Nitrogen is abundant in the soil and the atmosphere, while phosphorus is only available in the soil and in bedrock.

Phosphorus is generally scarce in the natural world.

Many plants are incapable of taking up available phosphorus, making it very difficult to have compost that is rich in phosphorus.

There are significantly more steps in the phosphorus cycle to make phosphorus usable for plants, limiting availability.

Atmospheric phosphorus disperses in the form of precipitation, and irrigating crops from a well source will be insufficient in replenishing phosphorus to the soil.

Waterlogging

Salinization

Sheet erosion

Gully erosion

Rill erosion

A farmer employs flood irrigation on a field with a slope, causing thin layers of soil to wash away over time.

Narrow channels develop in a farmer's field from running irrigation lines over exposed soil.

Wide channels develop in the soil where a stormwater discharge site has been established.

Light rain falls on a farmer's field that is mostly bare soil.

Soil is lost from irrigation ditches, causing the ditch to become wider and deeper.

Intensification

Contour plowing

Cover crops

Add legumes

Spread manure and then mulch to prevent leaching or erosion of nutrients.

Plant phosphorus-fixing cover crops to increase phosphorus presence in soil.

Deep-till the soil, bringing up phosphorus-rich soil and rock fragments from the subsoil layers.

Move the farm elsewhere. Once phosphorus is gone, it's gone.

Apply a substantial layer of chemical nitrates to the soil.