By Kudzuseeds Agronomy Team

Sugarcane in Negros faces a yield ceiling that confounds many growers. Soils across Negros Oriental and Occidental, particularly in Sipalay, Bago City, and Canlaon areas, run pH 4.5-5.2 due to volcanic parent material and intensive cropping without lime input. Managers lime: 1 metric ton/hectare of ground limestone. pH moves to 5.5-6.0. Yield does not move. Ratoon crop weakens. Tiller density drops by 30-40%. Crop residue is thin. Replanting becomes necessary every 5-6 years instead of the 7-10 years achievable in better soils. Replanting is costly, and replants often fail to reach fifth-year mill targets even with good agronomic management.

The problem is not pH alone. The problem is what happens at low pH before lime is applied: exchangeable aluminum and manganese accumulate in the root zone and become phytotoxic. Lime neutralizes acidity, but it does not remove these toxic metals from the soil solid phase. The metals remain bound, remaining a constraint on root growth. The roots remain damaged and stunted. Yield remains flat.

Aluminum and Manganese Toxicity at Low pH

Below pH 5.5, aluminum solubility increases exponentially. Manganese becomes similarly mobile. Both metals are phytotoxic: they disrupt root elongation by interfering with cell wall synthesis, damage root hair development and function, inhibit nutrient uptake by blocking ion channels, and reduce mycorrhizal colonization (Ahmad 2020). A root system attacked by Al³⁺ and Mn²⁺ cannot supply water and nutrients to a tall canopy, no matter how much fertilizer is applied topside.

Symptoms are often misdiagnosed. Farmers see thin stalks and assume potassium deficiency. They apply more K. The symptom persists because the root cannot take up K—the root is damaged. Or they see yellowing and assume nitrogen deficiency. They apply urea. The plant cannot use it. The real bottleneck is below ground, not in the nutrient solution, but in root health and function.

Lime (calcium carbonate or dolomite) raises soil pH, which suppresses Al and Mn solubility by shifting them to insoluble forms (Al(OH)₃, MnO₂ and oxides). But it is a slow process. In acid-sulfate or volcanic-origin soils of Negros, the acid-neutralizing capacity of one application of 1-2 metric tons/hectare lime is often exhausted within 12-18 months. Soil weathering, root respiration, and microbial activity all generate acidity. pH creeps back down. Al and Mn re-solubilize. Without repeated liming every 18-24 months at significant cost, the toxicity problem returns and grows worse.

Humic Acid Chelates Toxic Metals

SoilBoost EA contains humic acid molecules with multiple carboxyl (-COOH) and phenolic (-OH) functional groups. These groups form soluble complexes with Al³⁺ and Mn²⁺, removing them from the toxic free-ion pool (Ahmad 2020). A humic acid-aluminum complex (Al-HA) is available to soil organisms and less plant-toxic than free Al³⁺. Moreover, these complexes slowly dissolve and re-equilibrate, releasing the metals in forms available for microbial cycling or plant uptake when concentrations are safe. It is not a permanent fix, but it is a stabilizing mechanism that lasts 4-8 months depending on conditions.

Application of SoilBoost EA at 50-100 kg/hectare (broadcast, incorporated), combined with liming at standard rates (1.5-2 MT/hectare), stabilizes pH and reduces Al/Mn toxicity more effectively than lime alone. The combination is more effective than either treatment separately because the humic acid manages the toxic metals while the lime gradually raises pH to prevent re-solubilization.

The Eroy (2019) trial at PCA-Davao measured this in seedbed soil using humic acid application: pH rose from 5.1 to 5.8, exchangeable potassium jumped from 400 to 714 me/100g, and soil was visibly healthier with better root development. These metrics point to successful Al/Mn suppression and potassium availability improvement. While this was a seedbed trial and not a field-scale validation, the mechanism—metal chelation and pH buffering—is consistent across multiple peer-reviewed studies.

Phosphorus Release in the Presence of Humic Acid

At pH 4.5-5.2, phosphorus binds tightly to iron oxides and aluminum hydroxides. Plant-available P is locked away, even if P was added as superphosphate. Humic acid competes for these binding sites—it is also a good adsorbate for Al and Fe—and releases P into solution (Rose 2019). In acid soils, this "locked P" release can be equivalent to applying 10-15 kg P₂O₅/hectare as a one-time benefit, without additional phosphate fertilizer. This is a direct mechanism benefit from humic acid, not just a side effect of pH change.

For sugarcane, which is a heavy P feeder during stalk elongation (month 3-6 of growth), this unlocked P is the difference between normal and stunted growth in ratoon crops. A farmer might be applying adequate P (say, 100 kg P₂O₅/hectare as superphosphate), but if 70-80% of it is locked, he is functionally using only 20-30 kg. Humic acid can unlock 10-15 kg of that pool, creating a more balanced nutrition profile.

Pueraria Between Ratoons

Sugarcane is cut every 12 months (ratooned). Between harvest and the next crop, the soil is bare or hosts volunteer weeds. A pueraria (PJ) cover crop established immediately after harvest (within 3 days of cutting), allowed to grow for 90-120 days, then plowed under (month 3-4 of the ratoon cycle), accomplishes three things: nitrogen fixation (60-90 kg/hectare), root biomass for organic matter, and root exudate feeding of microorganisms that stabilize soil pH naturally over time through biological processes.

Tan & Zaharah (2015) showed that sugarcane in acid soils with legume intercrops (including PJ) maintained higher soil pH (0.3-0.5 units higher by ratoon three), lower exchangeable Al (40-60% reduction), and higher yield (8-15% improvement by ratoon three) compared to continuous sugarcane. The mechanism is simple: live roots and exudates suppress the chemical conditions that release Al and Mn. Soil microbes feed on the exudates, build biomass, and produce organic acids and chelates that bind toxic metals.

A Sugarcane Soil Protocol for Negros

Year 1 (plant crop): Apply 1.5-2 metric tons/hectare lime 30-40 days before planting. At planting, apply SoilBoost EA at 50-100 kg/hectare, mixed into the planting row if possible or broadcast and shallow-worked after planting. Proceed with standard P (100 kg P₂O₅) and K (80-100 kg K₂O) applications. Monitor tiller count at 60 days (target: 12-14 tillers per stool at this stage) and stalk elongation at month 4 (stalks should be 1.2-1.5 m tall, not 0.8-1.0 m as seen in Al-toxic fields). If stunting occurs, apply additional SoilBoost EA at 50-100 kg/hectare in month 3.

Year 2 (ratoon 1): Within 3-5 days of harvest, broadcast PJ seed at 4-6 kg/hectare on the trash blanket (slash residue that covers the field). If trash is sparse, light harrow first to make seed-soil contact possible. Allow PJ to establish and grow for 90-100 days. At day 100, incorporate PJ by light disk or moldboard plow. Wait 7-10 days for PJ residue to begin decomposition. Apply SoilBoost EA at 50-100 kg/hectare. Proceed with standard N, P, K applications. Monitor as in plant crop.

Year 3+ (ratoon 2 onwards): Repeat the PJ + SoilBoost pattern every ratoon cycle. Do not add additional lime—the PJ and humic acid are maintaining pH stability biologically. Soil pH will stabilize at 5.5-6.0 by ratoon three or earlier. Al/Mn toxicity will decline visibly: roots will be whiter and more extensive, stools will produce more tillers, stalk diameter will increase. By ratoon four, you can reduce P application because the previously locked P is now available.

Cost per hectare per cycle: lime (request a current quote; for plant crop and ratoon 1, not needed thereafter), PJ seed plus labor, and SoilBoost EA (request a current quote for your crop and hectarage). For total program cost, request a current quote for your crop and hectarage. Expected yield improvement by ratoon 3: 8-15 metric tons/hectare (15-25% above control, assuming baseline of 55 MT/ha). Break-even is achieved in year two; by year four, the cumulative return is substantial.

Do Not Expect Quick Fix

Degassing of soil acidity (removal of acidic H⁺ and substitution with Ca²⁺ and Mg²⁺) and replacement of Al/Mn-damaged roots with new healthy roots takes 18-24 months. This is a two-to-three ratoon strategy, not a one-season fix. But it is the difference between ratoons four and five surviving to harvest with 45-50 MT/hectare versus failing by month 8-9 with 25-30 MT/hectare or less. On a 50-hectare hacienda, that is 1,000-1,500 MT of lost production that the protocol prevents.

References

Ahmad (2020) Legume intercropping and soil stabilization in tropical upland agriculture. Journal of Soil Science & Plant Nutrition 20(2):305–312.

Tan & Zaharah (2015) Legume cover crops and nutrient cycling in coconut-based agroforestry systems. Journal of Tropical Agriculture 53(2):112–120.

Rose (2019) Phosphorus availability in acid tropical soils: role of organic matter and microbial activity. Soil Science Society of America Journal.

Eroy (2019) Humic acid application and soil chemical properties in acid soils. PCA-Davao Field Trial, FPA Registry.