The impacts of conservation agriculture practices on soil health

The sandy soils of the North-Western Free State are known for their inherent compaction problem, low organic matter content and low nutrient and water retention capability. Research in the 1970’s led to the implementation of the rip-on-the-row (ROR) tillage system, which comprised various depths of ripping, mostly on the row, in combination with secondary shallow tillage practices. Actual problems that still exist are the short-term residual effect of ROR, poor retention of crop residue, soil losses and seedling damage due to wind erosion and the impact of fluctuating water tables.

 

A comprehensive investigation based on Conservation Agricultural (CA) principles and practices was launched in 2016 by the Sandy Soil Development Committee to enhance and maintain the productivity of these soils in view of national food security. The emphasis will be on: (i) poor soil health, (ii) soil compaction, (iii) diversifying annual cropping systems to include legumes, perennial crops and forages in rotations, (iv) using cover crops in conjunction with row crops, (v) integrating livestock with cropping systems (vi) prevalence of crown and root rot, nematode infestation, microbial diversity and activity and (vii) profitability information of various CA systems and practices.

 

• Dry matter (DM) yields of the summer cover crops ranged from 14.5 to 17.1 t ha-1, while DM yields of the winter cover crops ranged from 11.3 and 16.1 t ha-1.
• The winter cover crops gave the highest water use efficiency (WUE) compared to the summer cover crops (36.6 vs. 25.9 kg dry matter mm-1 ha-1), while the monoculture maize attained a WUE of 14.3 kg grain mm-1 ha-1.

• The summer cover crop mixture led to a build-up of soil organic carbon (SOC) in the 0-100 mm layer of 0.74% C compared to, for example, 0.49% C under a grass land.
• Between rainfall events, soil water content (SWC) quickly approached the permanent wilting point. SWC was much lower under the summer cover crop mixture compared to the maize crop immediately after a rain event. This is probably due to the interception of the rain by the closed canopy of the former crops, followed by evaporative losses directly from the crop canopy. A full SWC profile was measured on the fallow land before planting of the winter cover crops.
• Leaching of costly and health threatening NO3-N on these sandy soils appears to be a serious problem. Another health threatening component was the presence of high concentrations NO2 (nitrite). Other plant nutrients, such as PO4, K, Ca and Mg, were also present in both water tables at all dates of sampling.

• Maize in 1.524 m rows had 2.5 times the number of tillers compared to plants in 1.016 m rows.
• Maize grain yield was affected by both plant population and row width. The mean yield of the 1.016 rows was 1.37 t ha-1 higher than that of the 1.524 m rows. The results on yield per plant as related to plant population and row width indicate that the cultivar displays tolerance to stress caused by increasing plant densities.
• The best margins were realised with narrow (1.106 m) rows at high population densities, compared to wider (1.524 m) rows. For example, R5082 ha-1 and R5153 ha-1 were realised with narrow rows at population densities of 30 000 and 50 000 plants ha-1, respectively. With wider rows (1.524 m), the highest margin (R3001 ha-1) was obtained at a population of 25 000 plants ha-1, compared to R1703 and R2356 ha-1 for 20 000 and 50 000 plants ha-1 respectively.

• The number of tillers per plant was not affected by seeding density.
• Each cultivar had a unique yield response to seeding density. Optimum population density appeared to be above 23000 ha-1.
• Cultivar 78-87Bt at plant population of 24 000 plants ha-1 gave a slightly higher (R12246 ha-1) margin than 26 000 plants ha-1 (R12187 ha-1). It appeared as if the economic optimum population for cultivar 78-87Bt is 22000-24000 plants ha-1.
• For cultivar 78-17Bt a plant population density of 26 000 plants ha-1 yielded a higher (R11443 ha-1) margin than 24 000 plants ha-1 (R10891 ha-1).
• Comparing the margins of the two cultivars, it can be seen that cultivar 78-87Bt with a population of 24 000 plants ha-1 yielded R803 ha-1 more than the 26 000 plants ha-1 of cultivar 78-17Bt.

• Grain yield increased linearly with ripping depths from 45 to 75 cm at a rate of 0.87 t ha-1 per 100 mm increase in ripping depth.
• No significant yield response was found by increasing the ripping depth beyond 75 cm.
• The 90 cm ripping yielded a slightly higher margin than the 75 cm ripping. However, the additional capital cost will eliminate this financial advantage. It can be concluded that 90 cm ripping would not have a financial gain over the current farm practice of ripping to 75 cm depth.
• Ripping shallower than 75 cm will not be economically viable.