Mineral and Organic Fertilizers

Sulphur Fertilizers

  • Several fertilizers contain sulphur; some act as both nitrogen and sulphate fertilizers.
  • Examples:
    • Ammonium sulfate: Contains more sulphate than ammonium. It is primarily a nitrogen fertilizer but contributes sulphur.
    • Gypsum (Calcium Sulphate): A natural product mined from the ground. Improves soil structure in heavy soils.
    • Sulphate of Potash: Also a potassium fertilizer.
    • Superphosphate: Contains a substantial proportion of sulphur due to sulfuric acid used in its production.
    • Monoammonium Phosphate (MAP) and Diammonium Phosphate (DAP): Contain sulfur in small proportions.
    • Triple Superphosphate: Has reduced sulphur content because phosphoric acid is used instead of sulfuric acid in its production.

Calcium

  • Calcium is typically used as a soil amendment rather than a fertilizer for nutrient supply.
  • Agricultural Lime: A natural product mined from the ground, varying in calcium content based on purity.
    • Used for liming acidic soils.
  • Dolomitic Lime: Contains magnesium in addition to calcium, lowering the calcium content.
  • Gypsum (Calcium Sulfate): Used as a soil ameliorant to improve soil structure, especially in heavy soils.
  • Calcium Nitrate: Used in Europe as an excellent source of nitrate nitrogen.
  • Calcium Ammonium Nitrate: Popular in Europe, contains a significant proportion of calcium.
  • Calcium Amendments: Added to improve soil chemistry and pH, not primarily to supply calcium as a nutrient; most soils already contain sufficient calcium.

Magnesium

  • Magnesium is usually sufficient in most soils; amendments are for soil improvement, not nutrient supply.
  • Dolomitic Lime: Contains a higher proportion of magnesium compared to agricultural lime.
  • Epsom Salt (Magnesium Sulphate): Occasionally used as a magnesium fertilizer in Europe.

Micronutrient Fertilizers

  • Micronutrients are required in small amounts; most soils contain sufficient quantities for many crops.
  • Availability issues may necessitate supplementation.
  • Application:
    • Micronutrient fertilizers are typically added to macronutrient fertilizers due to the difficulty of spreading small amounts accurately.
    • Examples include superphosphate blends with copper, zinc, and molybdenum (e.g., Super Copper Zinc Moly).
    • Vineyard and NPK Blue Special: Macronutrient fertilizers supplemented with variety of micronutrients.
    • The NPK Blue Special fertilizer, a rare instance of NPK fertilizer use in the region, approximates a complete fertilizer due to its comprehensive range of macronutrients and micronutrients

Organic Fertilizers

  • Organic fertilizers add organic sources of nutrients and organic matter to the soil.
  • Sources:
    • Animal Origin: Manure from different animals varies in nutrient content based on their feed quality.
    • Crop Residues: Excellent source of nutrients.
    • Compost: An excellent but often expensive source of nutrients, primarily used for cash crops due to cost considerations.
  • Nutrient Content in Manure: Varies significantly based on the animal type and feed, with human excreta being a rich source of nitrogen and phosphorus.

Comparison of Organic vs. Mineral Fertilizers

  • Studies comparing soil properties after application of organic vs. mineral fertilizers show mixed results (Mineral fertilizer assigned a value of 100 for reference).
  • pH: Generally, organic fertilizers are expected to increase pH, though results vary, with slight acidification or alkalinization observed.
  • Soil Carbon: Typically increases with organic fertilizer application, though exceptions exist due to variations in application rates and management.
  • Nitrogen: Organic nitrogen content often correlates with organic carbon.
  • Phosphorus: Shows a wide range of changes, reflecting different organic material types.
  • Potassium: Generally increases with organic fertilizer use.
  • Yield: Increases are not always guaranteed and depend on the crop and site.
  • Conclusion: Organic fertilizers are beneficial, especially in soils low in organic matter, but their effects vary based on several factors.

Crop Residues

  • Crop residues are an essential organic nutrient source, particularly in broadacre agriculture where animal manure is less accessible.
  • Decline in Soil Organic Matter: A continuous decline is observed globally due to reduced mixed enterprises and less manure production.
  • Nutrient Release: Crop residues provide a slow-release source of nutrients, requiring decomposition before nutrients are available.
  • Nutrient Content: Varies by crop and crop part; nutrient management during the crop phase influences residue nutrient concentrations; heavy fertilization increases nutrient concentrations; green manures provide high amounts of crop residues.
  • Economic Factors: Green manuring is recognized as beneficial but is limited by financial constraints due to income loss from foregoing a cropping cycle.

Factors Influencing Decomposition Rate

  • Nutrients: Microorganisms require nutrients to decompose organic matter effectively.
  • Water Availability: Adequate moisture is essential for microbial activity.
  • Physical and Chemical Nature of Residues: Different crop parts have varying nutrient content; finer chopping increases decomposition rates.
  • Soil Residue Contact: Limited in no-till systems; incorporation enhances decomposition.
  • Soil Types: Sandy soils decompose organic matter quickly; clay soils protect organic matter in small pores.
  • Temperature: Microorganisms prefer temperatures around 20-25°C.

Nutrient Content in Crop Residues

  • Significant differences exist in nutrient content between different crop residues.
  • Legumes vs. Cereals: Legumes like medic and Albizia have much higher nutrient content than cereals like wheat straw.
  • Nitrogen Content: Legumes can have tenfold higher nitrogen content compared to cereals.
  • Decomposition Rate: Microorganisms decompose legume residues faster due to their higher nutrient content.

Windrow Effects on Crop Growth

  • After canola swathing, stalks are piled to enhance ripening, resulting in uneven residue distribution and nutrient concentration affecting subsequent crop growth.
  • Observation: Uneven growth patterns in wheat following canola due to windrows affecting nutrient distribution.
  • Nutrient Redistribution: Piling of canola stalks redistributes nutrients, significantly impacting wheat yields.
  • Critical Nutrient: Potassium deficiency is often the most limiting factor in these scenarios.
  • Management Practices: Modern practices involve spreading chaff during harvest to avoid nutrient piling.

Nutrient Leaching from Crop Residues

  • Heavy Rainfall: A significant proportion of nutrients can be leached from crop residues by heavy rainfall without decomposition.
  • Potassium: Approximately 75% of potassium is readily leached out due to its presence in the cytoplasm without structural bonding.
  • Calcium: Up to 15% of calcium can be leached, surprisingly high.
  • Magnesium: Minimal leaching occurs, requiring structural breakdown for release.
  • Residue Size: Smaller residue sizes show only a minor and short-term increase in decomposition rates.
  • Full Decomposition: Eventually releases nearly 100% of nutrients, though magnesium and calcium may vary based on material type and size.

Compost as a Soil Amendment

  • Desirable organic matter source, but nutrient content is relatively low compared to mineral fertilizers.
  • Volume Requirements: Large volumes are needed to supply sufficient nutrients.
  • Cost: High transportation costs limit use in broadacre cropping; more feasible for cash crops.
  • On-Farm Composting: Minimizes transportation costs, commonly practiced in vineyards using grape marc.

Dynamics of Nutrient Mobilization and Immobilization

  • Fresh Organic Matter: Adding fresh organic matter affects inorganic nitrogen levels in soil.
  • Immobilization Phase: Microorganisms initially immobilize nitrogen from the surrounding soil during growth.
  • Mobilization Phase: As microorganisms decay, they release accumulated nutrients back into the soil.

Foliar Fertilizers

  • Commonly used in orchards and increasingly in broadacre agriculture.
  • Nutrient Uptake: Leaves can effectively absorb nutrients through stomata.
  • Urea: Adding urea can enhance nutrient uptake by leaf cells (make sure that burette is at very low rate, ideal zero, but that almost never happens).
  • Sulphates: High concentrations can burn leaves; recovery may be incomplete, leading to yield penalties.
  • Application Rates: Small amounts can be applied in one application, making it suitable for micronutrients.
  • Practical Considerations: Spray coverage and stomatal uptake efficiency must be considered.