Tillage and no-tillage effects on physical characteristics of a silt loam under 5 years of continuous oats-maize crop rotation : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Agricultural Engineering, Institute of Technology and Engineering, Massey University

Conservation tillage is one of the conserving practices recognized worldwide despite its empirical benefits still largely undergoing continuous research. This research is part of a sequence of studies carried out at Massey University tillage trial. The soil type is Ohakea silt loam representing youngest yellow-grey earth with poor natural drainage on fine texture material, and topsoil moderately to strong acid enleached soils. Selected soil physical properties under different tillage systems i.e. no-tillage (NT), moldboard plough (MP) and permanent pasture (PP) (as control) were measured and compared. The important soil properties considered were soil aggregate stability, soil penetration resistance, water infiltration rate, soil bulk density, soil water content, crop dry matter, water runoff and leachate and soil pH (H2O), total C and N. Results from both the field and laboratory experiments suggested that 5 years of continuous no-tillage have improved soil characteristics relative to conventional tillage. Soil penetration resistance was significantly lower in the MP plots soon after cultivation and at the early oats growing season, compared to the NT and PP plots. However, this trend was reversed within six months, following winter grazing and spring fallow when soil was recompacted. Bulk density measured during early oats growing season indicated a remarkably higher density at the top 0-5 cm soil layer under the NT compared to the MP treatment suggesting that NT plots' soils were more compacted at the time of planting and had lower total porosity than soils in the MP plots. On the other hand, water infiltration rates measured over one year period indicated an average value significantly higher under the NT and PP treatments than the MP plots. These results suggest that macropore continuity, water-filled porosity and other hydraulic properties were improved under NT. A substantially higher level (11%) of water content was found in the NT plots compared to that in the MP plot. These suggested that although the NT soils were more resistant to penetration and had high levels of bulk density, these soils retained more water. These further suggested that the water-filled porosity under the NT soil was higher, thus helped increase the water availability for plant growth. The results also demonstrated that the NT soil produced comparable winter oats and summer maize DM to those under MP treatment. Regression analysis results indicated, not unexpectedly, a strong linear relationship between bulk density and soil penetration resistance with R2 values of 0.97, 0.99, and 0.73 for the PP, MP, and NT treatments respectively. Similar analyses between soil water content and soil penetration resistance demonstrated a strong, moderate, and no correlations under the NT, MP and PP treatments respectively. The NT soils were substantially more stable than the MP soils but were similar to the PP soils. The surface soil (0-10 cm soil depth) water-stable aggregates remaining on sieve for the PP, MP, and NT were 75.2, 26.2 and 70.8% respectively. The macroaggregates (> 2 mm diameter) made up a large proportion of the pasture soil (54.7%) and the untilled soil (37.4%), whereas the ploughed soils had macroaggregates at 4.8%. The ploughed soil was consisted of 73.8% of 0.5 mm water-stable aggregates. Prolonged sieving for 60 minutes also confirmed the above results that the detachment of soils by water in the continuously ploughed land was much easier as compared to the NT and PP management. Thus making the MP soils most vulnerable to water erosion. Runoff and leachete experiments had produced rather inconclusive results as compared to the results on the same plots three years ago. However the trend was obvious that the MP treatment had caused more surface runoff than the other two treatments. By contrast, water runoff was lower in NT plots, which was also reflected by the occurrence of more water leaching under this treatment compared to the MP treatment. The NT soils were relatively more acidic (lower pH) both at 0-10 and 10-20 cm soil layers. Both the MP and NT had resulted in a marked decline in total C level compared to PP at the 0-10 cm soil layer. The decline of total C content after 5 years of continuous double cropping in the 0-20 cm soil layer was about 12% in the MP plots and 2.65% in the NT plots. At the 10-20 cm soil depth, total C and N showed no differences among all treatments. Total N at the 0-10 cm soil layer was significantly lower under MP treatment compared to the other two treatments