Basically, farmers provide fertilizer for crops at or just before planting, and they sometimes add a mid-season application of nitrogen during the summer. The availability of water in the site plays a deciding factor when fertilizer treatments are applied.
Water keeps plant tissues moist and provides a transportation system for much-needed chemical nutrition into plant cells. But it can have beneficial and detrimental effects to the fertilizer treatments if not applied at the right time.
Water allows granular fertilizers to enter solution and to become available to plants. Granular fertilizers rich in phosphate and potash dissolve in water and will become readily available to the root systems of plants. Phosphate and potash should be applied at the surface and allowed to soak into the soil, or they can be lightly worked into the soil. On very well-drained sites - sandy and normally dry - heavy rainfall will leach fertilizers high in phosphate and potash away if worked too deep into the soil before planting.
Water can have a dramatic effect on the way certain fertilizers are available to plants, and nitrogen is the main contributor to this potential gain and/or losses to nutrient availability.
Nitrogen is crucial for plant growth. Plants utilize nitrogen to produce protein and nucleic acids to make DNA and tissue growth. In crops such as corn, sorghum, oats and wheat, nitrogen is applied at planting and again after several weeks of growth, using split applications. But water availability in the soil and rainfall are critical factors when determining when to apply nitrogen-based fertilizers.
Nitrogen is applied to crops in either a liquid or solid format as mostly urea, ammonium nitrate, potassium nitrate, or ammonium sulfate. Plants can use both ammonium and nitrate forms of nitrogen if the right conditions are met within the soil, including: pH, moisture, temperature, etc.
If ammonium nitrate is applied - and it is recommended for mid-season applications - water dissolves the fertilizer into accessible nitrate and ammonium ions. Ammonium attaches to soil particles and can be inaccessible to plants if not captured immediately by their root systems. Nitrate ions are hydrophobic and essentially can be transported into the soil through water molecules and be readily usable to plants as long as in solution. Watering-in ammonium nitrate can be beneficial to plants when broadcast at the surface, especially when plants have shallow root systems.
If urea is applied, this type of fertilizer must be worked into the soil before coming into contact with significant moisture. Essentially, urea will react and convert to other chemicals immediately after being applied. If applied to a wet soil surface, the urea will convert to carbon dioxide and ammonia gas; the nitrogen will be wasted and lost into the atmosphere without every getting a chance to be used by the plants. Urea should be worked into the soil before ever becoming wet. Urea will still convert to ammonia even when incorporated into the soil and after receiving rain or irrigation, but the gas will combine with water in the soil and become available to plants. Additionally, nitrogen loss from wet soil during application can be reduced if soil temperatures are relatively low.
Plants will utilize nitrogen very quickly when presented in the right format. However, as soon as the nitrogen in solution dries out and heats up under the summer sun, the nitrate ions are absorbed into the atmosphere in the form of nitrous oxide when nitrate forms of nitrogen are used.
When applying fertilizer rich in phosphate, potash, and nitrogen, soil wetness and pending rainfall are critical factors for making sure that expensive fertilizers are applied at the right time to insure that food plots and field crops receive the greatest benefits.