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This article in AJ

  1. Vol. 78 No. 6, p. 991-994
    Received: Sept 19, 1985

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Phosphorus Uptake from Soil Layers Having Different Soil Test Phosphorus Levels1

  1. J. V. Pothuluri,
  2. D. E. Kissel,
  3. D. A. Whitney and
  4. S. J. Thien2



The effect of available P in subsoil on plant uptake of P from surface soil layers and consequently on fertilizer recommendations is largely unknown. Our objective was to determine how much P may be removed by crops from subsurface soil layers and how the subsurface P level influences crop drawdown of P from the surface layer. Soil columns were prepared that contained all four possible combinations of low or high P levels in the surface soil and low or high P levels in the subsurface layer. The soil used was a Farnum fine sandy loam (fine-loamy, mixed, thermic Pachic Argiustoll). Phosphorus removal was monitored by growing hybrid sorghum-sudangrass [Sorghum bicolor (L.) Moench cv. ‘Honey Chow’] 84 days with an intermediate harvest after 35 days. Compared to when both the surface and subsurface layers had low P levels, P uptake increased when either or both layers had high P levels. Root growth and morphology responded to the P level of the soil layers. For all conditions, P uptake demonstrated a high correlation (R2=0.94) to an accessibility factor defined as the product of the available P level and root length summed for both soil layers. Comparing ratios of crop P removal to the change in soil test showed that P drawdown from the surface layer was reduced when the P-supplying capacity of the subsurface layer was high. We concluded that available P in the subsurface layer can supply P to the crop, especially if the surface layer cannot supply sufficient P to meet crop demand. These results suggest that subsoil fertility should be considered in evaluating P available to crops and in estimating crop drawdown of available P.

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