Since some symptoms are more useful than others, relying solely on plant symptoms is often unsatisfactory. If plants fail to emerge, the cause may be low seed quality, root rots, use of certain chemicals, planting too deep, or severe crusting. Some leaf discolorations are fairly distinctive. Potassium deficiency causes yellowing of the leaves, starting at the outer edges. Manganese deficiency also causes yellowing at the leaf edges, but the veins stay green noticeably longer than the interveinal areas of the leaf.
Viruses often cause a bronze coloration of the leaves.
Nitrogen and sulfur deficiencies are typically indicated by lighter-colored to yellowish plants, but nitrogen deficiency can be created by several different problems. An uncommon but fairly distinctive leaf symptom is caused by bacterial chlorosis. The top leaves on isolated plants look like the plant has a combination of manganese and potassium deficiencies, but the discoloration develops almost immediately on normal-sized plants, and nodulation is usually poor. Holes in leaves suggest damage by foliage-feeding insects or hail, although hail damage looks more like a tear than a spot where the foliage was removed. Generally poor growth suggests poor growing conditions.