Originally Posted by Smallaxe
I still haven't been told whether those "soil tests" address the 'free P' only,,, or if it can detect how much 'bound P' is actually in the soil... grass develops a mutual connection with fungi over time that 'mines' the boundup P from the soil...
That means,,, that if the test only reveals the 'free P', that it doesn't adequately show what is actually available to turf... I hear that 80% of P is boundup in the soil...
Phasthound gave a great link for understanding soil test P (and laying the foundation for understanding soil tests for other nutrients). I've been raked over the coals on this board (by the usual suspects) for saying what is in this very passage (paragraph 2 of phastound's link):
"The soil P tests that are used today provide an indication of the level of soil P that is available to the plant. The tests do not determine the total concentration of P in the soil or even the actual concentration of available P
, but provide an index measurement
of the P that can be taken up by the plant."
You will never extract all the P from the soil complex without destroying the sample, but the total amount of P doesn't matter. The total amount of plant-available P doesn't even matter. The important part of soil testing is correlation-calibration -- knowing how plant responses correlate to different test values and knowing how fertilizer application rates influence soil test nutrient levels.
For example, the only reason we know if soil test values are high or low is that we've tested soils with plants that have exhibited deficiency symptoms and we've tested soils with plants that looked really healthy. The only reason we know how much fert is required to bring a soil back to the optimal range is that we've fertilized different rates on soils with particular soil test values and monitored nutrient levels and plant response after that.
Soil tests aren't the end-all-be-all of plant nutrition. But, they give us a darn good idea of where we are and how to get where we want to be.