

Natural inputs can also pollute when applied faster than crops and soils can retain their nutrients.