Phosphorus deficiency is by far the most difficult of deficiencies to recognize. An overall stunting of the plant and a gradual deepening of the green foliage color are the only visual signs. Except for size, such plants appear normal and look as if they had been planted several weeks later than comparable high-phosphorus plants. As the deficiency becomes more severe, the deep, green color often assumes a metallic lustre that ranges from dull, grayish-green to almost bluish-green. The purpling that is often associated with phosphorus deficiency of other crops seldom appears on sugar beets and is not a reliable criterion for diagnosis. In the early seedling stage, the cotyledons and the primary leaves are a deep green; and when the deficiency is severe, pitting occurs, followed by drying of the cotyledons. In older, severely deficient plants, a brown, netted veining forms in the tissues of older leaves upon drying, in contrast to the uniform yellowing that would occur for normal and nitrogen-deficient plants.

Stunting may occur any time during growth. At low soil temperatures (50' to 59' F), beets tend to become deficient. This is especially pronounced in some soils. As soil temperatures increase (to about 680 F), the symptoms often disappear because of improved phosphate absorption. Thus, during early-season cold weather, it is sometimes difficult to diagnose the reason for slow seedling growth. Phosphorus deficiency in seedlings can be detected, however, by a chemical analysis of the cotyledons, petioles, or bla&s of the first true leaves. The petiole is preferred, even though this plant part is more difficult to obtain at the early-seedling stage. Cotyledons should be used only when no other material is available. Values for nitrate-nitrogen will he lower for phosphorus-deficient seedlings than for comparable ones high in phosphorus.