Soil phosphorus availability to millet roots

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Abstract
Soil solution P level is believed to be important in determining P uptake rates from soil. The relation between initial P concentration in the soil solution and P flux into the root was investigated. Millet (Panicum miliaceum) was grown on 5 soils each of which was adjusted to 6 Cli [available soil P] levels by addition of P. Millet was also grown in solution culture and P influx vs. P concentration in solution measured. There was a curvilinear relation between P influx and relative yield of the Cli levels on each soil (R2 = 0.74). A P influx of at least 16 pmoles cm-1 s-1 was needed to obtain 90% of maximum yield. However, yield response was not correlated with Cli, indicating Cli was not a suitable indicator of P availability on these soils. Influx of P on soils with Cli less than 6 .mu.M was greater than occurred at similar P concentrations in solution culture indicating P influx was increased by the effect of the root on the soil.