crops- net primary productivity

'Many field crops have been bred to be responsive to high levels of fertiliser', but then how does this help increase productivity?

More fertilzer receptivity means increased nutrient uptake and hence greater production.

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These crops have been bred over generations to increase the allelelic frequency for the responsiveness to the nitrates in the minerals. Hence if more plants are able to express that allele in their Phenotype, a higher concentration of nitrates in the fertilizers will be up taken my by the plant. So, less nitrates will be wasted.
More nitrates taken up, leads to more protein syntheses and more amino acid production. This leads to DNA production. Hence cell growth and photosynthesis and increased.

Oh I thought by responsive they meant it ONLY responds to high levels and doesn't take up lower levels. So does it just take advantage of high levels while it's still there are you saying? So it's basically especially responsive to high levels of fertiliser?

Yes, high levels are an advantage to the crop. Low levels act as a limiting factor to they're growth. they still respond to Low levels but higher levels means a higher intensity of the stimulus. In this case the fertilizer , is the stimulus.

A fertilizer, (Very Strictly Speaking-VSS) cannot be a stimulus.

Note: Very high doses of micronutrients in fertilizers can also be damaging to plants.

Yes, high levels are an advantage to the crop. Low levels act as a limiting factor to they're growth. they still respond to Low levels but higher levels means a higher intensity of the stimulus. In this case the fertilizer , is the stimulus.