Table 1.

Soil nutrient requirements and critical concentrations.

Nutrient	Critical Concentration	Reference
Primary nutrients
Nitrogen (N)	25–50 mg/kg	[25]
Phosphorus (P)	1 µM *	[26]
Potassium (K)	141–370 mg/kg	[27]
Secondary nutrients
Calcium (Ca)	6–778 mg/kg *	[28]
Sulfide (S)	>15 mg/kg	[29]
Magnesium (Mg)	0.05–0.5 % *	[29]
Micronutrients
Cobalt (Co)	15–25 mg/kg *	[30]
Copper (Cu)	>0.04 mg/kg	[29]
Boron (B)	>0.75 mg/kg	[31]
Chlorine (Cl)	100 mg/kg *	[32]
Iron (Fe)	>7.5 mg/kg	[33]
Zinc (Zn)	>1.5 mg/kg	[33]
Manganese (Mn)	>4mg/kg	[34]
Molybdenum (Mo)	>0.2 mg/kg	[35]