Addressing biophysical constraints for Australian farmers applying low rates of composted dairy waste to soil

Abstract

This study examined the response of forage crops to composted dairy waste (compost) applied at low rates and investigated effects on soil health. The evenness of spreading compost by commercial machinery was also assessed. An experiment was established on a commercial dairy farm with target rates of compost up to 5 t ha −1 applied to a field containing millet [Echinochloa esculenta (A. Braun) H. Scholz] and Pasja leafy turnip (Brassica hybrid). A pot experiment was also conducted to monitor the response of a legume forage crop (vetch; Vicia sativa L.) on three soils with equivalent rates of compost up to 20 t ha −1 with and without 'additive blends' comprising gypsum, lime or other soil treatments. Few significant increases in forage biomass were observed with the application of low rates of compost in either the field or pot experiment. In the field experiment, compost had little impact on crop herbage mineral composition, soil chemical attributes or soil fungal and bacterial biomass. However, small but significant increases were observed in gravimetric water content resulting in up to 22.4 mm of additional plant available water calculated in the surface 0.45 m of soil, 2 years after compost was applied in the field at 6 t ha −1 dried (7.2 t ha −1 undried), compared with the nil control. In the pot experiment, where the soil was homogenized and compost incorporated into the soil prior to sowing, there were significant differences in mineral composition in herbage and in soil. A response in biomass yield to compost was only observed on the sandier and lower fertility soil type, and yields only exceeded that of the conventional fertilizer treatment where rates equivalent to 20 t ha −1 were applied. With few yield responses observed, the justification for applying low rates of compost to forage crops and pastures seems uncertain. Our collective experience from the field and the glasshouse suggests that farmers might increase the response to compost by: (i) increasing compost application rates; (ii) applying it prior to sowing a crop; (iii) incorporating the compost into the soil; (iv) applying only to responsive soil types; (v) growing only responsive crops; and (vi) reducing weed burdens in crops following application. Commercial machinery incorporating a centrifugal twin disc mechanism was shown to deliver double the quantity of compost in the area immediately behind the spreader compared with the edges of the spreading swathe. Spatial variability in the delivery of compost could be reduced but not eliminated by increased overlapping, but this might represent a potential 20% increase in spreading costs.

Prospects for improving perennial legume persistence in mixed grazed pastures of south-eastern Australia, with particular reference to white clover

“…There are several reports of long-term fertiliser application resulting in increased water-stable aggregation, porosity, infiltration capacity and hydraulic conductivity, and decreases in bulk density (Haynes and Naidu 1998), all factors that would favour increased plant-available water in the field. Applications of manures can also result in increased soil organic matter content, leading to similar improvements in soil physical condition and hydraulic properties (Haynes and Naidu 1998), with even low application rates leading to small but significant increases in plant-available water in the field (Hayes et al 2017). The effects of lime, fertiliser and manure applications on soil can be complex, with many interactions possible, some of which have potential negative consequences for plant-available water.…”

“…Increased NO 3 leaching under pasture swards dominated by legumes is associated with soil acidification Nodules formed in acidic soils typically contain more acid-tolerant strains (Bottomley 1992; Slattery et al 2001) Major plant nutrients (N, P, K and S) Perennial legumes vary in their nutrient foraging potential nutrients (e.g. P; Kidd et al 2016; Yang et al 2017)Soil with high N can support large populations of rhizobia, but strain persistence may be compromised where legume root nodulation is limited(Bottomley 1992;Brockwell et al 1995) Pasture legume biomass and N fixation under P, K and S deficit is similar, suggesting relatively equal importance(Divito and Sadras 2014) Generally, population size and persistence are positively correlated with available P(Brockwell et al 1995;Lindström et al 2010) and in some cases K(Parker et al 1977; Divito and Sadras 2014) Little evidence linking the long-term persistence of perennial legumes to soil fertility Available P, S, Mo, B positively affect nodulation, occupancy and N fixation(Brockwell et al 1995;McInnes et al 2004;Yates et al 2005; Divito and Sadras 2014) Nodulation and nodule function are more sensitive than biomass production to deficiency of P, K and S (Divito and Sadras 2014) SOC SOC is positively correlated with improved soil physical condition and hydraulic properties(Haynes and Naidu 1998), including increased plant-available water(Hayes et al 2017), but recent evidence questions the extent to which this occurs(Minasny and McBratney 2018) …”