Discuss how crop plants obtain the phosphorus that they need to grow and whether the supply of phosphorus to crops is sustainable.

Natural cycle

a. plants absorb phosphorus from the soil by the roots

b. soil phosphorus comes from weathered «phosphate» rocks

c. «soil/organic» phosphorus «also» comes from humus/plant/animal residues/guano/microorganisms

d. phosphorus is a limiting «macronutrient» factor in plant growth
OR
phosphorus is necessary for vital functions/ATP/DNA
OR
deficiency of phosphorus «in soil» limits plant growth/production 

Sustainability

e. «natural» phosphorus cycle is slow/not sustainable with increased crop production

f. phosphorus is replenished «in the soil» by use of mineral/organic fertilizers

g. phosphorus/phosphate is «mainly» mined from «phosphate» rocks

h. rocks are becoming used up/inaccessible so non-sustainable/non-renewable
OR
the use of fertilizers is non-sustainable «with increased crop production»

i. increased demand for food/meat/dairy/grain increases demand for fertilizers

j. runoff/erosion/leaching of fertilizers decreases potential supply for crops/OWTTE