By Kudzuseeds Agronomy Team

A millero in Negros managing 120 hectares of sugarcane watched his RAT (reflectance attenuation test) scores drop from 68° to 55° over four ratoon cycles. The decline meant slower ripening and lower sucrose—equivalent to a loss of 8–10 tonnes of sucrose per hectare per year. Soil testing showed phosphorus: total P 820 mg/kg, available P 28 mg/kg. The soil was rich in phosphorus but the cane could not use it.

This is a textbook Negros problem. The island's volcanic soils and decades of monoculture sugarcane have built up phosphorus bound to iron and aluminium oxides. The phosphorus is there; it is locked. Unlocking it requires understanding soil chemistry and timing the application window correctly.

Why Negros Soils Lock Phosphorus

Phosphorus exists in soil in multiple forms. Orthophosphate (PO₄³⁻) is the form roots absorb. But in acidic soils (pH 4.5–5.5, typical of Negros uplands), free Al³⁺ and Fe³⁺ ions dominate. These metals form extremely insoluble complexes with phosphate: 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 iron/aluminium content. Soils below pH 5.0 can lock 50–70% of their total phosphorus. A Negros hacienda soil with 800 mg/kg total P and pH 4.8 might have only 15–25 mg/kg available P—meaning 750+ mg/kg is locked away (Rose 2019).

Sugarcane has high phosphorus demand. During the grand growth period (months 4–8 after planting or ratoon sprouting), sucrose accumulation and root development require continuous P supply. A shortage at this point reduces tillering, delays flowering, and reduces juice quality (lower Brix, lower pol %). The effect is visible in the RAT test: locked phosphorus = lower maturity scores, later ripening, reduced mill recovery.

Mobilising Locked Phosphorus: The Humic Acid Mechanism

Humic acid molecules contain carboxyl groups (—COOH) that dissociate to negatively charged carboxylate (—COO⁻). In soil solution, these groups compete with phosphate for binding sites on Al and Fe oxides. The competition is energetically favourable for humic acid because the —COO⁻ form stronger ligand bonds than PO₄³⁻ (Nardi 2021).

In practical terms: when humic acid enters the soil solution, it displaces some of the bound phosphate. That displaced phosphate re-enters the soil water and becomes available to roots. The effect is not instantaneous—it takes 3–4 weeks for humic acid to permeate the root zone and achieve equilibrium. But once in place, the increase in available P can be substantial.

SoilBoost EA carries 60.6% humic acid by the CDFA method, with 0.45% sulfur. The sulfur is additional because it also addresses acidity: as soil microbes oxidise elemental sulfur to sulfuric acid, the pH drops slightly (0.2–0.5 units over 6–8 weeks), which further reduces Al and Fe solubility and reduces their phosphate-binding strength (Rose 2019). This is a secondary benefit, not a primary mechanism, but it is real.

Pairing Mobilised P with Fresh Phosphorus

Mobilising locked phosphorus is the first step. Adding fresh phosphorus is the second step. But standard DAP (18-46-0) and superphosphate in acidic Negros soils become partially locked themselves within days—some of the fresh P gets re-captured by Al and Fe oxides before roots can use it (Veneklaas 2017).

When a soluble phosphate fertiliser is applied with, or shortly after, SoilBoost EA, the humic acid's competition with phosphate receptors on the soil matrix keeps the fresh phosphorus in available form longer—an estimated 60–70% of the applied P remains in plant-available form for 4–6 weeks instead of being fixed. This is a modelled scenario based on competition kinetics (Nardi 2021); direct field comparison data at Philippine scale is not published.

Ratoon Stool Protocol: July Application Timing

Ratoon stools are the dormant buds on the stubble remaining after harvest. They sprout 7–14 days after harvest (or cutting) and enter rapid growth for 2–3 weeks. This is the critical window for root development and tiller emergence. Phosphorus demand during this phase is high but root contact with soil P is limited because the young roots are still extending into undisturbed soil.

Standard practice is to apply P-fertiliser within 7 days of harvest. In Negros, the off-season ratoon (plant-cane harvest in May–June, ratoon harvest in November–December) means the July pre-harvest application window is not available. But the main ratoon (harvest in May, growth into July–August) allows a 4–6-week window between ratoon sprouting and grand growth stage entry.

Recommended protocol for July ratoon feeding:

Week 1 post-harvest: Apply SoilBoost EA at 50–100 kg/hectare broadcast over the stool area. Irrigate lightly to incorporate into top 10 cm. Do not plough—this is a surface application to intercept young roots.
Week 2–3 post-harvest: Apply a soluble phosphate fertiliser in a band 15 cm from the stool row. This localises the fresh P near emerging roots. Water in immediately if dry.
Week 5 post-harvest: Apply foliar K (e.g., potassium chloride solution, 2% K) as a spray if ratoon growth looks sluggish or colour is off. This bridges to the soil-available K that will mineralise later.

The staggered timing (humic acid first, fresh P second, K foliar third) reflects the different mechanisms: humic acid needs time in solution to displace locked P, fresh P needs proximity to roots immediately, K can supplement foliar uptake if soil reserves look tight.

Yield Data and Realistic Expectations

Published field trials comparing conventional P+K versus SoilBoost plus a soluble phosphate source in Philippine sugarcane are limited. The millero case above reported a RAT increase from 55° to 62° over one ratoon cycle (6–8 months), corresponding to approximately 2–3 tonnes/ha sucrose gain—but this is one farm, one ratoon, and cannot be extrapolated to all Negros soils.

Conservatively, expect 5–8% cane yield increase on soils with available P below 30 mg/kg and pH between 4.5–5.2. Soils already at pH 5.8+ with available P above 40 mg/kg are less likely to respond because the phosphate-locking problem is already mitigated. Similarly, recent-ratoon cane (year 1 or 2 after plant-cane harvest) responds more than older ratoons (year 4+) because the soil structure and microbial community are more active in young ratoons.

Soil Testing and Baseline

Before starting a phosphorus mobilisation protocol, test soil for: pH, total P (via HCl-extraction or Mehlich-3), available P (via Bray-1 or Mehlich-3), and CEC. UPLB, PCA-Davao, or BSWM-accredited labs in Negros (e.g., Negros Oriental State University soil lab) can run these. A baseline soil with pH 4.5–5.2, total P 600–1000 mg/kg, available P 15–35 mg/kg is a clear candidate for humic acid + fresh P strategy.

Re-test after 6 months to check: pH (should increase 0.2–0.5 units), available P (should increase by 10–15 mg/kg), and CEC (should increase slightly as organic matter integrates). These confirmations validate whether the protocol is working on your specific soil.

Integration with Organic Matter Strategy

Phosphorus mobilisation is most durable when combined with organic matter addition. Stable organic matter (from legume cover crops or field residue composting) increases CEC and provides additional carboxyl groups that compete with Al and Fe for phosphate binding. A three-year approach (Year 1: SoilBoost plus fresh P, Years 2–3: legume cover crop cycles) builds cumulative P-availability gains that persist even when amendment inputs stop.

The Negros millero's RAT recovery over multiple ratoons will ultimately depend on this integration. Single-season phosphorus applications maintain availability for one ratoon cycle; long-term soil chemistry change requires ongoing organic matter and pH management.

References

Nardi, S., Renella, G., Muscolo, A., & Fabbri, C. (2021). Humic substance and microbial dynamics in soil: Role in organic matter processing and nutrient cycling. Soil Biology and Biochemistry, 152, 108052.
Rose, M. T., et al. (2019). Organic amendments for improving soil health: Do we know what we are applying? In Advances in Agronomy (Vol. 156, pp. 1–38). Academic Press.
Third-party laboratory analysis (2019). Humic acid content of SoilBoost EA, determined by the CDFA method.
Veneklaas, E. J., et al. (2017). Phosphorus dynamics in strongly weathered soils. In Phosphorus in Agriculture: 100% Zero (pp. 45–68). Springer.