LIME-INDUCED CHLOROSIS OF CITRUS IN RELATION TO SOIL FACTORS

Onie of the most iinportanit forms of chlorosis (3, 4) still widely distribuited in some of the best eitrus areas is that which may occur when the soil containis aii abunidainee of calcium carbonate. An extenisive survey of soils iu citrus orchards in sotutherni Californiia has showni that orchard soils may coiltaiui conisiderable caleium carboniate without the leaves of the trees show-iug" symptoms of lime-iniduced chlorosis. Mild syimptoms such as a slight clhangoe of the dark green color of the leaves to a more yellowish green mnay niot seriously impair the fruit production provided the growth is vioorous and abundaut. In fact, frequeitly some curtailmeit brougrht about in this mannlier in the quanitity of fruit produced may prove desirable in that the size aiid quality of the fruit miiay slhow improvemeit. When the leaf veins beeoiime colnspicuotus because of their oreeniiess in contrast to the pale yellow or yellowish green of the leaf blade, it is then that chlorosis may requiire inereasiiug consideration. The soils of maniy citrus orchards are caleareouLs anid the excellenee of some of these orchards is outstanding. The data collected during the survey of citrus orchards (8) have indicated that a soil may be caleareous aned yet not necessarily be alkaline in reaction (1) althotiuh potentially it may become so under certaini coniditions. Calcitum carbonate is onie of the buffer substanees in soil anid has hydrolytic properties that depend iiot only on the size and form of the particles anld the amouint of colloidal matter (1) but also on the percenitage of moistiire present, for it is this factor that permits hydrolysis to proceed. In tests of the pH of orehard soils it was conieluded (7) that no orchard soil has been eneouLntered wvhieh, wlheni sufficienitlyreduced in moisture coontenit, failed to showv an acid reaction. This was particularlyobvious in dealino with calcareous soils for, at moisture conitents greatly above the lmloisture equivalent, they were quite alkaline while at low moistuire contenits they were acid. In somie orelard soils the ealeiunm carbonate may oecur in a finely divided conditioni anid well distribtuted throughout the soil mass. In this case the color aned physical texture of the soil in addition may require the usual acid test in order to ascertain whether calcium carboniate is present. In other orchard soils the calcium carbonate, while abundantly present, maay occur in such a coarsely div-ided form that large pieces may be collectet. Thlus fre(quiently the pH of the soil (at the 1: 5 soil-water ratio) was found to be considerably hioher in the first thani in the second ease. 27