(342) Cool-season Living Mulchesfor Florida

Living mulches between beds of polyethylene-mulched vegetable crops may suppress weeds and decrease surface and ground water contamination by pesticides. They should be either low growing or amenable to mowing and should withstand traffic. Twelve winter cover crops were planted in north (N.) and north central (N.C.) Florida in Fall 2004: black oats (Avena strigosa cv. Soilsaver), annual ryegrass (Lolium multiflorum cv. Gulf), rye (Secale cereale cv. Wrens Abruzzi), hard fescue (Festuca longifolia cv. Oxford), white clover (Trifolium repens cvs. Dutch white and New Zealand white), berseem clover (T. alexandrinum cv. Bigbee), crimson clover (T. incarnatum cv. Dixie), subterranean clover (T. subterraneum cv. Mt. Barker), arrowleaf clover (T. vesiculosum cv. Yuchi), a barrel medic (Medicago trunculata cv. Parabinga), and a disc × strand medic (M. tornata × M. littoralis cv. Toreador). Black oats, rye, and annual ryegrass established quickly and suppressed winter annual weeds. Canopy development of the other species was poor. Shoot biomass was greater in N. Florida than in N.C. Florida. The highest shoot biomass occurred with black oats. By 8 weeks after planting (WAP) rye and annual ryegrass had similar amounts of biomass, but by 16 WAP the yield of rye was greater. At some harvests, biomass with wheel traffic or mowing was lower than without, but black oats, rye, and ryegrass did not succumb to these treatments. Of the legumes, only crimson clover and `Toreador' medic in N. Florida produced sufficient biomass by 16 WAP to permit a harvest. Black oats, rye, and annual ryegrass appear to be the best living mulch candidates; however, black oats would require more frequent mowing.

The relative contribution of plant residues and fertilizer to the phosphorus nutrition of wheat in a pasture cereal system

Wheat plants (Triticum aestivum cv. Warigal) here grown in a solonised brown soil (Calcixerollic xerochrept) which had been previously cropped to medic (Medicago trunculata cv. Paraggio). The 33P-labelled medic residues and 32P-labelled monocalcium phosphate were added to the soil in factorial combination. Amounts of 31P, 32P and 33P in the wheat plants and in the soil microbial biomass were determined. Addition of residues depressed wheat dry weight, 31P and 32P uptake, while simultaneously increasing amounts of 31P and 32P incorporated into the microbial biomass. Addition of fertiliser had no effect on the proportion of plant P taken up from the residues, but significantly increased the proportion of microbial P derived from this source. The 31P held in the microbial biomass was significantly increased by both residue and fertiliser P addition, with the former having the larger effect. Of the total P applied to the soil, medic residues contributed approximately one-quarter of that supplied by the fertiliser. Of the total P in the wheat plant, medic residues supplied approximately one-fifth of that supplied by the fertiliser.

Effect of phosphate supply and competition from grasses on growth and nitrogen fixation of Medicago trunculata

Medic plants and ryegrass were grown in small pots in a glasshouse in monoculture (six plants per pot) or in mixture (three plants of each species) at eight rates of phosphate application. After 10 weeks, the rate of nitrogen fixation was estimated by the acetylene reduction (AR) technique on the intact plants in soil, after which the plants were harvested, both shoots and roots. The yield of medic, in both monoculture and mixture, increased with increasing rate of phosphate application to an optimum at a level of 160 ppm phosphorus and then decreased again at higher phosphate levels. AR activity was similarly dependent on phosphate application and was optimal at the same phosphate level as was yield. AR activity was extremely low in the ryegrass monocultures, the yield of which was optimized at only 10 ppm phosphorus. The community x phosphate interaction on AR activity per plant of medic was not statistically significant, there being considerable variability in this measurement; the suppression of nitrogen fixation by competition from ryegrass near its own phosphate optimum was not therefore established in this study, but remains a possibility. Ryegrass plants benefited individually from growing in mixed culture with the legume, producing as much shoot dry matter from three plants in mixture as from six in monoculture. The advantage in mixture was due either to reduced competition offered by medic or to transfer of fixed nitrogen from legume to grass. Higher concentrations and contents of nitrogen per plant in mixture provided circumstantial evidence for the latter. However, on a pot basis, nitrogen content of ryegrass was similar in pure culture and in mixture. The relevance of the data to the results of a survey of farmers' fields is discussed.