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News: Proposed national standard for phosphorus derailed by critics
Proponents favor a one size fits all approach:
The proposal to have a single, national guideline for phosphorus application was part of an overhaul of the USDA's National Resource Conservation Service's nutrient management guidelines. The overhaul aimed to find a long-term, sustainable approach to guide the application of manure and synthetic fertilizer for both maximum crop growth and water protection.
It would have phased in an upper limit for the amount of phosphorus in soil. Once a soil test showed the phosphorus had reached that amount, farmers would have been discouraged from applying more. The proposed upper limit was 200 parts per million, a number believed to represent a point at or beyond which phosphorus begins to leave the field by dissolving in rain runoff or leaching into groundwater.
Opponents favor sticking to a site specific evaluation that considers the likelihood that the phosphorus will leave the land. It is in wide use.
20 years ago, NRCS began talking about three options to control the phosphorus problem. One would be a national standard; a second would be an approach that required farmers to use only as much manure as their crops need, and dispose of the rest by transporting it to other farmers who needed it; and a third would be a phosphorus index.
Most states settled on an index
Although the indexes and their phosphorus thresholds vary from state to state, all take into account factors such as how close a field is to water, how fast water drains from the field, how much phosphorus is in the soil, how sloped is the terrain, and how and when fertilizer and manure is applied.
The indexes include a soil test, but they also give farmers credit for conservation practices such as grass buffers and using no-till farming. So, if a particular field tests high for phosphorus but conservation practices are in place, and it is not close to waterways, a farmer may decide to go ahead and put more phosphorus on that field. The index, in other words, allows farmers to make decisions on what's best for them economically while taking water quality into consideration.
There are merits in both approaches, but a single standard serves soil scientists poorly. Consider that there is no single soil P test we can apply nationwide, leaving us to match the applicable test according to site specific soil chemistry using applicable standards. An across the board 200 ppm standard is too simplistic. That it was even considered speaks to the powerful need for simplification when politicizing science.
We ask different soils to perform different functions depending on the site and the needs of the surrounding community. There are good reasons for a community to want a soil to receive excess soil phosphorus, and there are clearly sites where excess soil phosphorus does not endanger water quality. At the other extreme are areas where running a phosphorus deficit is necessary to prevent eutrophication. Phosphorus heavy operations should gravitate toward the former and away from the latter.
Phosphorous is Tricky!
I agree with your post. Soil P should be measured depending on the other properties of the soil, and the most suitable method.
In Australia, most agriculturalists tend to get their soil tested by someone such as Incitec-Pivot or other fertilizer companies. Their methods are limited (usually Cowell, Bray), not site specific and are often inaccurate. I am sure this has impacted on water quality, and P in our riparian environments.
I also think that Australia needs to have better guidelines for the measurement of soil and water P.