Problem Set 3 Q2

For details on the P cycle see lecture #25
If you are uncertain on a specific process, please provide your thinking and we are happy to respond.
e.g. Q2a) Phosphorus can be lost from the soil by volatilization. False because.... (but also indicate why you are unsure)

specifically I was wondering about the following: Does chemical weathering of primary minerals release available P such as orthophosphate ions (H2PO4- and HPO42-)? Is is correct that the principle way of P uptake is in the form of Phosphate? Is organic residues plat available P? Because in the diagram it has a arrow going to soil organisms which makes me think its not plat available.

Q. Does chemical weathering of primary minerals release available P such as orthophosphate ions (H2PO4- and HPO42-)?
A. Yes, the weathering of primary minerals does release plant-available forms of P (ie H2PO4- and HPO42-), but this is a very slow process.

Q. Is it correct that the principle way of P uptake is in the form of Phosphate ions?
A. Yes, plants can ONLY take P (and any other element) in ionic form.

Q. Are organic residues plant available P?
A. No, organic compounds containing P, are not directly available to plants. ALL organic compounds need to be decomposed (ie mineralized) by soil organisms and through that decomposition organic forms of elements (eg P) need to be transformed into ionic (or mineral) form that is available to plants.

Hi Maja,

On Problem Set 3, I indicated that only inorganic P would be plant available (i.e., #2e - False) but this response was marked wrong. I am confused.

Thanks!

Prob Set #3 Q2e) Organic residues are a major source of plant available P in soil. The key word is source; i.e. the question does not say that organic P is plant available but that plant available P is derived from organic residues.