The effect of once-off tillage on selected soil physical and chemical properties and resultant crop response on a shale derived soil under no-till in the Swartland sub-region of the Western Cape

[摘要] ENGLISH ABSTRACT: Conservation agriculture is widely adopted by farmers who claim that any soil disturbance will be detrimental to both soil physical and chemical properties. However, as the effect of reduced tillage and maximum stubble retention becomes more prevalent, secondary effects, positive or negative, may develop at soil surface or in the upper layers of the soil profile. Therefore, an increased interest in strategic tillage have arose in order to address the emerging CA constraints. CA can be defined as the avoidance of mechanical soil disturbance combined with the maintenance of a permanent soil cover and the implementation of a crop rotation system. The study formed part of a long-term field trial established in 2007 investigating different crop and crop/pasture systems under no-tillage at the Langgewens Research Farm of the Western Cape Department of Agriculture, near Moorreesburg, Swartland, Western Cape, South Africa. The study was conducted during the 2014 and 2015 growing seasons to assess the effect of once-off tillage (till once in 10 years) in long-term medic/wheat/medic/wheat (McWMcW), wheat/lupin/wheat/canola (WLWC), lupin/wheat/canola/wheat (LWCW), wheat/medic/wheat/medic (WMcWMc) and canola/wheat/lupin/wheat (CWLW) cropping systems (the last letter in the sequence represents the crop that was in field at sampling time). Three once-off tillage treatments namely: continuous no-till (NT, soil left undisturbed until planting and then planted with an Ausplow), deep tine non-inversion tillage (DT) and mouldboard inversion tillage (MP) were conducted. Tillage treatments were conducted on 26 and 27 May, in 2014 and 2015, respectively. NT treatments were regarded as a control reference for this study. In February 2014 and May 2015, soil samples were taken at 0-50, 50-100, 100-200, 200-300 and 300-400 mm depth increments. Soil water content measurements were taken weekly (during growing season) and monthly (during fallow season) to a depth of 800 mm in order to determine the soil water balance and the resultant crop performance.The objectives of the study were to investigate the effect of once-off tillage of no-till soil on: i) the soil physical and chemical properties, ii) the soil water balance parameters, (iii) and the resultant crop performance (water use efficiency and rainfall use efficiency) as affected by physical, chemical and soil water balance related properties.Significant differences in particle size distribution between tillage treatments at different depths were found (P = 0.05). Tillage had no significant effect on coarse fragment percentage in both wheat systems investigated in the study (P = 0.05), although, in the canola after wheat system the DT treatment resulted in a significant higher coarse fragment percentage in the 200-300 mm soil depth (P ≤ 0.05). The general coarse fragment percentage trend observed was an increase with depth. This result is however not indicative of a mechanical sieving action which is usually expected after the repeatable conduction of conventional tillage practices. DT was the only treatment to result in significant aggregate stability decreases in both the WLWC and LWCW crop rotation systems. In the 300-400 mm soil depth (LWCW) a significantly higher aggregate stability was observed for the NT treatment compared to the DT treatment (P = 0.1514) while in the 0-100 mm (WLWC) soil depth a significantly higher aggregate stability was observed for the NT treatment compared to the DT treatment (P = 0.0078). The aggressive mechanical action of the deep tine implement was responsible for the aggregate stability decrease. Aggregate stability decreased with depth and therefore results correlated with SOC results due to increases in aggregate stability percentages in soil depths where increases in SOC were observed. According to results obtained once-off tillage had no significant effect on macro-aggregate density (P = 0.05) and it was concluded that the sample clods used for measurement were not representative of the prevailing soil conditions after tillage conduction. Hydraulic conductivity showed significant differences between treatments. Although not always significant, both the NT and DT treatments showed the highest hydraulic conductivity compared to MP for all cropping systems investigated (P = 0.05). The increase in hydraulic conductivity for the DT treatment can be explained by a more favourable soil structure created by the rip action, while the increase under NT was contributed to the preservation of soil macro-pores which is formed by earthworms and decayed plant roots as well as the present mulch layer. A MP tillage action leaves a soil surface bare which exposes the soil surface to compacting effects of rainfall and soil resettlement.MP tillage had a significant effect on pH (KCl and H₂O) while DT tillage had no significant effect. Significant differences were however only observed in the 0-50 mm and 50-100 mm soil depth increments (P = 0.05). The general trend was an increase in pH (KCl and H₂O) with depth and therefore it was concluded that differences were not attributed to a tillage effect but rather to the inherent mother material properties. It is well known that mother materials have higher pH values compared to weathered soil materials. A decreasing trend with soil depth was observed for electrical conductivity. DT proved to be the least favourable in terms of the leaching of salts due to EC increases while NT proved to be the most favourable. The SOC content was not influenced by the single tillage operation as no significant differences were observed between all tillage treatments at all measured depths (P = 0.05). The highest SOC content was observed in the 0-100 mm soil depth where after SOC decreased with each measured depth. The Active C content was not significantly influenced by a once-off tillage operation as was expected in 2014 (P = 0.0005). The prevailing low soil temperatures did not allow active microorganism activity. In 2015 (1 year after tillage) a significant increase in active C content (P = 0.0258) was observed for DT for both the medic MP tillage had a significant effect on pH (KCl and H₂O) while DT tillage had no significant effect. Significant differences were however only observed in the 0-50 mm and 50-100 mm soil depth increments (P = 0.05). The general trend was an increase in pH (KCl and H₂O) with depth and therefore it was concluded that differences were not attributed to a tillage effect but rather to the inherent mother material properties. It is well known that mother materials have higher pH values compared to weathered soil materials. A decreasing trend with soil depth was observed for electrical conductivity. DT proved to be the least favourable in terms of the leaching of salts due to EC increases while NT proved to be the most favourable. The SOC content was not influenced by the single tillage operation as no significant differences were observed between all tillage treatments at all measured depths (P = 0.05). The highest SOC content was observed in the 0-100 mm soil depth where after SOC decreased with each measured depth. The Active C content was not significantly influenced by a once-off tillage operation as was expected in 2014 (P = 0.0005). The prevailing low soil temperatures did not allow active microorganism activity. In 2015 (1 year after tillage) a significant increase in active C content (P = 0.0258) was observed for DT for both the medic after wheat and the wheat after canola systems which was explained by an increase in microbe activity due to favourable soil conditions.Tillage had a negligible small effect on SWC during the 2014 growing season. Significant differences were only observed between NT and DT after big rainfall events and at the end of the growing season (P = 0.05). SWC readings in 2014 ranged between 35-270 mm for all tillage treatments and crop rotation systems measured. ET values varied according to the crops developing stage as well as the available SWC and external environmental factors (rainfall and temperature). Tillage had no significant effect on ET as no significant difference in ΣET was observed at the end of the 2014 growing season (P = 0.05). Tillage had no effect on the ability of the soil to store water during the 2014/2015 fallow season as no significant difference in SWC were observed between treatments. More valuable results would have been obtained during the fallow season if technical difficulties could have been solved. The total amount of rainfall recorded during the 2015 growing season was 31% lower compared to rainfall recorded during 2014 and therefore SWC readings during 2015 ranged between 10-140 mm. Generally, the highest SWC was observed for the MP treatment for all crop rotation systems and on several dates significant higher SWC readings were observed for MP compared to NT and DT (P ≤ 0.05). MP resulted in a significantly higher ΣET at the end of the 2015 growing season (P = 0.05). Due to limited information on the topic a valuable explanation for the higher SWC and ΣET under MP was not found.In the 2014 growing season no significant differences in wheat yield were found between all tillage treatments and crop rotation systems investigated while for the 2015 growing season NT resulted in a significantly higher wheat yield (P =0.0274) compared to both DT and MP in the CWLW system which was attributed to the crop residues present. Although the lowest biomass was obtained for the MP treatment no significant difference between treatments were observed for medics (P = 0.9010). The lower biomass under medics was contributed to the transportation of the self-regenerating medic seeds during the inversion plough action which resulted in seedling emergence delays and poor crop stand. When comparing the 2014 and 2015 grain yield results a 71% decrease in canola yield was observed while a 57% decrease in wheat grain yield was observed in the 2015 growing season. Even though the crops underwent a longer growing season during 2015 a total of 31% less rainfall was recorded during the 2015 growing season compared to the 2014 growing season and therefore lower grain yields were expected. Once-off tillage had no significant effect on WUE and RUE in both the 2014 and 2015 growing seasons for all tillage treatments and crop rotation systems tested.

[发布机构] Stellenbosch University