Yellowing between the veins of my older spinach leaves with stunted growth

Yellowing between the veins of older lower leaves while veins stay green, with overall pale stunted plants, usually points to a nutrient deficiency. When it starts on older leaves it is often nitrogen or magnesium, since the plant moves these mobile nutrients to the new growth first. General paleness and slow growth lean toward nitrogen, while distinct interveinal yellowing on older leaves with green veins leans toward magnesium, common in sandy soils. A soil test confirms which. For nitrogen, top dress with a quick nitrogen source and the leafy crop will green up fast. For magnesium, a foliar spray of magnesium sulphate gives a quick response and a soil application corrects it longer term. Avoid over watering on sandy soil which leaches nutrients. Since spinach is harvested young and eaten fresh, prefer correcting through balanced nutrition and get a soil test at your KVK before heavy fertiliser use.