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Utilization of Pelleted Phosphorus by Flax, Wheat, Rape, and Buckwheat from a Calcareous Soil¹

Reaction zone root development and P absorption efficiency of roots of flax (Linum usltatissimum L. ‘Redwood’), wheat (Trlticum vulgate L. ‘Manitou’), rape (Brassica napus L. ‘Tanka’), and buckwheat (Fagopyrum esculentum Moench) have been contrasted in order to explain the large variability among them in utilizing P from pellets of KH₂PO₂ and K₂HPO₄ in a pot experiment. A large proportion of plant P was extracted from the pelleted applications by rape (40.8 to 57.6%) and buckwheat (33.1 to 55.5%). It is suggested that applied P utilization by rape was enhanced by e~tensive proliferation of its root system detected around points of application of fertilizer pellets. It is suggested for buckwheat, on the other hand, that its utilization of applied P was aided by the high efficiency of its root in absorbing P from high concentrations as indicated in absorption studies using excised roots. Relative to rape or buckwheat a much smaller proportion of plant P was extracted by flax (3.8 to 8.1%) and wheat (12.5 to 19.7%) from P applications. The poorer utilization of applied P by these two crops is considered due to the inability of their root systems to either proliferate extensively or to absorb P efficiently from high P concentrations.

It is reasoned that the greater P availability from a 40-mg P application as KH₂PO₄ than from a 20-mg P application as K₂HPO₄ was due to the higher water-soluble P level in the reaction zone of the former application, with the two applications spread through the same soil volume. A given P application as K₂HPO₄ spread through a larger volume of calcareous soil than the same xate of P as KH₂PO₄. The water-soluble P level, however, was lower in reaction zones of the former carrier. Thus, the effect of reaction zone size and of its watersoluble P level on the availability of P from KH₂PO₄, and K₂HPO₄ pellets offset one another, resulting in a similar availability of the two carriers to the four crops tested.

Key Words: Phosphate reaction zone • Excised roots • Root development • P absorption

² Graduate student and Professor, respectively. Present address of senior author is Queensland Wheat Research Institute, Toowoomba, Queensland, Australia.

Received for publication February 22, 1972.