Table 24. General guidelines for critical, sufficient, and toxic plant tissue levels of plant nutrients.

Elements	Critical level	Sufficient range	Toxicity level
N, %	<2.0	2.0-5.0	Nontoxic
P, %	<0.2	0.2-0.5	Nontoxic
K, %	<1.0	1.0-5.0	Nontoxic
Ca, %	<0.1	0.1-1.0	Nontoxic
Mg, %	<0.1	0.1-0.4	Nontoxic
S, %	<0.1	0.1-0.3	Nontoxic
Fe, ppm	<50	50-250	Nontoxic
Zn, ppm	15-20	20-100	>400
Mn, ppm	10-20	20-300	>300
Cu, ppm	3-5	5-20	>20
B, ppm	<10	10-100	>100
Mo, ppm	<0.1	0.1-0.5	>0.5
CI, %	<0.2	0.2-2.0	>2.0
Si, %	<0.2	0.2-2.0	Nontoxic
Na, %	<1.0	1.0-10	Nontoxic
Co, ppm	<0.2	0.2-0.5	>0.5
V, ppm	<0.2	0.2-0.5	>1

Data are from numerous references and numerous analyses based on the author's professional experiences. Levels of nutrients in certain crops can range to higher levels without toxicities. For example, the sufficient range for S is for grains and legumes, whereas the values for crucifers are generally three to five times greater.
Nutrients listed as nontoxic, when in excess, may cause imbalances and detrimentally affect growth, but they seldom are toxic.

Source: Nutrient deficiencies and toxicities in crop plants. William F. Bennett. APS Press p. 6.

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