What does the Olsen P level measure?

The Olsen P level specifically measures the concentration of plant-available phosphorus in soil, typically expressed in parts per million (PPM). This test is particularly relevant for soils that have a pH above 7.0, as it is designed to provide an accurate assessment of phosphorus availability to plants under those conditions. The Olsen method involves extracting phosphorus from soil using a sodium bicarbonate solution, allowing for an effective evaluation of how much phosphorus is accessible for plant uptake.

Other options, while related to soil health and fertility, do not represent the purpose of the Olsen P level. Soil moisture content refers to the amount of water contained in the soil; soil pH levels indicate the acidity or alkalinity of the soil; and soil potassium content measures another essential nutrient but not phosphorus. Therefore, the focus on phosphorus availability is what makes the provided answer the correct one in this context.