Fertigation stock tank: nutrient strength, calcium incompatibility and dose per hectare
A 1,000 L fertigation stock tank is being prepared for a tomato crop. The grower dissolves 40 kg urea (46-0-0), 25 kg monoammonium phosphate (12-61-0) and 30 kg potassium sulphate (0-0-50) in the tank, and the injector is set to deliver 1 L of stock into 200 L of irrigation water. The borewell used for irrigation contains 120 mg/L Ca²⁺ and 30 mg/L Mg²⁺, has a pH of 7.8 and a total alkalinity of 4.5 meq/L as CaCO3. Each irrigation event applies 20 mm of water, and the crop is irrigated twice a week. The farm manager now wants to add 20 kg of calcium nitrate (15.5% N, 19% Ca) to the same tank so that the fruit does not develop blossom-end rot, arguing that all fertilisers are soluble and will therefore mix without trouble.
1. Calculate the N, P2O5 and K2O concentrations delivered at the emitter, and comment on whether this is a sensible strength.
Read the worked answer
Work per litre of stock, then divide by the 1:200 dilution.
- Urea: 40 kg/1,000 L = 40 g/L; N = 40 × 0.46 = 18.4 g/L → 92 mg/L (ppm) after dilution.
- MAP: 25 g/L; N = 25 × 0.12 = 3.0 g/L → 15 ppm; P2O5 = 25 × 0.61 = 15.25 g/L → 76 ppm.
- SOP: 30 g/L; K2O = 30 × 0.50 = 15 g/L → 75 ppm.
Delivered solution: N ≈ 107 ppm, P2O5 ≈ 76 ppm, K2O ≈ 75 ppm. The whole tank (diluted to 200,000 L) carries 21.4 kg N, 15.2 kg P2O5 and 15 kg K2O.
Judgement: 107 ppm N is reasonable for a weekly fertigation but far too concentrated for continuous daily injection. The ratio 1 : 0.7 : 0.7 is a fruit-setting blend; it would be wrong for a young vegetative crop, which needs relatively more N and less K.
2. Will the calcium nitrate dissolve and stay in solution in this tank? Explain, and state what should be done instead.
Read the worked answer
No. The tank already holds about 15.25 kg P2O5 as dihydrogen phosphate and 30 kg K2SO4, i.e. roughly 16.5 kg sulphate in 1,000 L (about 16 g/L SO4²⁻). The proposed 20 kg of calcium nitrate adds 3.8 kg Ca. Calcium meets sulphate and phosphate immediately: CaSO4·2H2O solubility is only about 2 g/L, and calcium phosphates are less soluble still, so a white precipitate forms in which both calcium and phosphate are lost, and the sludge blocks filters and drippers. Urea hydrolysis also raises tank pH, pushing still more calcium phosphate out of solution.
Remedy: split into two tanks. Tank A - MAP, SOP, urea. Tank B - calcium nitrate alone (or with potassium nitrate or ammonium nitrate). Inject B after A, flush the line between, and never allow the two concentrates to meet. If a single tank is unavoidable, supply calcium separately (gypsum in soil, or a chelated foliar calcium targeted at the fruit).
3. Using the dilution and irrigation depth given, work out how much N one full tank delivers per hectare and judge whether the tank should be split.
Read the worked answer
Diluted volume = 1,000 L × 200 = 200,000 L. Since 1 mm over 1 ha = 10,000 L, that is exactly 20 mm - one full tank matches one irrigation event.
Nitrogen delivered per event = 18.4 kg (urea) + 3.0 kg (MAP) = 21.4 kg N/ha, with 15.25 kg P2O5 and 15 kg K2O.
Judgement: a heavy-fruiting tomato crop takes up roughly 2–4 kg N/ha/day. At two events per week this schedule supplies about 43 kg N/ha/week against a demand of roughly 15–28 kg - close to double. Options, in order of preference: cut urea to about 20–22 kg per tank; or halve the stock strength (1:400, giving ~53 ppm N) and spread the same tank over two events; or reduce the MAP, since 76 ppm P2O5 is generous for a fertigated crop. The bag weights should be set by this arithmetic, not by habit.