Skeleton weed (Chondrilla juncea) in the Victorian Mallee. 3. The effects of applied phosphorus and nitrogen on wheat on infested land

Eight fertilizer experiments were conducted at five locations in the Mallee from 1963 to 1966, following both lucerne and volunteer pastures, to study responses to applied phosphorus and nitrogen in wheat on sandy soils infested with skeleton weed (Chondrilla juncea). Each year two separate sites were selected-one having no history of sown pasture, the other having supported a poor, but typical lucerne stand for several years. In most years, grain yields were markedly increased by both nutrients following either lucerne or volunteer pasture, and a large positive interaction occurred between phosphorus and nitrogen at five sites. The response to nitrogen obtained after lucerne was due to the low density of the lucerne stands, which had had little effect on skeleton weed populations and soil nitrogen levels. Total yield variation depended mainly on the number of fertile tillers (66 per cent) and grain number per ear (19 per cent). On the other hand, the yield increases to phosphorus and nitrogen were attributed mainly to increases in the number of grains per ear, although increased tiller number also contributed. Grain weight was of little importance in both total yield and yield response. The level of grain protein content depended mainly on the growing season rainfall (r = -0.832). Application of phosphorus reduced grain protein content, and nitrogen increased it. The response to applied nitrogen was related to growing season rainfall (r = 0.783) and the difficulty of determining optimum rates of application is discussed.