Abstract Ultisols represent one of the most common soil orders utilized for oil palm cultivation in Johor and Peninsular Malaysia. A 2012 study by PHC Ng tracked the chemical evolution of these soils under the oil palm agro-ecosystem. The findings revealed a consistent downward trend in key fertility indicators: Soil pH, Organic Carbon (C), and Total Nitrogen (N). This article analyzes these changes and discusses the management strategies required to arrest soil degradation in mature plantations.

Introduction

Johor is a powerhouse of Malaysian palm oil production, with vast tracts of land situated on Ultisols. These soils (often including the Rengam and Jerangau series) are naturally weathered, acidic, and possessed of low fertility.

When a forest is converted to a plantation, the soil ecosystem undergoes a radical shock. The study in question highlights a critical trajectory: over time, the intensive cultivation of oil palm strips the soil of its chemical reserves faster than natural processes can replenish them. Understanding this decline is the first step toward sustainable soil management.

1. The Acidification Trend (Soil pH)

The study noted a decline in soil pH over time, although the drop was described as “slight.”

  • The Observation: Ultisols are already acidic (typically pH 4.0 – 4.5). The cultivation of oil palm tends to push this lower, often towards pH 3.8 or 3.5.
  • The Cause:
    1. Nitrogen Fertilizers: The extensive use of ammoniacal fertilizers (like Ammonium Sulphate or Urea) releases hydrogen ions ($H^+$) into the soil during nitrification, driving acidity up.
    2. Base Cation Removal: Oil palms are heavy feeders. They extract large amounts of Potassium (K), Magnesium (Mg), and Calcium (Ca) for fruit bunch production. These “basic cations” act as a buffer against acidity. When they are removed in the harvest and not fully replaced, the soil loses its ability to neutralize acid.
  • Why “Slight”? The decline might be slight only because Ultisols are highly buffered by Aluminum. Once the pH drops below 4.2, aluminum becomes soluble, which resists further rapid pH changes but becomes toxic to plant roots.

2. The Loss of Organic Carbon (Total C)

Organic Carbon is the “battery” of the soil—it holds water, feeds microbes, and retains nutrients. The study found a decrease in Organic C with time.

  • The Mechanism: In a natural forest, leaf litter is diverse and continuous. In an oil palm plantation, the canopy is more open (especially in early years), increasing soil temperature and accelerating the microbial decomposition (mineralization) of existing organic matter.
  • The Implication: A drop in Organic C leads to soil compaction. Ultisols, which contain clay, rely on organic matter to keep the soil “fluffy” (porous). As Carbon levels drop, the soil becomes harder, reducing root penetration and water infiltration.

3. The Nitrogen Drawdown (Total N)

Total Nitrogen levels were observed to fall alongside Carbon. This is expected, as Nitrogen in the soil is largely tied up in organic matter (the C:N ratio).

  • The Cycle: As organic matter decomposes and Carbon is lost as CO2, the Nitrogen is mineralized into ammonium or nitrate. While this makes it available for the palm to eat, it also makes it vulnerable to leaching. In Johor’s high-rainfall climate, mineralized Nitrogen that isn’t immediately grabbed by roots is often washed away into the groundwater.

Management Strategies: Reversing the Trend

The findings by PHC Ng serve as a warning: standard NPK fertilization is not enough to maintain soil health on Ultisols. To counter these declines, agronomic practices must pivot toward replenishment.

  1. Liming is Mandatory: To combat the pH drop, the application of Ground Magnesium Limestone (GML) or Kieserite is essential. This not only supplies Magnesium but raises the pH, precipitating toxic aluminum and making other nutrients more available.
  2. Importing Organic Matter: The decline in Organic C cannot be fixed with chemical bags. It requires volume.
    • EFB Application: Empty Fruit Bunches must be returned to the field, specifically on the inter-rows.
    • Frond Stacking: Pruned fronds should be stacked broadly to cover more surface area, protecting the soil from the sun and slowly rebuilding the carbon stock.
  3. Leguminous Cover Crops (LCC): In replanting scenarios, establishing a thick carpet of Mucuna or Pueraria is critical to inject Nitrogen and Carbon back into the soil before the canopy closes.

Conclusion

The “slight” decline in pH and the loss of C and N reported in Johor’s Ultisols are indicators of a system under stress. While oil palm is highly productive, it is an extractive crop. Sustainable farming on weathered tropical soils requires us to treat the soil not just as a physical anchor for the tree, but as a chemical bank account that must be constantly topped up with organic capital to remain solvent.

Key Takeaways

The Fix: Chemical fertilizers must be paired with organic inputs (EFB/Compost) and liming to sustain long-term fertility.

Soil Type: Ultisols (common in Johor) are prone to acidification and nutrient stripping.

Key Losses: Organic Carbon and Nitrogen decrease over time without intervention.

The Ultisol Prescription: Targeted Fertilization for Mature Oil Palms

Abstract

Following the findings that Ultisols under oil palm cultivation suffer from time-dependent acidification and nutrient depletion (Ng, 2012), this article outlines the specific corrective fertilizer regimes required for mature palms (>8 years). It contrasts standard maintenance dosages with “remedial” dosages necessary to arrest the decline in soil pH and organic carbon.

The “Steady State” Fallacy

Many planters assume that once a palm reaches maturity, the fertilizer regime enters a “steady state.” The research proves this is dangerous. On Ultisols, the soil’s buffering capacity weakens over time. Therefore, the fertilizer program for mature palms (Year 9+) must be higher and more strategic than in the early mature phase (Year 4-8).

1. Correcting Acidity: The Foundation

As noted, soil pH in Johor’s Ultisols often drifts below 4.0. At this level, root uptake of Potassium (K) and Magnesium (Mg) is blocked by Aluminum toxicity. Throwing more fertilizer at acidic soil is a waste of money; you must unlock the soil first.

  • The Prescription:
    • Ground Magnesium Limestone (GML): Apply 2.0 – 3.0 kg/palm/year.
    • Placement: Broadcast widely in the inter-row, not just the weeded circle. This encourages roots to explore the inter-row where the soil structure is better preserved.
    • Timing: Must be applied at least 4 weeks before any Urea or Ammonium fertilizer to prevent volatilization losses (turning nitrogen into ammonia gas).

2. Nitrogen (N): The Balancing Act

The study highlighted a decline in Total N. On Ultisols, leaching is the enemy.

  • The Prescription:
    • Dosage: 3.0 – 4.0 kg/palm/year of Ammonium Sulphate (AS) OR 1.5 – 2.0 kg/palm/year of Urea.
    • Selection Strategy: If your soil pH is critically low (<3.8), switch to Urea. Although Urea is volatile, Ammonium Sulphate is significantly more acidifying (3x more acid-forming than Urea). Using AS on already acidic Rengam series soil accelerates the pH drop.
    • Frequency: Split into 4 rounds per year to minimize leaching during Johor’s monsoon seasons (Nov-Jan).

3. Potassium (K): The Yield Driver

Potassium is the nutrient removed in the largest quantities by the Fresh Fruit Bunch (FFB). Ultisols have low clay activity (low CEC) and struggle to hold K.

  • The Prescription:
    • Dosage: 2.5 – 3.5 kg/palm/year of Muriate of Potash (MOP).
    • The “Ash” Alternative: If available, apply bunch ash (from the mill) at 3.0 kg/palm. Bunch ash is alkaline (pH 12) and rich in K (30%), making it the perfect dual-purpose fertilizer for acidic Ultisols—it feeds K while neutralizing acidity.

4. Phosphate (P): The Root Builder

Ultisols are “P-fixing” soils—iron and aluminum in the soil lock up phosphate, making it unavailable to the plant.

  • The Prescription:
    • Dosage: 1.0 – 1.5 kg/palm/year of Christmas Island Rock Phosphate (CIRP).
    • Why Rock Phosphate? Unlike water-soluble superphosphates (TSP), Rock Phosphate releases P slowly and relies on the soil’s natural acidity to dissolve. It also adds a small amount of Calcium, which helps with the pH decline.

Summary: Annual Dosage Table (Mature Palms >8 Years)

NutrientSourceDosage (kg/palm/yr)Rounds/YearNote
NitrogenAmmonium Sulphate*3.5 – 4.04*Switch to Urea (1.75 kg) if pH <3.8
PotassiumMOP2.5 – 3.03apply on weeded circle
PhosphateRock Phosphate1.0 – 1.51apply on frond heap
MagnesiumKieserite1.0 – 1.21essential for “Orange Frond” prevention
BoronHGF Borate0.10 – 0.151critical for fruit set

Conclusion

The “slight” decline in soil health observed in research is a slow-moving crisis. For planters on Johor’s Rengam and Jerangau series, the key to longevity is not just replacing what the crop takes out, but actively fighting the soil’s natural tendency toward acidity and compaction. A targeted regime of Liming + Organic Matter + Split Fertilizer Applications is the only way to sustain yields of 25-30 MT/ha on these weathered soils.