Calibrating a Knapsack for Cabbage
Marcia has a 16 L knapsack sprayer with one flat-fan nozzle. She calibrates on a 50 m course: average swath 0.60 m, walking time 40 s, and nozzle output collected over 60 s is 1.20 L. She will spray mancozeb 80% WP on cabbage at the current label rate of 2.0 kg product/ha in 300 L water/ha. The tank has no mechanical agitation beyond shaking, and she wants to add a non-ionic surfactant at 0.05% v/v. Wind is 8 km/h, temperature is 31°C, and rain is forecast in 4 hours. The field is 0.35 ha. Her older label says 2.5 kg/ha; the current label says 2.0 kg/ha. She has enough product for 1.0 kg and a 100 mL bottle of surfactant.
1. Calculate the actual spray volume, the area covered per full tank, the product and surfactant needed per full tank, and the number of tanks for the 0.35 ha field. Show your reasoning.
Read the worked answer
Step 1 - speed and area per minute Speed = 50 m ÷ 40 s = 1.25 m/s. Area per minute = swath × speed × 60 = 0.60 m × 1.25 m/s × 60 s = 45 m²/min.
Step 2 - actual application volume Nozzle output = 1.20 L/min. Volume per hectare = (1.20 L/min ÷ 45 m²/min) × 10,000 m²/ha = 266.7 L/ha, about 267 L/ha. This is below the label's 300 L/ha. If the label treats 300 L/ha as a requirement rather than a guide, she must recalibrate; if it is a guide and coverage is good, 267 L/ha is acceptable provided the product per hectare is correct.
Step 3 - area per full tank 16 L ÷ 266.7 L/ha = 0.060 ha per full tank, or 600 m².
Step 4 - product per full tank Use the current label, not the older 2.5 kg/ha label. 2.0 kg/ha × 0.060 ha = 0.120 kg = 120 g mancozeb per 16 L tank. Total product for 0.35 ha = 2.0 kg/ha × 0.35 ha = 0.70 kg, which is within her 1.0 kg stock.
Step 5 - number of tanks 0.35 ha ÷ 0.060 ha/tank = 5.83, so she needs six loads, with the last load only partly filled. She should mark the 0.060 ha area and avoid mixing a full sixth tank unless she can use it.
Step 6 - surfactant 0.05% v/v = 0.0005 × 16 L = 0.008 L = 8 mL per full 16 L tank. For a partial tank, add 0.5 mL per litre actually mixed. She should not exceed the label's surfactant instruction.
2. Should Marcia spray now? Consider rain, wind, heat, compatibility and agitation, and state how she should adjust the job.
Read the worked answer
Rain: Heavy rain is forecast in 4 hours. Mancozeb is a protectant fungicide, and wash-off before the spray dries will reduce control and increase runoff risk. She should check the label's rainfastness statement. If rain is likely before the deposit dries, she should delay until a drier window rather than spray and hope.
Wind: 8 km/h is generally manageable, but she should spray early in the day, keep the boom or nozzle low, avoid gusts, and maintain a buffer beside ditches, houses and sensitive crops. Wind can increase at midday.
Heat: 31°C raises heat-stress risk for a worker wearing a respirator, coveralls, boots and gloves. She should spray in the cooler early morning, take breaks, drink water, and not work alone in full PPE for long periods.
Compatibility and agitation: Mancozeb WP needs good agitation. A knapsack with only shaking is a weakness: the powder will settle, and the last tankful can be too concentrated. She should pre-mix the product in a small amount of water, add it to a partly filled tank, shake frequently, and use a jar test before adding the non-ionic surfactant. The surfactant is compatible with many WP products, but the label must allow it; if it causes thickening, flocculation or heat, she should not use it.
Decision: Do not spray just before rain. Recalibrate or confirm the 267 L/ha volume, mix 120 g per full tank, use 8 mL surfactant only after a jar test, and spray in a cooler, rain-free window with frequent agitation. She must also use the current 2.0 kg/ha label and record the older label as superseded.