Some aspects of the cation status of soil moisture

Summary A study was made of the effect of soil moisture tension (pF 0–3), regarding the soil moisture status and soil oxygen status as interrelated, on the growth and cation uptake byRaphanus sativus using a tension plate apparatus. A similar study was made usingBrassica sinensis in pots.Results suggested that the tension-plate apparatus would be useful in such studies, whereas the lack of adequate soil moisture control in the pot experiment made this type of method unsuitable.Increasing soil moisture tension resulted in an increase in the cation concentrations of both soil solution and plant material. The dilution and valency effect in the soil solution was demonstrated.The growth response to soil moisture tension was a reflection of changes in moisture status which in turn affected the soil oxygen tension.The value of logK–1/2log [Ca+Mg] in the plant was constant over the soil moisture tension range studied and was a reflection of the characteristic potassium intensity status of the soil solution, defined by pK–1/2p (Ca+Mg) which was also constant. The concentration ratio, K/Ca+Mg, in the plant varied with moisture tension in the same way as the ratio varied in the soil solution and was therefore a reflection of both the soil solution composition and soil moisture tension.The plant reflected the composition of the soil solution and it was concluded that both the adsorption and contact-exchange theories played no significant role in plant ion uptake.