Add in ability to estimate total soil P from soil test P data

[LeahJB]

Most soil P measurements are in the form of soil test P (some measure of bioavailable P) rather than total soil P. The link between the two is often not straightforward, depending on a range of soil properties. However, it would be useful to have this link explicitly made in the model for cases where soil test P is the only measure of soil P available. This could be a user-specified linear relationship in the first instance. Future improvements could include incorporating more advanced geochemical representations. The benefit of making these parameters explicit in the model is that they can then be included in any calibration or uncertainty analysis in a transparent way.

[LeahJB]

Looked into this, but there is no linear relationship between soil total P and P-AL (the soil test P commonly used in Scandinavia). There may be a relationship between some other extractable P fractions and total soil P, but the absence of one for P-AL makes me think that a better approach may be to build a database of total soil P values (given soil type, climate and land use history), and use these as defaults if data aren't available.