Hi hoping for people’s thoughts/comments on the generalised relationship between soil horizons (sensu lato soil depth) and the concentration of exchangeable bases as well as the regulation of inorganic ion concentrations (esp. K+ and P species) in soil solution? Specifically, is it fair to say that: (i) reserves of Ca, Mg, K and P species are highest within the subsoil due to a greater abundance of layer silicates and sesquioxides, and; (ii) that the greater abundance of layers silicates and sesquioxides results in stronger/greater regulation of the concentration or these ions in soil solution (buffering?) I realise that organic matter has by far the highest charge density (pH dependent) and yet I did not think it was particularly effective at regulating/retaining K, P or base cations relative to clays. I’m thinking particularly of fine texture (leossial) soils developed a temperate-humid climate. Clint
In broad terms P "reserves" at a specific site in the temperate zone (rainfall above say 300mm per year; not water limited) depend on soil parent material and/or land cover/land use history.
For crops and natural vegetation the plant extractable P reserves are likely to be the limiting growth factor.
In other words how we measure reserves is crucial.
Article Nitrogen and phosphorus limited production of cereals and se...
The abundance of nutrient ions in soil solution depends especially on pH. In case the topsoil is much more acidic compared to subsoil your hypothesis may be right. Yet, in other cases and especially in somehow managed farmland one will find higher concentrations of nutrients in the topsoil with much higher exchange capacity due to soil organic matter occurring in addition to clay meinerals and pedogenic oxides.. This is even more the case if soils have external input of nutrients ...
These cations get preserved in soils of arid and semi-arid climates where leaching of cations are restricted by less rainfall.
All, thank you for your replies. Some more thinking for comment… One of the main reasons I ask this question is for an assessment of water (ground and surface) chemistry/quality. Specifically, waters I have collected the top soils (Teflon suction cups), interflow, subsoil drainage, and overland flow (soil zone) show much higher concentrations of K, SO4, DOC, NH4, and E.coli counts but low Si concentrations when contrast with groundwater and groundwater derived baseflow. Groundwaters show tightly regulated K concentrations, much lower and less variable DOC and SO4 no E.coli and elevated Si. This all makes sense to me in terms of the vertical filtering of DOC and selective retention of and regulation of ions as waters percolate deeper into the soil and unsaturated zone before reaching the underlying aquifer. In other words waters reaching the underlying aquifer have been subject to more regulation (interacted with far greater volumes of fine textured materials) than waters derived from shallow (upper 600 mm) lateral movement of water. So although concentrations of ‘nutrients’ (K, SO4, TAM, P etc) may be higher within the upper 600 mm they do not appear to be as strongly regulated by ion exchange or sorption processes. It would therefore appear that buffering (regulation) of ‘nutrients’ is proportionately greater within the subsoil and unsaturated zone than within the upper 600 mm of the soil profile.. not necessarily the concentration of ‘nutrients’. To me this suggest either that organic matter is poor at regulating ion concentrations and or at retaining ions or that the shear abundance of nutrients from intensive land use overwhelm the regulatory (buffering) capacity of the upper portion of the soil. Regards, Clint.
HI P Shand,
Thanks for your comment. We do see soil zone bypass in areas of shrink swell soils but we do not see elevated DOC, SO4, K in shallow groundwater underlying these soils. In these areas vertical fractures open in the summer months. (These are strongly reduced (redoximorphic) soils with much mottling and gleying and smectites). In the autumn (austral) under recharge conditions we see instantaneous aquifer response to recharge due to bypass flow. Further, nitrate concentrations under these reducing soils are very high (median of 9.7 mg/l NO3-N; N = 27 unconfined wells) as are D.O. concentrations and ORP (+170 mV). It would appear that excess NO3 accumulated over the summer or autumn is flushed from the top soil through cracks to the underlying aquifer - bypassing the reducing soil zone. However, groundwaters are not particularly elevated in soil zone constituents such as DOC, SO4, K or other such indicators of intensive land use and poor regulation by ion exchange/mechanical filtration suggesting sufficient material within the deeper vadose zone for regulation (ion exchange/mechanical filtration). Interestingly, as these soils wet up during the winter the cracks close and we see little if any aquifer (gw level) response. Rather soil waters transition to lateral movement mediated by subsurface artificial drainage (mole-pipe drainage). Waters samples from tile discharges show elevated DOC, K, SO4 and perhaps more interestingly elevated MnII and FeIi and low nitrate consistent with reducing conditions within the soil zone. The seasonality of DOC, K, SO4 and dissolved Mn and Fe is apparent in the lowland streams associated with this area reflecting the shift in soil drainage mechanism and associated biogeochem.. well I think it does 😊 Cheers Clint
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