Water erosion occurs only when water runoff takes place, but we treat erosion and runoff as two separate factors in the assessment of the risk of P transport. Erosion refers to sediment loss, while runoff refers to liquid loss.
Runoff from cropland typically carries less P than erosion, but runoff P consists of a higher proportion of bio-available P, compared to total P, than does erosion P (Fig. 9).
The transport of P in runoff involves desorption or dissolution of readily desorbable P from soil, fertilizer or manure, resulting in an increase in the level of dissolved P in the runoff. Soil P conditions within the top one inch of soil have the biggest effect on runoff, as this is the soil exposed to mixing with the runoff water. Runoff becomes more important relative to erosion with no-tillage systems and from grasslands where erosion is well controlled but runoff is still significant.
Discussion Question: How might the soil P conditions at the 0-1” depth differ from deeper soil?
Answer: In no-till and grassland systems, soil P level is often very stratified with much higher levels at the surface than in deeper soil. As the surface soil is most exposed to the effects of runoff, the stratification has implications for P loss.
Discussion Question: Will the ratio of dissolved P to total P for runoff from a no-till field be typically higher or lower than the ratio for a comparable tilled field?
Answer: We expect the ratio to be higher for no-till land. The total P load will be much reduced but dissolved P load higher for no-till than for tilled land.