AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 2002, No. 4
Phosphorus uptake is greatest in the early spring. Uptake by sugarbeet tops and roots is greatest early in the season. The concentration of phosphorus in both plant fractions decreases as the season progresses. Tops have much higher concentrations than the roots. This means that actively growing tops place a demand on the roots to provide phosphorus. Given this pattern, adequate supplies of phosphorus early in the season appear critical for helping the sugarbeet plant reach its attainable yield potential.
Adequate nutrition depends on roots finding soil phosphorus supplies. Phosphorus is taken up from the soil by the sugarbeet root system. For a sugarbeet plant to be sufficiently fed with phosphorus, two conditions must be present: 1) enough phosphorus must be in the soil, and 2) soil phosphorus must be close to growing roots to be available for uptake. Phosphorus moves very little in most soils. The highest concentration tends to be within inches of where it was originally placed. The trick then is to place phosphorus where the plant roots can get to it at the time when the plant needs it. For sugarbeets, that demand appears to be greatest early in the growing season.
The sugarbeet root system does not produce many fibrous roots until later in the season. For the first couple of months after planting, the taproot dominates the root system. Short laterals form from the taproot, but do not extend very far. After two months, the fibrous root system begins proliferating in the upper foot and a half of soil. Under irrigated conditions, root branching can be fairly extensive close to the soil surface. Approximately three months after planting, root proliferation shifts from the upper to the lower soil profile. During this period, fibrous root growth occurs primarily below a depth of about 18 inches and continues to maturity.
Early phosphorus uptake relies on proper placement. Placing phosphorus with or directly below the seed puts phosphorus near the actively growing taproot early in the season. Placement to the side means the crop must rely on lateral root growth early in the season, which does not appear to occur significantly until two or more months after planting. Recent studies from the Red River Valley in Minnesota have shown that maximum yield response to phosphorus can occur from low rates placed with the seed at planting.
Phosphorus uptake later in the season may depend on soil phosphorus reserves. Research has shown that the fertility level determines to some degree the probability that sugarbeets will respond to additional phosphorus inputs. Responses to broadcast applications in the Red River Valley have not been as consistent as those from placement with or below the seed. Given the growth of the root system, overall phosphorus supplies at various depths may be important for determining the probability of response to broadcast additions. Such relationships have not been well studied. Research for other crops such as barley and wheat has shown that while annual responses are greater at lower soil test phosphorus levels, the highest yields are attained on soils with adequate supplies. The importance of soil fertility levels for attaining higher yields has yet to be fully explored for sugarbeets.
Soil fertility management needs to consider other crops in the rotation. Placing small amounts of phosphorus with or below the seed may be the best approach for early season phosphorus requirements of sugarbeets. However, rotational crops, such as wheat and corn, require a combination of phosphorus placement with the seed and adequate soil fertility levels to realize their yield potential.