Choosing a Fertiliser for a Nitrogen-Deficient Crop
A grower has two paddocks. Paddock A has yellowing older leaves and stunted growth, suggesting nitrogen deficiency. Paddock B has purple-tinged older leaves, suggesting phosphorus deficiency. The grower has 100 kg of ammonium sulfate ((NH4)2SO4, M = 132 g/mol) and 100 kg of superphosphate (treated here as Ca(H2PO4)2, M = 234 g/mol). A soil test shows paddock A also has a low pH of 5.0, and ammonium sulfate is known to acidify soil further. The grower can only afford to treat one paddock this season.
1. Calculate the mass of nitrogen in 100 kg of ammonium sulfate and the mass of phosphorus in 100 kg of superphosphate.
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Ammonium sulfate: N fraction = (2 x 14)/132 = 28/132 = 0.212. Mass N = 100 x 0.212 = 21.2 kg. Superphosphate: P fraction = (2 x 31)/234 = 62/234 = 0.265. Mass P = 100 x 0.265 = 26.5 kg. So each 100 kg bag supplies roughly 21 kg N or 26 kg P respectively.
2. Recommend which paddock to treat and justify, considering both the deficiency symptoms and the soil pH information.
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Treat paddock B with superphosphate. Its purple older leaves match phosphorus deficiency, and superphosphate supplies phosphorus (about 26.5 kg per 100 kg). Paddock A does show nitrogen deficiency, and ammonium sulfate would supply nitrogen (21.2 kg per 100 kg), but the soil is already acidic at pH 5.0 and ammonium sulfate acidifies further, which could harm the crop and reduce nutrient availability. So the better judgement is to treat B now and address A's nitrogen and pH together later (e.g. with lime plus a less acidifying nitrogen source).
3. Explain why the deficiency symptoms appear on older leaves rather than new growth for both nutrients.
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Nitrogen and phosphorus are mobile nutrients. When supply is short, the plant redistributes them from older leaves to support new growth. So the older leaves show the deficiency (yellowing for nitrogen, purple tinges for phosphorus) while newer leaves stay greener longer. This contrasts with immobile nutrients, whose deficiency shows first in new growth.