In recent years, fertilizer usage in India has seen a significant rise. However, this has not translated into proportional increases in crop yields. One major reason is the growing deficiency of secondary and micronutrients, which contributes to the gradual decline in soil fertility. As India aims to produce 300 million tonnes of food grains by 2025, improving crop productivity becomes essential, especially given the limited scope for expanding cultivated land.
Low fertilizer use efficiency and declining factor productivity have become serious concerns. This highlights the urgent need to adopt best management practices (BMPs) for fertilizers, focusing not just on primary nutrients but also on secondary nutrients like Calcium (Ca), Magnesium (Mg), and Sulphur (S).
1. Role of Secondary Nutrients in Yield and Nutrient Efficiency
Secondary nutrients, although required in smaller quantities than primary nutrients (N, P, K), are equally vital for plant growth and yield optimization.
a. Calcium (Ca)
Uptake Form: Ca²⁺ ions
Functions:
Facilitates nutrient absorption and translocation within plants
Activates several enzyme systems related to growth
Essential for cell wall formation and cell division
Deficiency in properly limed soils is rare, but acidic soils can restrict growth due to aluminum and manganese toxicity, not calcium shortage.
Symptoms of Calcium Deficiency:
Death of terminal buds and root tips
Premature shedding of flowers and buds
Weak stems
Blossom-end rot in tomatoes, peppers, melons; bitter pit in apples
b. Magnesium (Mg)
Uptake Form: Mg²⁺ ions
Functions:
Core element of chlorophyll, essential for photosynthesis
Activates numerous enzymes in growth processes
Mobile nutrient: translocated from old to young tissues during deficiency
Symptoms of Magnesium Deficiency:
Interveinal chlorosis (yellowing between leaf veins)
Leaf curling upward at margins
Marginal yellowing near the midrib
Recommendation: Apply soluble Mg fertilizers if soil test shows Mg < 50 ppm. Avoid dolomite in high pH soils.
c. Sulphur (S)
Uptake Form: Sulphate (SO₄²⁻)
Functions:
Component of amino acids (cysteine and methionine) and proteins
Supports photosynthesis and winter hardiness
Crucial for nitrogen fixation in legumes
Participates in nitrate reduction
Symptoms of Sulphur Deficiency:
Slow growth and delayed maturity
Yellowing of young leaves (S is immobile)
Stunted, spindly plants
Note: S-deficiency resembles N-deficiency, but shows up first in younger leaves.
Common Secondary Nutrient Fertilizer Sources
Calcium Fertilizers
| Source | Ca Content (%) |
|---|---|
| Calcium Nitrate | 19.4 |
| Calcium Chloride | 15.0 |
| Gypsum | 23.3 |
| Basic Slag | 29.0 |
| SSP (Single Super Phosphate) | 20.0 |
| Bone Meal | 23.1 |
| CAN (Calcium Ammonium Nitrate) | 8.0 |
| Ca EDTA | 3–5 |
Magnesium Fertilizers
| Source | Mg Content (%) |
|---|---|
| Magnesium Sulphate | 9–20 |
| Magnesium Nitrate | 16 |
| Potassium Magnesium Sulphate | 11 |
| Dolomite (MgCO₃) | 20 |
| CAN | 4.5 |
| Magnesium Chloride | 7.5 |
Sulphur Fertilizers
| Source | S Content (%) |
|---|---|
| Elemental Sulphur | 90 |
| Ammonium Sulphate | 23 |
| SSP | 11 |
| Gypsum | 18.6 |
| Pyrite | 22 |
| Zinc Sulphate | 10 |
| Ferrous Sulphate | 10.5 |
| Manganese Sulphate | 17 |
| Copper Sulphate | 12 |
| Magnesium Sulphate | 12 |
Fertilizer Recommendations
Calcium
Soil application: Gypsum (CaSO₄·2H₂O), Calcium nitrate, or Calcium chloride
Low pH soils: Use lime or calcium carbonate
Foliar spray: 1% calcium nitrate for general use
Specific Crops:
Grapes: 0.2% Ca-EDTA + 0.1% boric acid
Tomato: Soil: 1–2 kg/ac calcium sulphate; Foliar: 0.5% calcium chloride
Rose, Carnation, Anthurium, Lilium: Calcium nitrate
Tapioca: 25 kg ZnSO₄ + 20 kg S (gypsum) + 10 kg Borax/ha
Delhi Crossandra: 100 kg/ha gypsum
Magnesium
Soil and foliar: Magnesium sulphate or magnesium chloride
Cotton: Soil: 20 kg/ha MgSO₄; Foliar: 1% MgSO₄
Sulphur
Foliar spray: 1% Potassium sulphate (K₂SO₄) – two sprays at fortnightly intervals
Soil application: Elemental sulphur or gypsum
Foliar sources: Ammonium sulphate, Magnesium sulphate, or Potassium sulphate
Conclusion
Achieving high crop productivity and profitability demands balanced and adequate nutrition, including secondary nutrients. Though these nutrients are often available in many soils, their deficiency is increasingly noticed in light soils and with sensitive crops. Timely application, guided by deficiency symptoms or soil tests, can prevent yield loss and improve fertilizer efficiency.
Dr. P. Malathi & Dr. K. M. Sellamuthu, Department of Soil Science & Agricultural Chemistry, Tamil Nadu Agricultural University, Coimbatore – 641 003