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Calcium Deficiency as a Causal Agent of Ammonium Phosphate Injury to Cotton Seedlings¹

ABSTRACT

Short-term experiments were conducted with cotton seedlings (Gossypium hirsutum L.) on a Lakeland loamy sand at different pH levels and at different rates of (NH₄)₂HPO₄ and NH₄H₂PO₄. Seed was planted in a layer of untreated soil that was underlain by treated soil. The primary roots were allowed to grow into the treated soil for 48 hr. Root growth into treated soil was inhibited by several treatments, and apparent death of the primary root was effected by several other treatments. Root growth into the treated soils could not be explained in terms of soil pH, P-rate, or individual concentrations of soil solution cations. However, if all solution cations were expressed in terms of molar activities, then a striking relationship between root growth and ratio of Ca/total-cation or NH₄/total-cation in the soil solution became evident. Root growth was inhibited by all NH₄-phosphate treatments that resulted in Ca/total-cation molar activity ratios in soil solution of less than about 0.15. Apparent death of root was caused by all NH₄-phosphate treatments that resulted in Ca/total-cation molar activity ratios in soil solution of less than about 0.05. It was concluded that the injurious effect of NH₄-phosphate on cotton seedlings was a Ca deficiency in the ambient soil solution. This resulted from the precipitation of Ca phosphates and from the antagonistic action of NH⁺₄ on Ca absorption by the root.

¹ Contribution from Department of Agronomy and Soils, Auburn University Agr Exp Sta, Auburn, Ala.

² Professor of Soils.

Received for publication February 4, 1966. Accepted for publication March 21, 1966.