Many of the reasons for using one method of fertilizer application over another are based on the nutrient's potential for movement in the soil. For example, nitrogen is quite mobile, so it will come in contact with plant roots more easily. Potassium or phosphorus, which are less mobile need to be place closer to plant roots.
There are many ways to get the fertilizer in contact with the plant roots, and there are advantages and disadvantages for each method.
Banding fertilizer is especially important for phosphorus fertilizers. In this way, the roots after germination will have to grow through the fertilizer. The negative effects of the immobility of phosphorus are reduced by the close association of roots with the banded fertilizer. Care is needed to avoid salt injury to seeds and seedlings with banding. Potatoes are a crop that will frequently have the fertilizer banded. In the above photo a band has been placed on both sides of the potato plant.
This ridge till planter also has a tube that can band a fertilizer while planting.
Starter fertilizer is usually applied as a band. This is especially important for phosphorus since it aids in establishing a vigorous root system. For more information on root zone banding go to :Root Zone Banding Increases Yields
Broadcasting fertilizer is frequently done after the crop has been harvested, in preparation for the next crop. These materials should then be incorporated by plowing to avoid water pollution by runoff. Generally, more P and K will be needed when broadcasting than if the fertilizer was banded.
Top dressing refers to adding fertilizer to the surface of a crop already growing. Fertilizing established turf grass is also done with top dressing. When applying water-soluble materials, care is needed to avoid burning the turf. Also, overlap the application to avoid missed spots; otherwise, strips of the lawn will be different colors of green.
If a large quantity of fertilizer is spilled on turf, sweep it up and do not try to wash it away with water.
Side dressing is often done with anhydrous ammonia. It must be accomplished before the crop is too high for the implements. Ammonia is injected into the soil at least 3" deep by these knife blades. It is rapidly changed to ammonium and can be taken up by roots, attached to cation exchange sites, or converted to nitrate. Urea can also be side dressed with cultivators.
In order for the anhydrous to be applied most efficiently the soil must be the correct moisture, the depth of application must be adequate, and the application rate can not be too high. See the diagram Ammonia Injection for details.
Foliar spray can also be used to apply nutrients where it is absorbed by the leaves. Only small amounts of nutrients can be absorbed by the leaves, so if you need a large amount of nutrient, soil application is generally best. Generally micro-nutrients such as iron or zinc are applied in a foliar spray. This orchard is receiving a zinc sulfate spray to increase zinc in the plant, due to a high pH soil which reduces the Zn availability in the soil.
The quantity of nutrients to apply will be dependent on the unit yield increase with each additional unit of fertilizer. Eventually, the point is reached where it is not economically feasible to add any additional fertilizer because the yield increase will not be sufficient to pay for the fertilizer (the point of diminishing returns). Today, farmers also need to consider the potential pollution hazards from applying more fertilizer than the plant can use.
See Fertilizer for Trees and Shrubs
Chapter 7 Nutrient Needs
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