Maize is the primary crop grown on arable land in the Waikato region, predominantly established on Allophanic soils due to their well-drained and resilient properties. Full cultivation (FC) is universally adopted in cropping systems and associated with increased soil aeration and successful seed establishment, however, has been shown to reduce soil quality through declines in soil organic matter (SOM) and soil structure. SOM (TC & TN) decline and aggregate stability are considered important indicators of soil quality in cropping systems as they pose the greatest risk to long term productivity and profitability. Soil degradation can be reduced through conservation tillage such as no-till (NT) and strip-till (ST). Previous studies have investigated the effect of cultivation intensity on soil quality and consistently found that NT systems have greater carbon (C) levels and aggregate stability at the soil surface than higher intensity cultivation systems.
The main aim of this thesis was to determine whether there were significant differences in soil quality between varying intensity cultivation systems (FC, ST, NT) on Allophanic soils in the Hamilton Basin. Further aims were to determine whether differences in the inherent soil properties of the Horotiu silt loam and Bruntwood silt loam would influence the soil quality within the study area, and to identify whether soil quality was influenced by an interaction between cultivation intensity and soil type.
Twelve plots with four replicates of each cultivation treatment were sampled and soil quality measured using seven soil quality indicators (total C (TC), total nitrogen (TN), mineralisable nitrogen (N), soil pH, Olsen P, bulk density (BD), and macroporosity (MP)) and three additional cropping indicators (aggregate stability, penetration resistance, and visual soil assessment). Mechanically driven cores for TC and TN analysis were taken from 0 – 7.5 cm, 7.5 – 15 cm, and 15 – 30 cm. Significant differences in TC, TN, and aggregate stability in the top 10 cm were detected between cultivation treatments (p < 0.05). NT was shown to be the most beneficial cultivation for a maize cropping system, indicated by significantly greater TC (3.98 %), TN (0.41 %), and aggregate stability (0.97 mm, MWD) at the soil surface than higher intensity cultivation systems (For FC; TC = 3.56 %, TN = 0.37 %, aggregate stability = 0.62 mm). Additionally, significant differences in TC and TN were found between soil types, where Horotiu silt loam had significantly greater TC and TN (e.g. for 0 – 10 cm; Horotiu: TC = 4.02 %, TN = 0.42 %; Bruntwood TC = 3.52 %, TN = 0.36 %). There were also significant differences in aggregate stability, MP, and BD between soil types, where the Horotiu silt loam had higher aggregate stability (0.82 mm, MWD), MP (14 %) and lower BD (0.96 t m-3) and Olsen P (82.9 μg g-1) than the Bruntwood silt loam (Aggregate stability = 0.73 mm, MP = 12 %, BD = 1.05 t m-3, Olsen P = 105.5 g g-1). Where there were interactions between cultivation intensity and soil type, significant differences were detected in aggregate stability and penetration resistance, where the Horotiu silt loam under NT had higher aggregate stability (1.07 mm, MWD) and penetration resistance (2.00 MPa) than all other combinations. Conversely, the Bruntwood silt loam under FC had the lowest aggregate stability (0.55 mm, MWD) and penetration resistance (1.63 MPa).
Many of the soil quality values in the study area fell below or exceeded target ranges set for cropping systems, regardless of cultivation treatment or soil type. This is due to the intensive nature of cropping systems, use of heavy machinery, removal during harvest, and poorly defined target ranges. This study highlights how differences in inherent soil properties between two soil types within the same soil order can greatly influence soil quality. Previous data showed cultivation intensity did not significantly influence maize yield at this stage in the trial. This research therefore suggests NT systems result in higher SOM content and greater aggregate stability and therefore may be a more suitable cultivation system for continuous maize cropping without decreasing productivity or profitability.
