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I will assume that this is meant to be measured in cm, since a bed that size would be massive! With this being said- To get the capacity of each bed, you'll need to find out how many plants that you can fit going down the length and width. At 400cm, and at a spacing of 75 cm per plant (Both cultivars have this requirement), that would be 5 plants per row, since you'd have to round down a little.

Going down a length of 300cm, and a gap of 75cm, you can fit 4 potatoes. This will be 20 plants per bed, multiplying the rows going down and rows going across.

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