Calibrating a custom sprayer before a tomato job
Anita Devi runs a small custom-spraying business with a 16-litre battery-operated knapsack sprayer fitted with a single flat-fan nozzle. Today she is booked for 0.4 ha of tomato (Pusa Ruby) at early fruiting, where you have advised mancozeb 75 WP at 2.0 kg/ha plus chlorantraniliprole 18.5 SC at 150 ml/ha plus a sticker at 0.5 ml/litre. The label volume for this canopy is 400 L/ha. To check the machine you collect the nozzle output for two minutes and measure 2.3 litres; she walks at about 1.2 km/h with an effective swath of 0.9 m. She fills a 16-litre tank, adds both products and the sticker, and starts. She has no gloves or mask; the wind is light and drifting toward a neighbouring okra field. Rain is forecast in about two hours, and the block's next harvest is in five days. She asks whether the spray will "hold".
1. Calculate the spray volume she is actually applying, decide how to bring it to 400 L/ha, and work out the water and tank count for the job.
Read the worked answer
Step 1 - actual application rate.
- Nozzle output: 2.3 L in 2 minutes = 1.15 L/min.
- Area per minute = speed × swath = (1200 m/h ÷ 60) × 0.9 m = 18 m²/min.
- Application rate = (1.15 L/min ÷ 18 m²/min) × 10 000 m²/ha = 639 L/ha.
She is applying about 1.6 times the label volume. If she puts the per-hectare dose into each tank she will under-dose; if she calculates per-tank amounts for 400 L she will over-dose. Both errors are common and both drive resistance and residues.
Step 2 - reaching 400 L/ha. Three levers:
- Nozzle (best): fit a flat-fan nozzle delivering about 0.72 L/min at 2.5 bar - roughly two sizes smaller than the present one. Speed and pressure (and therefore droplet spectrum) stay unchanged.
- Speed: keep the nozzle and walk at about 1.9 km/h. Cheaper but hard to hold steady across a whole field.
- Pressure: drop pressure to reach 0.72 L/min, but this coarsens the droplets and worsens underside coverage; pressure tuning alone is the worst of the three.
Whichever lever is used, re-measure output and check the pattern on water-sensitive paper before spraying a client's crop.
Step 3 - water and tanks. 400 L/ha × 0.4 ha = 160 L, i.e. 10 tanks of 16 litres.
2. Calculate the product quantity per tank and set out the mixing sequence and compatibility checks.
Read the worked answer
Per tank (16 L covers 0.04 ha at 400 L/ha):
- Mancozeb 75 WP: 2.0 kg/ha × 0.04 ha = 80 g per tank (5 g/L).
- Chlorantraniliprole 18.5 SC: 150 ml/ha × 0.04 ha = 6 ml per tank (confirm the figure on the label in hand).
- Sticker: 0.5 ml/L × 16 L = 8 ml per tank.
Whole job (0.4 ha): 800 g mancozeb, 60 ml chlorantraniliprole, 80 ml sticker in 160 L water.
Mixing sequence.
- Half-fill the tank with clean water.
- Add the wettable powder first, pre-mixed as a slurry - mancozeb needs the longest to disperse, and a lumpy sprinkle blocks nozzles.
- Stir, then add the suspension concentrate (chlorantraniliprole); SCs disperse well in an already wetted tank.
- Add the sticker/adjuvant last.
- Top up to 16 L and keep the tank agitated. Never add products to a full tank and never pre-mix dry products together.
Jar test before the first tank. Use 500 ml of the same water at the same dilution, stir, and stand for 30 minutes. Flocculation, a cream layer, heat or sediment means do not mix - apply as two separate passes.
Water quality. Mancozeb is hydrolysed in strongly alkaline water, and calcium and magnesium in hard borewell water tie up surfactants. Check pH and hardness with the water she will actually use, and add a buffering or hard-water adjuvant if needed. The jar test must be done with that same water, not with tap water at home.
3. Given the rain forecast, the five-day harvest, the missing PPE and the drift toward okra, what do you tell her to do?
Read the worked answer
Rain in two hours - do not start. Mancozeb is a protectant with limited rainfastness; the deposit needs several rain-free hours to dry and set. Starting now wastes the product, the water and the labour. Wait for the rain to pass and the foliage to dry, then spray.
Harvest in five days - do not spray this block with mancozeb. Check the label PHI (commonly around 7 days for mancozeb products on tomato, but the label governs). Five days sits inside that window, which means residue at harvest, a rejected consignment and a wasted spray. Spray only blocks that are not being picked within the PHI; for the block picked in five days use sanitation and, if truly needed, a product whose registered PHI fits.
PPE and drift. No gloves and no mask is unacceptable for someone mixing a WP and an SC. Gloves, mask, full sleeves, soap and water at the site. Spray in the early morning or evening when wind is minimal; keep a 5–10 m buffer from the okra, use low-drift nozzles at the correct pressure, and warn the neighbour before starting. Never spray when the wind carries toward people, animals or a neighbouring crop.
Record keeping. Date, field, crop stage, products, dose per tank, number of tanks, area covered, weather and PHI - she needs these both for the farmer's residue compliance and to price and defend her service.
Bottom line: delay, skip the harvest block, correct the machine to 400 L/ha, and do not accept the job on the current terms.