EFFECT OF AN ACIDIFYING NITROGEN-FERTILIZER AND LIME ON SOIL-PH AND WHEAT YIELDS .2. PLANT-RESPONSE

This paper reports the effects of an acidifying fertiliser on wheat yi elds and mineral composition in the first 10 years of trials at Merred in, Wongan Hills, and Newdegate, described in the previous paper. Ther e were grain yield increases from application (kg/ha.year) of 17.5 N 7.6 P and 35 N + 15.2 P in all years except 1980 (Wongan Hills and Ne wdegate) and 1989 (Merredin). At Merredin, 3 t/ha of limestone increas ed grain yields in all years except 1982 and 1983. At Wongan Hills, li mestone increased yields in the continuous cropping system, in the pre sence of high N + P, during 1986 and 1989. At Newdegate, grain yield r esponses to lime were recorded in 1980, 1981, 1982, 1986, 1988, and 19 89; limestone resulted in decreased wheat yields in 1984 because of an increase in the severity of the root disease take-all (caused by Gaeu mannomyces graminis var. tritici), and decreased lupin yields in 1987 because of manganese deficiency. There were a few responses at Merredi n and Newdegate to low rates of limestone with each N + P application. Application of the magnesium (Mg) + potassium (K) + molybdenum (Mo) i n addition to limestone gave isolated responses at all sites. Concentr ations of calcium (Ca), Mg, and K at the 1989 plant sampling were high est at Wongan Hills and lowest at Merredin. Calcium concentrations wer e higher at Merredin and Newdegate following N + P application, but lo wer at Wongan Hills. Application of N + P reduced the concentration of Mg at Merredin and Wongan Hills, but increased Mg at Newdegate. The c oncentration of K was decreased at all sites by N + P. Limestone had n o consistent effect on Ca concentrations but increased the concentrati ons of Mg and K at Merredin and K at Newdegate. Where Mg + K + Mo had been applied in 1980, concentrations of Ca decreased and K increased, while there was no consistent effect on Mg concentration. The Mo conce ntration in plant shoots was reduced at all sites by increasing rate o f N + P and increased by application of lime and Mo in 1980. Grain Mo concentrations were very low at Merredin and Wongan Hills, even in the absence of N + P. Concentrations at Newdegate were reduced by increas ing N + P rate. Application of limestone increased grain Mo concentrat ions at all sites, but these were still lowered by N + P application. Concentrations were also increased by the application of Mo in 1980, b ut fell with time, particularly with application of N + P.