Don't rule out nutrient deficiencies if your crop yields are below normal.

Nutrient deficiencies are sneaky. They can limit yields or quality without apparent symptoms, according to the Potash and Phosphate Institute (PPI), Norcross, GA.

This "hidden hunger" is the most likely consequence of inadequate soil nutrient levels. It can be prevented with intensive soil testing programs and plant tissue testing. Plant tissue testing is an under-used tool that could help identify and correct problems during the growing season, say PPI experts.

Deficiency symptoms aren't common in well-managed fields, but they occasionally show up. And if nitrogen (N), phosphorus (P), potassium (K) or sulfur (S) deficiency symptoms appear during the growing season, yield probably has already been hurt. But it's still important for growers to know the symptoms so problems can be corrected.

Here are PPI's guidelines on how to recognize deficiencies in corn:

Nitrogen deficiency in young plants makes the entire plant look pale and yellowish green, with spindly stalks. V-shaped yellowing may later appear on leaf tips and along the midrib of lower, older leaves. It progresses up the plant if deficiency persists.

Phosphorus-deficient corn may be dark green with reddish-purple leaf tips and margins. However, in the seedling stage, some corn hybrids show purple colors similar to P deficiency when P is adequate. And some don't show purple whether the P is limiting or not.

Deficient plants may be smaller and grow more slowly than healthy plants.

Potassium deficiency in corn, beginning on lower leaves, turns them yellow, with dying tissue on the leaf margins. If the deficiency isn't corrected, those symptoms will progress up the plant. With severe K deficiency, lower leaves will turn yellow while the upper leaves may stay green.

Sulfur deficiency in small corn plants may appear as a general yellowing of foliage, similar to N deficiency. But the yellowing is more pronounced with an S deficiency than with a lack of N because S isn't easily translocated in the plant. Other symptoms may include interveinal chlorosis, stunting of plants or delayed maturity. This deficiency is more likely in acid, sandy soils, in soils low in organic matter or in cold, wet soils.

Learn the signs of soybean deficiencies, too:

N-deficient soybeans are pale green; leaves may later turn distinctly and uniformly yellow. Symptoms appear first on basal leaves and spread to upper parts. Eventually, plants defoliate, are spindly and stunted.

Although soybeans create their own N, N fertilization may help in high-yield environments or other special conditions, says PPI.

Phosphorus-deficiency symptoms aren't well-defined, but plants are spindly, with small leaflets and retarded growth. Leaves may be dark green or bluish green.

Potassium-deficiency symptoms are easier to spot. The first appear on older leaves. In early growth stages, an irregular yellow mottling appears around leaflet margins. The yellow areas may join to form an irregular yellow border.

Sulfur-deficient plants are pale green, with the youngest leaves often appearing more yellow. Stems are thin, hard and elongated, with small, yellow-green leaves at the plant top.

According to the institute, here's what you need to know if you grow cotton:

A lack of nitrogen on cotton early in the season shows up as a yellowish-green discoloration of older leaves. Younger leaves may be smaller than usual. Plants are shorter, with few vegetative branches, short fruiting branches and bolls that may shed soon after flowering.

When N deficiency occurs in late season on plants with a moderate load of maturing bolls, leaves redden in the middle of the canopy. Few bolls make it to late fruiting.

Phosphorus-deficiency symptoms rarely occur in early growth and aren't distinct. Plants may be stunted, leaves darker green than normal, flowering delayed and boll retention poor. Leaves on P-deficient plants late in season undergo premature senescence.

Potassium-deficiency on cotton before peak bloom may show interveinal light-green to gold mottling, first on older leaves. Yellowing and necrosis may develop at leaf margins under severe deficiency. After peak bloom, K deficiency may first appear on younger mature leaves in the top third of the canopy.

Sulfur deficiency can be spotted on younger leaves in the upper canopy. Older leaves retain a normal green color. S-deficient leaves turn pale green, then a yellowish-green similar to N-deficient leaves, and leaf veins tend to stay greener than interveinal tissue. Plants are short, have few vegetative branches and small bolls.