A common pattern on acid cane soils: a soil test shows plenty of total phosphorus, little of it is in available form, and the cane responds poorly. The phosphorus is there, but much of it is bound in forms roots cannot take up.
On acid soils, phosphorus binds to iron and aluminium. Getting more of it to the crop starts with understanding that soil chemistry and the measures that are known to work.
Why Negros Soils Lock Phosphorus
Phosphorus exists in soil in multiple forms. Orthophosphate (PO₄³⁻) is the form roots absorb. But in acidic soils, free Al³⁺ and Fe³⁺ ions dominate. These metals form highly insoluble compounds with phosphate, such as AlPO₄ and FePO₄. Once bound, the phosphorus is unavailable to roots even though it is chemically present.
The extent of the problem depends on soil pH and on iron and aluminium content. In strongly acid soils, a soil test can show high total phosphorus while only a small fraction is available to the crop.
Sugarcane needs a steady phosphorus supply for root development and tillering, in plant cane and in each ratoon. A shortage reduces tillering and stalk growth, and with them cane yield.
Can Humic Acid Mobilise Locked Phosphorus? What the Evidence Says
Humic acid molecules carry carboxyl groups, and in principle these organic anions can compete with phosphate for binding sites on aluminium and iron oxides. The evidence that this releases much phosphorus in the field is weak. A laboratory study found that humic and fulvic acids had only a limited effect on phosphate adsorption by aluminium and iron oxides (Borggaard et al., 2005), and a review of competitive sorption concluded that the effect of organic matter breakdown products on phosphorus sorption is likely to be negligible at the rates applied in the field (Guppy et al., 2005).
Where organic amendments have raised available phosphorus in acid soil, the main route was soil chemistry rather than direct release: in a laboratory incubation of an acid soil, biochar and compost raised soil pH and lowered exchangeable acidity, aluminium and iron, and available phosphorus rose with them (Ch'ng et al., 2014).
That points to the measures with the strongest basis: correct soil acidity with lime at the rate your soil analysis recommends, place phosphate fertiliser close to the roots, and build soil organic matter over time. SoilBoost EA is a humic acid soil conditioner that may support soil structure and root-zone conditions as part of that programme; it is not shown to release bound phosphorus.
Placing Fresh Phosphorus
Soluble phosphate fertiliser applied to acid soil is also fixed by aluminium and iron over time (Ch'ng et al., 2014). Placing it in a band close to the emerging roots, rather than broadcasting it, gives the crop a better chance to take it up before it is fixed.
An Example Sequence After Ratoon Harvest
- After harvest: If you use SoilBoost EA, apply it over the stool area at the rate on the product label or your quotation, as part of your soil programme.
- As the ratoon sprouts: Apply phosphate fertiliser in a band close to the stool row, at the rate your soil test and local fertiliser recommendation set. Water in if dry.
- During early growth: Check ratoon growth and leaf colour, and correct potassium or other deficiencies as your soil and leaf analysis indicate.
Follow your mill district or Sugar Regulatory Administration (SRA) recommendation for fertiliser rates and timing on your soil.
Realistic Expectations
No published field trial has measured the effect of SoilBoost EA on phosphorus availability or cane yield in Philippine sugarcane, so we do not quote an expected yield gain. To judge it on your farm, treat one block, keep a comparable untreated block, and compare soil-test available phosphorus and cane yield at harvest.
Soil Testing and Baseline
Before changing your phosphorus programme, test the soil for pH, total P (by acid digestion), available P (for example by Bray-1 or Mehlich-3) and CEC. A Department of Agriculture regional soils laboratory or another accredited soil laboratory can run these. An acid soil with high total P but low available P is the case this article describes.
Re-test after the crop cycle and compare pH, available P and CEC on treated and untreated blocks. That comparison shows whether the programme is working on your soil.
Integration with Organic Matter Strategy
Phosphorus management is most durable when combined with organic matter addition. Organic matter (from legume cover crops between cane plantings or from composted field residue) increases CEC and, as Ch'ng et al. (2014) found for organic amendments in acid soil, can raise pH and lower exchangeable aluminium and iron. Long-term change in soil chemistry requires ongoing organic matter and pH management.
References
Borggaard, O.K., Raben-Lange, B., Gimsing, A.L. & Strobel, B.W. (2005). Influence of humic substances on phosphate adsorption by aluminium and iron oxides. Geoderma 127(3-4): 270-279. https://doi.org/10.1016/j.geoderma.2004.12.011
Ch'ng, H.Y., Ahmed, O.H. & Majid, N.M.A. (2014). Improving phosphorus availability in an acid soil using organic amendments produced from agroindustrial wastes. The Scientific World Journal 2014: 506356. https://doi.org/10.1155/2014/506356
Guppy, C.N., Menzies, N.W., Moody, P.W. & Blamey, F.P.C. (2005). Competitive sorption reactions between phosphorus and organic matter in soil: a review. Australian Journal of Soil Research (now Soil Research) 43(2): 189-202. https://doi.org/10.1071/SR04049