It doesn't "percolate" up into the plant. If that were the case, we'd be dead long ago from the chicken manure that is often used to fertilize.
Nutrient ions such as nitrate, chloride and sulphate, are not absorbed by the soil colloids and are mainly in solution. Such nutrient ions are absorbed by the roots along with soil water.
The second mechanism is mass flow or convection, which is considered to be the important mode of nutrient uptake. This mechanism relates to nutrient mobility with the movement of soil water towards the root surface where absorption through the roots takes place along with water. Some are called mobile nutrients. Others which move only a few millimetres are called immobile nutrients. Nutrient ions such as nitrate, chloride and sulphate, are not absorbed by the soil colloids and are mainly in solution. Such nutrient ions are absorbed by the roots along with soil water. The nutrient uptake through this mechanism is directly related to the amount of water used by the plants (transpiration). It may, however, be mentioned that the exchangeable nutrient cations and anions other than nitrate, chloride and sulphate, which are absorbed on soil colloids are in equilibrium with the soil solution do not move freely with water when it is absorbed by the plant roots.
Many organic gardeners prefer to use manures instead of chemical fertilizers. Ferretti warns gardeners never to apply manure after crops have been planted. "Manure is home to many food-borne pathogens such as E. coli 0157: H7, and bacteria can be spread to the vegetables by rain, watering or by the gardener," he says. "If you use manure, it should always be applied the previous fall, to give it a chance to break down within the soil."