Effects of grain legumes on succeeding non-legumes in a rotation and their response to source of phosphorus

To determine the potential of barley, lentils, lupins and peas and the soil incorporation of their different plant parts to maintain soil fertility, soil chemical and physical properties, a trial was sown during the 1996/97 growing season at Lincoln University, Canterbury, New Zealand (43 °S) on a Wakanui silt loam on 1 November 1996. Plots were sown to barley (Hordeum vulgare cv. Liberty), lentils (Lens culinaris cv. Titore), narrow leafed lupins (Lupinus angustifolius cv. Fest), peas (Pisum sativum cv. Allure), or were left in fallow. At crop maturity, plots which contained plants had all above ground vegetation removed (R), had seed only removed (HR) or had no vegetation removed (HRS) and the residual vegetation, was soil incorporated. The trial area was then sown to a winter active cultivar of Italian ryegrass (Lolium multiflorum cv. Moata). On the fallow plots and on the barley plots (all vegetation removed) sub-sub plots were top-dressed with 0 50, 100, 150 or 200 kg/ha of N as urea. After the ryegrass was harvested, the area was sown with forage maize (Zea mays cv. Janna). Lentils, lupins, peas and barley produced 5.4, 31.3, 15.2 and 14.0 t/ha DM (shoot + root + seed) respectively and this was equivalent to 0.2, 1.5, 0.5, and 0.1 t of N added when all residues were incorporated. Ex- lupin plots, where all DM was incorporated (HRS), yielded considerably more ryegrass DM (19.2 t/ha) than fallow plots, which were given 200 kg N/ha (16.3 t/ha). Even when seed was removed, (HR) ex-lupin plots still produced as much DM as ex-fallow plots with 200 kg N/ha. Ex-lentil and pea plots gave a DM yield equivalent to 150 kg N/ha (13.8 t/ha). After the removal of all above ground DM, all ex-legume plots still gave more DM than ex-fallow plots and considerably more than barley plots without nitrogen (N). There was considerable benefit of all preceeding crops to the following maize crop. After incorporation of whole lupin plants (HRS) the forage maize yield at 100 DAS was 21.00 t/ha and from seed harvested plots (HR) it was 19.0 t/ha which was higher than the fallow plots with 200 kg N/ha (14.0 t/ha). The yield of maize DM after lupins was substantially higher than after barley, peas or lentils. Soil aggregation increased after cultivation of the preceding crops in the order of lupins > lentils = barley > peas > fallow. After the residue incorporation, lupin soil was more friable with high amounts of organic carbon (OC) and stronger aggregates (78%) than barley (61%), lentils (63%) and peas (50%). These increments in soil aggregation were due to the increased amounts of soil organic matter following residue incorporation, which also improved the soil bulk density. The total nitrogen (TN), mineral N, organic carbon (OC), and cation exchange capacity (CEC) were relatively higher after the growth of the preceding legume crops. Lupins generally left more TN, mineral N, and OC in the soil than the other two legumes as it has the capacity to fix greater amounts of N combined with a high level of DM production. The levels of incorporations such as HRS and HR improved these chemical properties of the soil. The high N content of legume residues enhanced N mineralization. A greenhouse study was carried out to investigate the effects of rock phosphate and superphosphate at P rates of 0, 15 and 30 kg/ha on shoot, root growth and nodulation of lupins (Lupin us angustifolius cv. Fest) and peas (Pisum sativum cv. Allure). Application of P generally increased leaf area, shoot and root dry weights and the number of nodules. Phosphorus concentration of herbage was increased especially in the lupins with both P sources. While, peas did not respond to rock phosphate at the equivalent of 15 kg/ha, lupins responded nearly as much to rock phosphate as to superphosphate.