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All fertilizers are analyzed to determine the fertilizer grade. This grade is represented by three numbers that correspond to the percentage by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). It is often simplified as nitrogen, phosphorus, and potassium (N-P-K), however it is really based on the percentage weight of N-P2O5-K2O.

For example, a 100-pound bag of 10-10-10 fertilizer contains 10% or 10 pounds of nitrogen (N), 10% or 10 pounds of phosphate (P2O5), and 10% or 10 pounds of potash (K2O). Of the 10 pounds of phosphate, only 4.4 pounds are phosphorus (multiply the weight of the phosphate by 0.44). Of the 10 pounds of potash, 8.3 pounds is potassium (multiply the weight of the potash by 0.83). So only 22.7 pounds of the 100 pounds is made of nitrogen, phosphorus, and potassium elements.

Fertilizers contain other substances as well. They can contain conditioners to keep the fertilizer in granular form for easy spreading. They contain absorbents to slow the release of nutrients into the soil. In some cases, filler may be added to dilute the concentration of nutrients to yield a low analysis fertilizer. Premium fertilizers can also contain compounds like limestone, which act both as a filler and to neutralize acidic soils.

Fertilizer Ratio

As previously explained, the fertilizer ratio describes the relative proportions of nitrogen, phosphate, and potash in a fertilizer. The first number always represents the percentage of nitrogen by weight. The second number always represents the percentage of phosphate, but the element phosphorus makes up only 44% of the weight of the compound phosphate. The third number always represents the percentage of potash, but the element potassium makes up only 83% of the weight of the compound potash.

Complete Fertilizers

Complete fertilizers are fertilizers that contain all three of the major plant nutrients (nitrogen, phosphorus, and potassium) such as 10-10-10, 5-10-10, or 16-4-8. Soils requiring only one or two of the three major plant nutrients do not require a complete fertilizer.

Balanced Fertilizers

Balanced fertilizers are those fertilizers which contain equal amounts of nitrogen, phosphate, and potash. For example, 10-10-10 and 17-17-17 are balanced fertilizers.

Incomplete Fertilizers

An incomplete fertilizer is a fertilizer that lacks one or two of the three major plant nutrients. Multiple incomplete fertilizers can be combined to create complete fertilizers to meet the need of a specific soil as determined by a soil test.

Special-Purpose Fertilizers

Special-purpose fertilizers are fertilizers created for specific type of plants, such as “Camellia Food” or “Azalea Food”. Camellias and azaleas are acid-loving plants, so the ingredients in these fertilizers are chosen because of the acidifying reactions they provide to soil that is naturally neutral or alkaline.

Slow-Release Fertilizers

Slow-release fertilizers are designed to release their essential elements into the soil over an extended period. This provides plants with a balance of nutrients throughout their growth cycle. Because they are less susceptible to leaching, slow-release fertilizers are especially effective in sandy soils. They are also useful for areas prone to runoff like slopes, compacted soil, and areas with little vegetation reducing the potential for runoff and water contamination.

There are three categories of slow-release fertilizers:

  1. Those consisting of materials which dissolve slowly like granite meal, greensand, or rock phosphate
  2. Those consisting of materials from which nitrogen is released by microorganisms
  3. Those consisting of granules coated in resin or sulfur that control how fast nutrients are released into the soil.

Slow-release fertilizers can be applied less frequently than other fertilizers, as well as in higher amounts, because they are less likely to cause burning of plants. While more expensive to buy than other fertilizers, slow-release fertilizers save time as they do not need to be applied as frequently as conventional fertilizers.

Inorganic Fertilizers

Inorganic fertilizers consist of certain salts and minerals that can be added to soil. They fertilize the soil in the same way that naturally occurring rocks might release similar salts and minerals into the soil as they weather over time. These inorganic fertilizers include compounds like ammonium sulfate, superphosphate, potassium nitrate, potassium chloride, and potassium sulfate to name a few.

Synthetic Organic Fertilizers

Synthetic organic fertilizers are human-made organic materials that can be used for fertilization. These include urea, ureaform, and isobutylidene diurea (IBDU).

Natural Organic Fertilizers

Organic fertilizers are those fertilizers made from the remains of once living organisms or as a by-product of the decomposition of once living organisms. Waste and by-products from plant and animal industries can be used to produce organic fertilizers. Blood meal, bone meal, horn meal, hoof meal, and manures are all examples of such organic fertilizers.

When packaged as fertilizers, these products will include a nutrient guarantee or fertilizer grade on the label. However, some such products, such as composted manures and sludges, are sold as “soil conditioners” and do not include a grade on the label, even though they contain small amounts of nutrients.

Organic fertilizers generally release their nutrients over a fairly long period. This is good as there is a low potential for burning your plants. However, it could mean the main nutrient is not release fast enough to give the plant what it needs to grow. Organic fertilizers depend on organisms in the soil to break them down and release the nutrients, so they are often most effective in moist soils where the climate is warm enough for living organisms to be active. Soil pH and aeration can also influence the effectiveness of organic fertilizers as they affect microbial activity. Microbial activity decreases drastically in soils with a pH of less than 6.

Organic fertilizers generally contain low concentrations of nutrients when compared to synthetic fertilizers. However, organic fertilizers provide other benefits to the soil that their synthetic counterparts do no. They increase the percentage of organic matter in the soil, improve the physical structure of the soil, and increase microbial (bacterial and fungal) activity in the soil, especially that of mycorrhiza fungi which assists plants in absorbing all nutrients from the soil.

Fertilizers Combined with Pesticides/Herbicides

Fertilizers are available that also contain certain pesticides and/or herbicides. The main advantage of using such fertilizers is convenience since one application can solve two problems. However, the timing for applying fertilizers often does not coincide with the optimal time for applying pesticides/herbicides. Care should be taken to closely follow the directions on such fertilizers.

Fertilizer Formulations

Fertilizers come in many formulations, such as water-soluble powders, slow-release granules, slow-release spikes, liquids, granular solids, and more. Regardless of formulation, all fertilizers must be labeled with the amount of each nutrient and may even disclose how quickly the nutrient will be available in the soil.

Liquid fertilizers are meant to be applied in liquid form, but can be sold in different formulations such powder, pellets, or concentrated liquid. They must be mixed with water before application. Such fertilizers can be applied to the soil so that the nutrients can be absorbed through the roots or, in some cases, can be applied directly to the leaves, which is called foliar feeding.

Nutrients in foliar fed fertilizers are absorbed and used by plants quickly. Absorption starts within minutes of application and is typically complete within 1 to 2 days of application. Plants, however, are better equipped to absorb nutrients through their roots. Only a few nutrients like potassium, zinc, and iron are capable of absorption through the leaves, fruit, and other portions of the plant above the ground. Others are blocked by waxes, oils, hairs that coat the exterior of the above the ground portion of plants.

The effects of foliar feeding are short lived and only affect those portions of the plant that exist above ground at the time of application. It treats the symptoms (typically a deficiency in micronutrients like iron, zinc, manganese and copper) and not the problem with the soil that led to the symptoms like a pH imbalance or lack of nutrients available in the soil.

What Type of Fertilizer Should I Buy?

When buying fertilizer, it is important to look at not only the content and cost of the fertilizer, but also how fast the nutrients are delivered to the soil. For example, if you need fast results, then you will want to buy a fertilizer which has nutrients that can be readily absorbed by plants, such as nitrate or urea. For long-lasting results, you should consider a fertilizer with slow release. It is often best to use a combination of products with fast and slow-release nitrogen.

Once you choose a nitrogen source, then consider which secondary nutrients, if any, are needed to when making your final fertilizer choice. While many fertilizer producers exploit the presence of micronutrients in their products as a differentiator, lawns and gardens rarely receive any noticeable benefit from them. If a lawn or garden is seriously deficient in one or more micronutrients, the small quantities found in popular fertilizers will rarely be sufficient to correct such a problem.