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Phosphorus Distribution in Calcareous Soil Profiles of the Central Plains

USDA-ARS, P.O. Box 400, Akron, CO 80720

D. J. Savory

Agronomy Dep., Colorado State Univ., Fort Collins, CO 80523

* Corresponding author.

ABSTRACT

Deep-rooted crops benefit from high subsurface P content. While some documentation exists for high levels of available inorganic P well below the soil solum, very little information exists on organic-P contents deep in the soil profile. This research was conducted to document the presence of large pools of available organic and inorganic P on top of a carbonate layer 2.2 m below the soil surface. Additionally, soil organic C, N, texture, and selected mineral concentrations were determined to assess their correlations with total P. Soil samples (Aridic Argiustolls) from two fields that have been out of cultivation for at least 10 yr were taken in 0.15-m increments to a depth of =2.5 m. The surface soils (0.0–0.3 m) were high in nutrients (11.0 g C and 1.1 g N/kg soil) and negligible in carbonates; the middle soil layers (0.30–0.25 m) were low in nutrients (4.0 g C and 0.3 g N/kg) and intermediate in carbonates (90 g/kg); and the deep soil layers (2.25–2.55 m) were high in available inorganic and total organic P, and very high in carbonates (> 300 g/kg). The total P concentration in the profile was inversely correlated with the percentages of sand and Ca, and directly correlated with the total content of Fe and Al. Total organic C and N and, to a greater extent, total organic P accumulated near or on top of a carbonate layer. Most of the organic C and P below 0.45 m were soluble in 2 M HCl. For the surface soil (0.0–0.45 m), the bulk of the organic P was extracted only after 2 M HCl pretreatment. Alfalfa and similar deep-rooted crops may benefit from this large reserve of available P near the water table.

Received for publication June 18, 1990.