Nutrient status and fertility constraints of degraded soils in Luzon, Philippines (Calubaquib, Navarrete, Sanchez, 2016)

Introduction

• Prerequisite to soil management: knowledge of degraded soils’ properties and fertility status to plan appropriate soil management for crop production.

• Aim: determine physico-chemical and mineralogical properties and fertility constraints of degraded soils in Luzon, Philippines.

• Study scope: 10 surface soil samples representing dominant soil series on Luzon; analyses cover physical, chemical, and mineralogical properties.

Materials and Methods

• Soils studied (selected degraded soils with parent materials): Alaminos (basalt, old alluvium), Annam (basalt/andesite/shale), Antipolo (basalt), Bantay (old alluvium, shale, sandstone), Bolinao (limestone), Cervantes (old alluvium, shale), Mirador (limestone-derived), Luisiana (andesitic/basaltic), Tacdian (limestone), Tigaon (volcanic ash/pyroclastic).

• Sampling: $10$ representative points per soil, randomly scattered ~$25 ext{ m}$ apart; surface soil (0-25 cm) collected and composited; air-dried, free of gravel/roots, ground to pass $2 ext{ mm}$ mesh.

• Analyses:

  • Particle size: clay (<$2 ext{ µm}$), silt ($2-200 ext{ µm}$), sand ($200-2000 ext{ µm}$) by hydrometer method.

  • pH: measured in 1:1 soil suspensions with $1 ext{ M KCl}$ and in water.

  • Organic matter (OM): Walkley-Black; total N (Kjeldahl); available P (Bray 2); exchangeable cations (Ca, Mg, K, Na) by $1 ext{ M NH}<em>{4} ext{OAc (pH 7.0)}$; CEC{pot} by steam distillation; CEC{ef} as sum of exchangeables + exchangeable acidity; base saturation (%BSpot) = $rac{ ext{Ca}+ ext{Mg}+ ext{K}+ ext{Na}}{CEC</em>{pot}} imes 100$; exchangeable Al^{3+} and H^{+} by displacement with $1 ext{ M KCl}.$

  • Available Fe by DTPA extraction; Fe in solution by AAS.

  • Mineralogy: XRD of powdered samples; dominant minerals identified.

• Fertility evaluation: match soil properties with published crop thresholds; signs (+) indicate favorable, (-) indicate constraints. Thresholds from Tisdale et al. 1985; Landon 1991; Schlichting et al. 1995.

• Statistics: Principal Component Analysis (PCA) with SPSS v16; first two PCs account for part of total variance; components with eigenvalues > 0.5 highlighted.

• Key PCA notes: PC1 loadings linked to exchangeable acidity and CEC_{ef} vs pH factors; PC2 linked to OM, exchangeable bases, available Fe, and mineralogy differences.

Results

• Physical characteristics:

  • All soils, except Bantay, are clayey; Bantay is sandy loam.

  • Clay content highest in Bolinao; lowest in Bantay.

• Chemical characteristics:

  • All soils acidic (low pH); ΔpH (pHKCl vs pHH2O) negative.

  • OM and total N generally low; OM/ N positively correlated (r ≈ 0.808, p < 0.001).

  • Available P very low (≈ $2-7 ext{ mg kg}^{-1}$).

  • Exchangeable bases (Ca, Mg, K, Na) low; exchangeable Ca generally adequate; Mg sometimes low (Bantay, Luisiana); K frequently limiting in Annam, Bolinao, Bantay, Cervantes.

  • Exchangeable Al^{3+} and H^{+} form most of exchangeable acidity; Tacdian shows highest exchangeable acidity.

  • CEC{pot} typically 3–5× higher than CEC{ef} in several soils (e.g., Alaminos, Bolinao, Mirador, Tigaon).

• Mineralogy (XRD):

  • Dominance of halloysite/kaolinite, quartz, and hematite across soils.

  • Halloysite/kaolinite more evident in some soils (e.g., Alaminos, Antipolo, Bantay, Cervantes); hematite more evident in Bolinao, Luisiana, Mirador, Tacdian, Tigaon.

  • Quartz present in all soils; some soils show higher quartz relative to others, reflecting parent material and advanced weathering stage.

• Fertility constraints (summary):

  • All soils: physical and chemical fertility constraints for crop production.

  • Primary chemical constraints: acidity, low OM, low total N, very low available P, low exchangeable bases, and high exchangeable Al.

  • Some soils have K and Mg limitations (notably Annam, Bolinao, Bantay, Cervantes).

  • PCA effectively differentiated soils by mineralogy and fertility constraints; soil pH exerts strong control over fertility and Al/Fe activity.

• Fertility evaluation results (positive/negative; thresholds):

  • pH and ΔpH: generally unfavorable (acidic) for most crops.

  • OM > 3% and Nt > 0.2% mostly not met; Available P often below 8-15 mg kg^{-1}.

  • Ca generally adequate; Mg and K often limiting in several soils; Na generally low/adequate.

Discussion

• OM variation reflects past land use, vegetation, and management; strong OM–N correlation due to most N being bound to OM.

• High exchangeable acidity dominated by Al^{3+} indicates strong acidity and weathering; acidity constrains crop production and lime requirement.

• Mineralogy indicates advanced weathering with halloysite/kaolinite and hematite; quartz abundance likely from parent material.

• Soil properties and variance show limited differentiation by parent material; soils at advanced weathering stages show similar mineralogical trends.

• PCA demonstrates that pH-related factors and mineralogy are key in differentiating soils and in predicting fertility constraints.

• The study suggests that standard N-P-K focus in Philippine soil fertility research is insufficient for degraded soils; a broader set of properties are needed for effective management.

Conclusion

• Physical properties: except Bantay, soils are highly clayey; Bantay is sandy with wind erosion risk.

• All soils are acidic with low exchangeable bases, low available P, and high exchangeable Al; these are major constraints on crop production.

• Lime application to raise pH to within $5.5 ext{ to } 6.0$ (i.e., $5.5 \le pH \le 6.0$) is recommended; lime can improve P availability and reduce Al/Fe toxicity.

• Addition of organic matter is advised to improve soil fertility and structure; use highly acid-tolerant crops where liming is impractical.

• Long-term soil management should integrate knowledge of both physical and chemical constraints to sustainably utilize degraded soils.

Quick reference thresholds (summary)

• pH (H2O): $ext{target} <br>ightarrow 5.5 ext{ to } 6.0$

• OM: >3 ext{ %} for productive soils (typical threshold varies by crop)

• Available P: $8-15 ext{ mg kg}^{-1}$ (threshold for crop needs)

• CEC{pot} vs CEC{ef}: $CEC<em>{pot} ext{ is typically } 3-5 imes CEC</em>{ef}$

• Major acidity: dominated by $Al^{3+}$ and $H^{+}$ in exchange sites

• Mineralogy: halloysite/kaolinite + quartz + hematite common across soils