Sugarcane on acidic soil: how combination treatments are changing yields in the Philippines
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Quick takeaways
- Much Philippine sugarcane is grown on acid, highly weathered soils, where aluminium toxicity, phosphorus fixation and poor root development can hold yields well below the crop's potential.
- Single inputs (lime alone, fertiliser alone) address only one constraint at a time on degraded acid soils, while the others remain.
- Combination treatments (liming, humic acid and a legume green manure) are designed to address pH, organic matter, soil biology and nutrient retention together.
- Published, block-level trial data for this combination in the Philippines is limited. The sensible way to judge it is a paired strip trial on your own blocks, measured over more than one ratoon.
- What we will not claim: that combination treatments work identically on every soil type, that SoilBoost EA alone fixes acidic soil, or that these approaches replace proper variety selection and harvest management. Soil rehabilitation is one layer of a complete production system.
Why this guide exists
Cane yields on many acid blocks in the Philippines sit well below what the same varieties achieve on better soils. Variety and management matter, but on degraded acid ground the soil is often the main constraint.
Decades of continuous monoculture, heavy nitrogen fertilisation without organic matter return, and neglect of soil pH have degraded many of the Ultisols and Alfisols used for cane. Soil tests from such blocks commonly show high aluminium saturation, low organic matter and weak biological activity.
This guide looks at combination treatments that address these compounding soil constraints together rather than one at a time, and at how to test them on your own blocks.
1) Understanding the acidic soil yield trap
What happens to sugarcane on strongly acid soil
Sugarcane tolerates mildly acid conditions, but as pH falls further, several constraints compound:
- Aluminium toxicity: Exchangeable Al³⁺ rises sharply in strongly acid soil, damaging root tips and inhibiting root elongation. Cane with a restricted root system cannot reach subsoil water or nutrients.
- Phosphorus fixation: In strongly acid soil, aluminium and iron bind phosphorus into insoluble forms, so much of the applied P is not available to the crop.
- Calcium and magnesium deficiency: Acid soils are typically depleted of these base cations, which the crop needs for growth and stalk development.
- Reduced soil biology: Many beneficial soil bacteria and mycorrhizal fungi decline in strongly acid conditions, which slows nutrient cycling.
Why the problem is getting worse
Standard sugarcane fertiliser programmes in the Philippines rely heavily on urea (46-0-0) and ammonium sulphate (21-0-0). Both acidify the soil, ammonium sulphate more strongly than urea for each kilogram of nitrogen applied. Without lime to replace the lost bases, soil pH drifts down crop after crop.
After many years of continuous cane with acidifying fertilisers and no lime, some blocks become strongly acid. At that point, raising fertiliser rates gives little or no extra yield, because root-zone chemistry, not nutrient supply, is limiting the crop.
2) Why single inputs fall short on degraded soil
Lime alone
Lime corrects pH and reduces aluminium toxicity, but on soils very low in organic matter the correction can fade as acidifying fertilisers continue, because the soil has little buffering capacity. Surface lime also moves slowly, so an acid subsoil can remain under a limed topsoil, and heavy surface applications can leave a calcium-rich topsoil that upsets nutrient balance.
Fertiliser alone
Increasing NPK rates on acid soil is like filling a leaky bucket. Nitrogen is lost by leaching (and, from surface-applied urea, by volatilisation), phosphorus is fixed by aluminium and iron, and potassium leaches through low-CEC soils. The yield response flattens at high rates.
Organic matter alone
Compost or filter cake adds organic carbon but does not correct pH in the near term. On strongly acid soils, organic acids released during decomposition can briefly add to acidity before the buffering effect builds.
None of these works well alone because each addresses only part of the problem: pH, organic matter, biology and nutrient retention. Combination treatments aim to address all four together.
3) The combination treatment approach
Three components, one system
- Liming: Dolomitic limestone at the rate your soil test recommends, applied to the furrow or inter-row at planting or ratoon initiation. It corrects pH, supplies Ca and Mg and reduces aluminium toxicity.
- Humic acid (SoilBoost EA): Applied with or after lime, at the rate on your quotation. A humic acid soil conditioner can add to cation exchange capacity, help retain nutrients and bind some aluminium; it does not raise pH or supply calcium.
- Legume green manure: Crotalaria juncea or Vigna radiata (mung bean) grown during the fallow between the final ratoon and replanting. It adds biologically fixed nitrogen and organic matter and supports soil biology.
Why the components work together
Each component supports the others:
- Lime creates the pH conditions in which roots and soil biology can function.
- Humic acid adds exchange sites that help hold nutrients in the root zone.
- Legume green manure provides carbon and nitrogen that build organic matter and sustain biological activity.
- Together they can support a gradual improvement: more organic matter improves buffering, which helps keep pH stable, which in turn supports soil biology.
4) What the evidence shows
The individual components are well documented: liming acid soils at the rate a soil test indicates is standard sugarcane practice (Calcino et al., 2018), and a 2024 meta-analysis of humic acid fertilisers reported higher crop yields and nitrogen use efficiency across many trials (Ma, Cheng and Zhang, 2024). Published, block-level trial data for the full lime, humic acid and green manure combination in Philippine cane is limited, so we do not quote yield gains for it.
How to test it on your own blocks
Lay out paired strips on a representative acid block: your standard programme against the combination treatment, with the same variety and management. Record cane yield and, where available, sugar content at each harvest, and repeat soil tests on both strips. The most useful comparison comes over more than one ratoon, because the effect on ratoon-yield decline, which drives replanting decisions, only shows over several crops.
5) Practical implementation
At replanting (best opportunity)
- Soil test first. Measure pH, exchangeable Al, organic matter, available P and base cations (Ca, Mg, K).
- Apply dolomitic lime at the rate the soil test recommends and incorporate it into the furrow zone.
- Apply SoilBoost EA in the furrow at planting, at the rate on your quotation.
- Grow a green manure (Crotalaria or mung bean) during the pre-plant fallow and incorporate it a few weeks before planting.
At ratoon initiation (maintenance)
- Apply SoilBoost EA to the inter-row after trash management, at the rate on your quotation.
- Check pH every year and apply maintenance lime when the soil test calls for it.
- Retain trash mulch where possible (green-cane harvest). Trash mulch returns organic matter to the soil and helps suppress weeds.
Frequently asked questions
Q: Can I use combination treatments on ratoon cane, or only at replanting?
A: Both. Replanting offers the best opportunity because you can incorporate lime and green manure into the soil profile. For standing ratoons, apply lime to the soil surface (inter-row), apply SoilBoost EA and retain trash mulch. The benefits build more slowly than at replanting, so judge them over more than one ratoon.
Q: How much does the combination treatment cost per hectare?
A: It depends on the lime rate your soil test calls for, local lime and seed prices, and the SoilBoost EA rate for your blocks. We quote SoilBoost EA per project: request a quote with your soil test and area, and weigh the cost against the yield you record in your own paired strips.
Q: Does SoilBoost EA replace lime?
A: No. SoilBoost EA does not supply calcium or magnesium and does not raise pH directly. It may help the soil hold nutrients and support biological recovery alongside the pH correction that lime provides. Use both together.
Q: Will this work on all Philippine sugarcane soils?
A: Combination treatments are most likely to help on degraded acid soils with low organic matter. On soils that are only mildly acid and already have adequate organic matter, the extra benefit is likely to be smaller. A soil test shows whether your blocks are likely to respond.
Sources
- Calcino, D., Schroeder, B., Panitz, J., Hurney, A., Skocaj, D., Wood, A. and Salter, B. (2018). Australian sugarcane nutrition manual. Sugar Research Australia Limited. https://elibrary.sugarresearch.com.au/handle/11079/17260
- Ma, Y., Cheng, X. and Zhang, Y. (2024). The impact of humic acid fertilizers on crop yield and nitrogen use efficiency: a meta-analysis. Agronomy 14(12): 2763. https://doi.org/10.3390/agronomy14122763
About this article
This guide is part of the evidence-based content published by Chemiseed Sdn. Bhd. (Malaysia) and its Philippines-based sister company Kudzu Seeds Trading. We separate field-supported claims from mechanism-based ones and are transparent about where evidence gaps remain.