BULLS SOIL-PH AND RHIZOSPHERE PH OF PEACH-TREES IN CALCAREOUS AND ALKALINE SOILS AS AFFECTED BY THE FORM OF NITROGEN FERTILIZERS

One-year old nectarine trees [Prunus persica, Batsch var. nectarina (A it.) Maxim.], cv Nectaross grafted on PS.B2 peach seedlings [Prunus pe rsica (L.) Batsch] were grown for five months in 4-litre pots filled w ith two alkaline soils, one of which was also calcareous. Soils were r egularly subjected to fertigation with either ammonium sulphate or cal cium nitrate providing a total of 550 mg N/tree. Trees were also grown in such soils receiving only deionized water, as controls. Rhizospher e pH, measured by the use of a microelectrode inserted in agar sheet c ontaining a bromocresol purple as pH indicator and placed on selected roots, was decreased by about 2-3 units compared to the bulk soil pH i n all treatments. This decrease was slightly less marked when plants w ere supplied with calcium nitrate rather than ammonium sulphate or con trol. Measurements conducted during the course of the experiment indic ated that ammonium concentration was similar in the solution of soils receiving the two N fertilizers. During the experiment, soil solution nitrate-N averaged 115 mg L(-1) in soil fertilized with calcium nitrat e, 68 mg L(-1) in those receiving ammonium sulphate and 1 mg L(-1) in control soils. At the end of the experiment nitrate concentrations wer e similar in soils receiving the two N sources and bulk soil pH was de creased by about 0.4 units by ammonium sulphate fertigation: these evi dences suggest a rapid soil nitriflcation activity of added ammonium. Symptoms of interveinal chlorosis in apical leaves appeared during the course of the experiment in trees planted in the alkaline-calcareous soil when calcium nitrate was added. The slightly higher rhizosphere p H for calcium nitrate-fed plants may have contributed to this. The fin dings suggest that using ammonium sulphate in a liquid form (e.g, by f ertigation) in high-pH soils leads to their acidification and the micr onutrient availability may be improved.