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This chapter provides the latest knowledge on phosphorus (P acquisition strategies by cotton plants. Knowledge on P acquisition characteristics helps understanding the responsiveness of cotton crop to P fertilizers as well as developing an effective P placement strategy. Physiologically, the cotton plant excretes phosphatase enzymes, but lacks the ability to manipulate its rhizosphere chemistry, and to mobilize non-labile inorganic P sources. Significant exudation of carboxylates is not detected in the rhizosphere of cotton in response to P deficiency. In addition, cotton is not superior in using sparingly soluble P sources such as Al-, Fe- and Ca-phosphates compared with other species such as wheat and white lupin. Thus, P acquisition by cotton plants mainly depends on root exploration of relatively labile inorganic P and organic P sources from the soil profile. Root morphological traits, especially mycorrhizal associations, would result in a high absorbing surface area. Traditionally, it is desirable to place P fertilizer in the way that favors a higher probability of root-P contact. In contrast to the low responsiveness of cotton to the P fertilizer banded in the topsoil, significant yield responses are detected for cotton to deep P placement. The effectiveness of deep P placement is possibly due to an enhanced contact between root and P fertilizer during the later stages of growth, and also to a sustained P availability under periodic surface drought conditions. The placement depth of P fertilizers depends to a large extent on the soil's wetting and drying patterns during the growing season. Alternatively, the low responsiveness of cotton to P fertilizers applied in the surface soil indicates that cotton plants are able to access subsoil P pools. Since subsoil P pools cannot be replenished by routine inorganic P fertilizer input. the extent to which these pools can be depleted with regard to the long-term sustainability and productivity of crop production are questions for further research. The role of mycorrhizae in P acquisition is also discussed. |
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