|Sourced from AccessScience
topic of soil chemistry.
Water moves within plant-soil system from wettest to driest. This gradient is termed water potential, and explains why irrigation works. Basically, the water potential gradient between the soil and the plant, from wettest to driest, would be wet soil, dryer soil, root, stem/trunk, branch, leaf, and finally, air. Water, when applied through irrigation or rain, will follow that path, moving through the soil, into and up the tree, and out the leaves through the stomata.The final destination for the water molecule is the driest environment, which is air. As an analogy, think of the plant as a wick of an oil lamp, as the oil is burned, oil moves up the wick out of the reservoir to the point of being burned, keeping the lamp lit. For more information, please follow the links for water potential and the process of mass flow.
Since nitrate, the only form of nitrogen that is used by plants, is a water-soluble negatively charged particle (anion), it does not bind with most soils, and therefore remains in the water solution within soils. Once in solution, it moves with the water. As water moves into the plant’s roots and up into the plant, nitrate enters the root and plant as well. If there is no or low water demand by the plant due to conditions of dormancy, very little of the nitrate-water solution will move into the plant. Only when the process of transpiration begins does nitrate move into the growing tips of the plant, providing nitrogen, an essential element for plant anabolism.
Since leaves are needed on the tree in order for transpiration of water to occur, applications of nitrogen before leaf-out are at risk for loss due to leaching from the root-zone. For example, lets say that 20 units of nitrogen was applied in mid-January to a sandy soil during a pre-irrigation. The pre-irrigation filled the soil profile, and now there is ample nitrogen within the system. Rain moves into the area before leaf out and brings 1-2 inches of rain. With that rate of rain falling on a soil with a full profile, nitrogen is most likely lost from the system since it is in the water solution and the water moves beyond the reach of the roots. In this situation, it would have been more efficient to apply the nitrogen later, after leaf out, when the tree is using water.