Supplying Soil Nutrients
As crops grow they pull soil nutrients out of the soil to produce plant tissue in the form of roots, stems, flowers and fruits. Some crops are very heavy feeders pulling a lot of soil nutrients out of the soil and other crops are light feeders. At some point home gardeners realize that they need to replenish the soil nutrients in their soils. There are many ways of doing this after the level of nutrient deficiency is determined by a soil test.
There are conventional and organic based fertilizers, water soluble and water insoluble fertilizers, complete and incomplete fertilizers, special purpose fertilizers, and slow-release fertilizers.
Fertilizers will be labeled with the nutrients that they contain. At least three numbers appear on the label and always refer to the percentage of Nitrogen, Phosphorous and Potash in this order. For example a 5-10-10 conventional fertilizer would have 5% Nitrogen, 10% Phosphorous, 10% Potash and 75% filler. The filler is important so that the fertilizer can be evenly spread and avoid burning plants with too much fertilizer.
Water soluble fertilizers are usually mixed with water and applied to plants. The nutrients particularly nitrogen are more readily available for plants. Water insoluble granular fertilizers that we apply to the surface of soils don’t move to plant roots until they are rained or irrigated into the soil.
A complete fertilizer has all three nutrient components nitrogen, phosphorous and potash such as 10-10-10 or 5-10-10. An incomplete fertilizer has one of these components missing such as 0-10-10. Special purpose fertilizers are formulated for certain types of plants such as Azalea fertilizer that has an acid reaction that is beneficial to the plant.
Slow release fertilizers release nutrients at a rate that makes them available to plants over a long period of time. There are three types of slow release fertilizers. Those that dissolve slowly, those that contain microorganisms that release nitrogen and those with membranes that control the rate of nutrient release.
Organic fertilizers are derived from the remains or by-products of once-living organisms. Cottonseed meal, alfalfa meal, compost and manure are examples of organic fertilizers. In general organic fertilizers release nutrients over a long period; the potential drawback is that they may not release enough of the needed nutrient for the crop needs. The high level of organic matter in organic fertilizers act as soil conditioners and help improve soil tilth.
An important point to remember is that the plant does not differentiate between conventional and organic fertilizer sources for their basic nutrient elements. Research has shown that sources of basic mineral plant nutrients have no impact on plant growth, vigor, flavor or human nutritional value.
Source: Maine Master Gardener Manual, 1999, pgs.10 to 13.