Comparing how Lupinus angustifolius and Lupinus luteus use zinc fertilizer for seed production

Zinc (Zn) deficiency is a common problem on the sandy acidic soils in south Western Australia (WA) for seed (grain) production of Lupinus angustifolius, and L. luteus. The Zn requirement of L. luteus is not known; while that of L. angustifolius has only been measured in one field experiment in WA. The effectiveness of Zn fertilizer was measured in 1997 for grain production of L. angustifolius cv. Gungurru, and L. luteus cvv. Motiv and Teo, for Zn applied once only to plots not treated with Zn before, either in 1997 (current Zn) or in a previous year (previous Zn, applied in one of the following years: 1983, 1984, 1986, 1990, 1992). For each lupin species, the effectiveness of previous Zn decreased relative to the effectiveness of current Zn, the decrease being larger with increasing time since application. Fourteen years after application, the decrease was 96% for L. luteus cv. Teo compared with about 65% for the other two lupins. When no Zn was applied, L. luteus produced larger grain yields than L. angustifolius. For Zn applied in the current year, relative to L. angustifolius cv. Gungurru, L. luteus cv. Motiv used the Zn about 45% less effectively and so required about twice as much Zn to produce the same percentage of the maximum (relative) yield. L. luteus cv. Teo used the Zn about 70% more effectively so only required about two thirds the Zn needed by L. angustifolius to produce the same relative yield. For Zn applied in each of the previous years, and relative to L. angustifolius cv Gungurru in each of those years, L. luteus cv. Motiv used the previously applied Zn from about 3 to 33% more effectively for producing grain, whereas L. luteus cv. Teo used previously applied Zn about 15 to 88%, less effectively. Both currently and previously applied Zn fertilizer increases lupin seed yields. The effectiveness of the previously applied Zn fertilizer declines with time since it was originally applied.