Spinach has a high demand for nutrients. Apply the following rates of magnesium and trace elements (per hectare) every 18 months.
50kg magnesium sulphate
20kg manganese sulphate
18kg ferrous sulphate
18kg copper sulphate
18kg zinc sulphate
2kg sodium molybdate
Spinach has one the highest requirements for phosphorus of all vegetable crops. Your program needs to take account of this but also recognise that the crop is often harvested as little as 30 days after planting. Phosphorus present during this short time needs to be immediately available to the crop, but not so excessive that large quantities are left in the soil after the crop is removed, potentially to be leached away and wasted.
If soil test P levels are less than 40mg/kg Colwell P, broadcast a high phosphorus product such as triple superphosphate at 500kg/ha before planting and incorporate to 15cm. Immediately after sowing, broadcast 100kg/ha MAP fertiliser.
Spinach also has a high demand for nitrogen and potassium. Starting three days after sowing, spray urea at 20kg/ha plus potassium nitrate at 20kg/ha in 1000L/ha of water. Repeat this every 3-4 days until the crop is ready for harvest. To avoid foliar damage, these applications should be washed from the leaves as soon as possible with no more than 3mm of irrigation.
Salad processors, the main buyers for baby spinach, have almost no tolerance for damaged leaves.
If these rates prove to be insufficient, fertigation with urea at 50kg/ha or calcium nitrate at 150kg/ha may be required after 21 days. The downside of doing this is there is often overspray on adjacent younger plantings, leading to fertiliser waste and leaching.
On alkaline soils, a foliar spray of manganese sulphate at 5g/L may be required in the first 14 days after emergence if the leaves show a mottled yellowing and don’t go green after nitrogen is applied.