When soil is tested in the lab, the reported soil test values are so many PPMs or lbs/acre of P (i.e. elemental P). Crop P guidelines for fertilizer (and manure), on the other hand, are given in terms of P205. This is a legacy of old chemistry used in the fertilizer industry. The P205 molecule doesn't actually exist, but it's the standard! You can convert between P and P205 using the following method:
Fertilizers are designed to have a known "grade" or concentration of nutrients. The N - P205 - K2O analysis of the fertilizer is always listed on the fertilizer bag in that order. For example, the fertilizer shown below has a 20-5-10 grade, meaning it is 20% N, 5% P205, and 10% K2O by weight.
For example, if 100 lbs of the fertilizer above were applied evenly over an acre, you would apply:
20% N x 100 lbs fertilizer/acre = 20 lbs N/acre
5% P205 x 100 lbs fertilizer/acre = 5 lbs P205/acre
10% K2O x 100 lbs fertilizer/acre = 10 lbs K2O/acre
The concentration of nutrients in the fertilizer is multiplied by the amount of the fertilizer material applied per acre to find the actual amount of N, P205, and K2O applied per acre.
On the other hand, if you wanted to satisfy a crop P guideline of 15 lbs P205/acre with that same fertilizer, you would....
(15 lbs P205/acre) / (0.05) = 300 lbs/acre of the fertilizer would need to be applied.
In this case, the crop P205 guideline is divided by the concentration of P205 in the fertilizer and the amount of fertilizer material to be applied per acre is calculated!
Try out some fertilizer math, below...
Next, try meeting crop P guidelines with manure.
The best way to know how much phosphorus can be supplied by manure is to have the manure tested. The manure nutrient analysis, below, provides an example for satisfying crop P guidelines with manure.
Phosphorus in manure is usually expressed both as P and as P205. Since crop P guidelines are always in terms of P205, use the P205 value. It's also often expressed in %, lbs/ton, and lbs/1000 gallons, so you can choose the units that fit best with the particular farm you're working with.
When satisfying a crop P guideline with manure, no adjustments are made for residual P from sod or past manure applications (the soil test P reflects past applications). But, guidelines should be adjusted for other manure and fertilizer applications during the current year.
If a crop P guideline calls for 40 lbs P205/acre and there are 4 lbs P205/ton (based on the manure analysis), then...
(40 lbs P205/acre) / (4 lbs P205/ton) = 10 tons/acre of manure should be applied!
The crop P205 guideline is divided by the P205 concentration of the manure to calculate how much manure to apply per acre to meet the guideline.
Go to the next page to complete the Manure and Fertilizer Phosphorus Management section.