Applied and Environmental Soil Science

Review Article

Managing the Nutrition of Plants and People

Table 1

The main chemical forms in which mineral elements are acquired from the soil solution by roots, and the critical leaf concentrations for their sufficiency and toxicity in nontolerant crop plants. The critical concentration for sufficiency is defined as the concentration in a diagnostic tissue that allows a crop to achieve 90% of its maximum yield. The critical concentration for toxicity is defined as the concentration in a diagnostic tissue above which yield is decreased by more than 10%. It should be recognized that critical tissue concentrations depend upon the exact solute composition of the soil solution and can differ greatly both between and within plant species. The latter differences reflect both ancestral habitats and ecological strategies. Data are compiled from references [4, 9, 10].


Element	Form acquired	Critical leaf concentrations (mg g⁻¹ DM)
Element	Form acquired	Sufficiency	Toxicity

Nitrogen (N)	NH₄ ⁺, NO₃ ⁻	15–40
Potassium (K)	K⁺	5–40	>50
Phosphorus (P)	H₂PO₄ ⁻	2–5	>10
Calcium (Ca)	Ca²⁺	0.5–10	>100
Magnesium (Mg)	Mg²⁺	1.5–3.5	>15
Sulphur (S)	SO₄ ²⁻	1.0–5.0
Chlorine (Cl)	Cl⁻	0.1–6.0	4.0–7.0
Boron (B)	B(OH)₃	5–100 × 10⁻³	0.1–1.0
Iron (Fe)	Fe²⁺ Fe³⁺-chelates	50–150 × 10⁻³	>0.5
Manganese (Mn)	Mn²⁺ Mn-chelates	10–20 × 10⁻³	0.2–5.3
Copper (Cu)	Cu⁺, Cu²⁺ Cu-chelates	1–5 × 10⁻³	15–30 × 10⁻³
Zinc (Zn)	Zn²⁺ Zn-chelates	15–30 × 10⁻³	100–300 × 10⁻³
Nickel (Ni)	Ni²⁺ Ni-chelates	0.1 × 10⁻³	20–30 × 10⁻³
Molybdenum (Mo)	MoO₄ ²⁻	0.1–1.0 × 10⁻³	>1
Sodium (Na)	Na⁺	—	2–5
Aluminium (Al)	Al³⁺	—	40–200 × 10⁻³
Cobalt (Co)	Co²⁺	—	10–20 × 10⁻³
Lead (Pb)	Pb²⁺	—	10–20 × 10⁻³
Cadmium (Cd)	Cd²⁺ Cd-chelates	—	5–10 × 10⁻³
Mercury (Hg)	Hg²⁺	—	2–5 × 10⁻³
Arsenic (As)	H₂AsO ₄⁻, H₃AsO₃	—	1–20 × 10⁻³
Chromium (Cr)	Cr³⁺, CrO₄ ²⁻, Cr₂O₇ ²⁻	—	1-2 × 10⁻³