The research team, led by Johanah Mmasekeli Thema and colleagues at the University of Limpopo, set out to address a gap that has persisted in the conservation agriculture literature. While numerous studies have demonstrated separately that compost application and minimum tillage can mitigate soil degradation, their interaction effects on crop performance and soil properties at different depths remain poorly explored. The question is not trivial: compost application may fundamentally alter how a crop responds to the tillage system around it, meaning that recommendations developed for each practice in isolation could mislead farmers who combine them. The team therefore designed a trial that would test tillage system, compost rate, and their interaction simultaneously, measuring everything from leaf dimensions to soil organic carbon stocks.

The experimental design was a split-plot arrangement within a randomized complete block design with three replications. Tillage system served as the main plot factor, with conventional tillage and minimum tillage assigned to main plots, while three compost rates of 0, 10, and 20 tonnes per hectare were assigned to sub-plots. The researchers note that this assignment was based on practical and operational requirements rather than on the relative importance of the two factors. Each sub-plot measured 3 by 2 metres. The compost itself was made from 70% chicken manure, 15% cattle manure, and 15% sawdust on a dry weight basis, with a pH of 8.0 in potassium chloride, a carbon-to-nitrogen ratio of 18:1, and total nitrogen of 1.10%. The chosen compost rates fall within the range of 10 to 40 tonnes per hectare commonly used in practice.

The two tillage treatments differed sharply in intensity. Minimum tillage involved ploughing to approximately 12 centimetres using a heavy-duty chisel plough with three tines, which loosens and fractures the soil without completely inverting it. Conventional tillage, by contrast, ploughed to approximately 30 centimetres using a sequence of chisel plough, mouldboard plough, and an offset disc harrow with notched discs at the front and smooth-edged discs at the back, pulverizing the soil into a well-levelled seedbed. Compost was applied at the specified rates 10 days before sowing to allow nutrient mineralization. Soya bean variety Y657 was planted at an intra-row spacing of 15 centimetres and an inter-row spacing of 40 centimetres, under purely rainfed conditions with no supplementary irrigation, and weeding was done manually with a hand hoe.

The growth results at flowering stage were striking. Tillage system significantly influenced total biomass and every measured growth parameter. Plants under minimum tillage recorded leaf length, leaf width, plant height, and number of leaves per plant that were 13%, 15%, 41%, and 51% higher respectively than those under conventional tillage. Total biomass at flowering was 44% higher under minimum tillage. The interaction between tillage and compost also significantly influenced leaf width, plant height, leaf number, and total biomass. Notably, plants under minimum tillage with or without compost were considerably taller, ranging from 31.81 to 32.33 centimetres, than plants under conventional tillage, which ranged from 22.28 to 23.57 centimetres. Chlorophyll content, measured with a portable CCM-200 plus meter, was unaffected by any treatment.

Yet compost told a more complicated story. Under minimum tillage, applying 20 tonnes per hectare of compost caused a significant 46% reduction in total biomass at flowering compared with the untreated control. Under conventional tillage, compost produced only a smaller, statistically insignificant decrease of up to 12%. The authors suggest this may indicate a rate-dependent interaction between high compost inputs and minimum tillage conditions, with the greater soil disturbance under conventional tillage possibly moderating the effect. However, they caution that confirming this explanation would require more detailed and frequent assessments of soil conditions and plant growth to detect short-lived changes. The absence of a similar response at physiological maturity suggests the flowering-stage effect may have been transient, or that the crop partially compensated as the season progressed.

At harvest, the yield picture crystallized. Tillage system and its interaction with compost rate significantly influenced grain yield, while root-to-shoot ratio, total biomass at maturity, and harvest index were unaffected by any treatment. The headline number is the 62% yield advantage of minimum tillage over conventional tillage. Within that pattern, compost behaved differently depending on the tillage system: under conventional tillage, compost at 10 and 20 tonnes per hectare produced small, statistically insignificant yield increases of up to 7% over the control, whereas under minimum tillage, compost reduced grain yield, with a significant 31% decrease at the 10-tonne rate. The control under minimum tillage outperformed every compost treatment under conventional tillage. The authors note that variable short-term yield responses to compost are consistent with previous reports showing that compost effects depend heavily on the management system, and may be neutral or negative in the short term due to nitrogen immobilization and asynchronous nutrient release, particularly under minimum tillage.

The soil measurements added another layer of nuance. Tillage system significantly affected soil organic carbon, organic carbon stock, nitrate-nitrogen, and total mineral nitrogen, with minimum tillage plots recording values 20%, 27%, 56%, and 48% higher respectively than conventional tillage plots. Soil organic carbon reached 0.51% under minimum tillage versus 0.43% under conventional tillage, which the authors say affirms that reducing tillage intensity increases organic matter accumulation. Soil depth also mattered: bulk density was significantly higher in the 15 to 30 centimetre layer than in the top 15 centimetres, while organic carbon, nitrate, ammonium, total mineral nitrogen, and Bray-1 phosphorus were all significantly higher in the surface layer. Intriguingly, organic carbon stock ran counter to this pattern, being significantly higher in the deeper layer, which the authors attribute to slower decomposition at depth providing more opportunity for long-term carbon storage. Compost at 20 tonnes per hectare significantly lowered residual nitrate and total mineral nitrogen compared with the control, possibly reflecting short-term immobilization of mineral nitrogen during organic matter decomposition.

Crucially, the three-way interaction of tillage system, compost rate, and soil depth had no significant effect on any of the measured soil properties, including bulk density, organic carbon, nitrate, ammonium, total mineral nitrogen, and available phosphorus. This means that, under the conditions and duration of the experiment, increasing compost rate did not produce a differential soil response across tillage systems or depths. The authors attribute the limited short-term soil response partly to the relatively brief period of compost application, noting that changes in soil organic matter and nutrient pools may require longer periods to become pronounced. Correlation analysis offered one more insight: grain yield was significantly and positively associated with soil organic carbon and organic carbon stock measured at 15 to 30 centimetres depth, suggesting that subsoil carbon may play an outsized role in crop performance, likely through its influence on root activity, moisture retention, and nutrient availability in the active rooting zone. Grain yield also correlated positively with total biomass, while harvest index correlated negatively with it.

The authors are careful about the limits of their evidence. The study covered a single year, a single crop, and a single site with one climatic environment and one soil type, dominated by Hutton soils corresponding to Ferrasols in the World Reference Base system. They frame the findings as a useful foundation for future multi-site and multi-year studies needed to support more conclusive recommendations on sustainable soil quality and crop productivity. Still, the message for farmers in similar rainfed environments is concrete: minimum tillage delivered substantially better soya bean growth, soil organic carbon, mineral nitrogen, and grain yield than conventional tillage within one season, but compost application under minimum tillage demands careful management, because the same amendment celebrated for rebuilding degraded soils can, at these rates and in this timeframe, measurably reduce the harvest it is meant to secure.

Subject of Research: Interactive effects of tillage system and compost rate on soya bean growth, yield, and soil physico-chemical properties

Article Title: Short-term interactive effects of tillage system and compost rate on soya bean growth, yield, and physico-chemical characteristics of soil

Article References: Thema, J. M., Zerizghy, M. G., Dlamini, P., & Masowa, M. M. (2026). Short-term interactive effects of tillage system and compost rate on soya bean growth, yield, and physico-chemical characteristics of soil. Discover Soil, 3(1), Article 198. https://doi.org/10.1007/s44378-026-00356-0

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00356-0

Keywords: minimum tillage, conventional tillage, compost, soya bean, soil organic carbon, grain yield, soil phosphorus, mineral nitrogen, bulk density, Limpopo, rainfed agriculture, soil health