Benefits of CA

Trials conducted by the Agricultural Research Council at its Zeekoegat experimental farm in Roodeplaat have compared conventional farming practices with conservation agriculture, and have identified many benefits to CA. Peter Hittersay reports.

Benefits of CA
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The Zeekoegat conservation agriculture (CA) project originated in 2007 to investigate CA principles in an on-station trial under rainfed conditions. The ARC experimental farm at Zeekoegat, north of Pretoria, was selected for the trial. The multi-disciplinary study, which it is hoped will continue for another three years, involves researchers from various ARC institutes.

Focus areas include soil parameters (aggregate stability, soil water and temperature, nutrient dynamics, and greenhouse gas emissions), plants (biomass, grain yield, leaf analyses and weeds) and biology (nematodes, soil bacteria, mycorrhizae and root pathogens).

Trial components
The Zeekoegat CA trial was designed to enable researchers to compare CA practices with conventional farming practices. The aspects included were:

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  • Tillage systems of RT – crop residue slashed and shallow planting furrows drawn by a four-tine cultivator – and conventional tillage (CT) – crop residue slashed, soil mouldboard-ploughed and disc-harrowed, and shallow planting furrows drawn by a four-tine cultivator;
  • Cropping systems of maize monoculture, maize/cowpea rotation, maize/soya bean rotation, maize/cowpea intercropping, maize/oats delayed intercropping, and maize/grazing vetch delayed intercropping;
  • Fertiliser systems of high fertiliser application rates (optimal for rainfed crops) and low fertiliser rates (50% of high fertiliser rates).

The trial layout was a split-plot randomised complete block design, replicated three times, with each replicate split into two tillage systems (whole plots, 8m x 8m each) and each whole plot (conventional and reduced tillage) subdivided in turn into 12 treatments (six crops x two fertilisers).

The maize grain yields under different tillage systems were compared. The average grain yield over four years was 4,13t/ha under reduced tillage compared with 3,89t/ha under CT. Comparing different years, however, shows that in some years, reduced tillage performed better (7,04t/ha in season one, compared with 6,29t/ha under CT), while in other years CT was better (5,06t/ha in season two under CT compared with 4,8t/ha under RT).

When the effects of the cropping systems were analysed, it was clear that the maize monoculture plots had performed the worst, while intercropping and rotation plots did better. The maize/grazing vetch intercropping system was the best for grain yields.

Weed population, density and composition are closely associated with tillage practices, and therefore any change in tillage will change the dynamics of weed occurrence. In the Zeekoegat trial, weed biomass, density and dominant species were monitored for three years. Large thorn apple or olieboom (Datura ferox) was particularly abundant on the ploughed plots (60,4% of weed biomass), but very low (5,1%) on RT plots.

The second-most abundant weed under CT was sedge (Cyperus rotundus), which contributed 18,1% of total weed biomass under CT, but only 8,3% under RT. Dominant weeds under RT were more varied and included flax-leaf fleabane (Conyaza bonariensis) at 35,7%, blackjack (Bidens pilosa) at 10,9% and narrow-leaved ribwort (Plantago lanceolata) at 10,2%. The weed biomass was, however, higher on RT soils, while both the biomass and variety of weed species were lower under CT.

Soil aggregate
Aggregate stability is an indication of the strength of a soil, its ability to hold water, and its erosion resistance. In general, it is known that a high soil organic carbon (C) content results in a high aggregate stability and stronger, more resistant soils. CA practices aim to increase soil organic C, and thus the hypothesis was that CA practices would benefit aggregate stability. 

And indeed, results from the trial confirmed expectations: higher soil organic C content as well as aggregate stability was measured on RT plots than on CT plots. CA practices proved to promote increased aggregate formation and stabilisation, as well as improved soil water retention, compared with CT practices.

Soil, water and temperature
The effects of tillage and cropping systems on soil water status and soil temperature are currently being measured. The results show that soil water content is higher on RT plots than on CT plots. The effect is most visible up to a depth of 400mm, after which the difference is not so apparent. The first 400mm is, however, critical for experimental crops, as the soil water data shows that the crops extract water primarily from this zone.

Soil temperature on RT plots was lower in summer. The mulch or crop residue left on the soil of RT plots acts as an insulator, reducing temperature extremes in the soil and creating a more favourable growing environment.

Greenhouse gases
Climate change is expected to affect agriculture severely, but at the same time, the sector influences climate change, contributing an estimated 12% of total greenhouse gas (GHG) emissions. As part of the research, two GHGs, nitrous oxide (N2O) and carbon dioxide (CO2), were measured, as these gases are closely associated with agricultural soils. The preliminary results show that N2O emissions were very low and not affected by tillage or cropping systems.

This is no surprise as N2O is produced under anaerobic soil conditions, and the emission rate from dryland agriculture was not expected to be high. CO2 on the other hand did show some differences in emissions, with the ploughed soils emitting more than the RT plots. This could be due to higher mineralisation rates under CT as a result of incorporation of organic material into the soil during ploughing.

Soil microbiology
Microbial biodiversity and activity are integral to soil health, and were thus included in the study. Carbon source utilisation profiles indicated differences between various rotation systems and tillage practices. Enzyme activities increased since 2009, and by 2011 overall enzyme activity was higher under RT than under CT. Enzyme activities also differed between rotational crops. Increased enzyme activity indicates an increased potential of soil to degrade or convert substrates, which in turn can be used as an indication of soil fertility.

The purpose of studying nematodes is to monitor, over an extended period, the succession of nematodes in some of the RT and CT treatments to better understand the effect of tillage on nematodes under South African conditions. Long-term land-use cycles can influence the structure of nematode communities.

The succession of both plant-feeding (plant-parasitic) and free-living nematodes are monitored. The latter, also called beneficial or non-parasitic nematodes, play an important role in soil fertility and agricultural productivity. The composition of both types of nematode communities correlates well with nitrogen cycling and residue decomposition, two of the most critical ecological processes in soil. In particular, free-living nematodes are killed easily by relatively low doses of chemicals and are therefore useful indicator organisms of pollution in soil or of soil health.

The plant-feeding nematodes observed in the samples are typically those associated with maize soils in South Africa and include Criconemoides sphaerocephalus, Helicotylenchus dihystera, Meloidogyne javanica, Nanidorus minor, Pratylenchus brachyurus, Pratylenchus zeae, Scutellonema brachyurus, Rotylenchulus parvus and Tylenchorhynchus brevilineatus. No trend could be observed in the population numbers of the various plant-feeding nematodes.

There were also no consistent differences between the plant nematodes in the RT plots and those in the CT plots. If the results of the last three seasons are considered, the only trend observed was a decline in most plots in the incidence of free-living nematodes with numbers generally lower than those of the plant-parasitic nematodes. The numbers of free-living nematodes in the CT plots were also generally higher than in the RT plots.

Arbuscular mycorrhizal (AM) fungi play an important role in crop growth, as they facilitate the transportation of nutrients and water to plants, thus helping to protect them against drought stress and pathogens. In this study it was hypothesised that AM fungi are good indicators of soil health. Initial counts indicated very low spore numbers and glomalin concentrations in all treatments.

Commercial inoculant (Mycoroot SuperGro Inoculum) was applied in November 2010, and the spore count and glomalin levels increased significantly as a result. The colonisation percentage was higher under RT than under CT, and trends are starting to develop that differentiate between CT and RT.

Research team
The researchers of the Agricultural Research Council-Institute for Soil, Climate and Water (ARC-ISCW) are Corrie Swanepoel, project leader and researcher; Dr Danie Beukes, specialist researcher; and Reedah Mampana, student. Those from the ARC- Plant Protection Research Institute are: Dr Mariette Marais, senior researcher; Dr Antointette Swart, specialist scientist;  Dr Susan Koch, senior researcher; Wendy Sekgota, student; and Johan Habig, researcher.

Source: Synopsis of the Information Day by Corrie Swanepoel, project leader and researcher of ARC-ISCW.
For more information, contact Corrie on 012 310 2630 or at
[email protected]