Fertilizer

Three Primary Nutrients

The three major nutrients in fertilizer are nitrogen (N), phosphorus (P) , and potassium (K). Fertilizers are usually labeled with three numbers in the form N–P–K (eg: 2–8–16) to indicate the relative amounts of nitrogen, phosphorus, and potassium.

Technically, the "N" in N–P–K indicates the percentage by mass of nitrogen. The "P", however, indicates the percentage by mass of phosphorus pentoxide if the potassium were in that form and the "K" indicates the percentage by mass of potash or potassium oxide if the potassium were in that form. In practice, the phosphorus may be in the form of the negatively-charged phosphate ion. A fertilizer marked 2–8–16 should contain the equivalent primary nutrients of at least 2% nitrogen, 8% phosphorus pentoxide, and 16% potash. The remaining 52% may be referred to as ballast or filler.

Nitrogen, phosphorus, and potassium are all necessary for healthy plants but nitrogen is more closely associated with encouraging green growth; phosphorus is more closely associated with encouraging rooting, blooming and fruit production; while potassium is more closely associated with cold hardiness, disease resistance, and general health.

Fertilizers used in horticulture typically have some mixture of all three primary nutrients and are called complete. Fertilizers used in agriculture typically are missing at least one of these primary nutrients and are called incomplete.

The relative amounts of each primary nutrient needed depends on the amounts already in the soil and the particular needs of the flora being cultivated. A sandy soil with no organic matter may need all nutrients so a 10–10–10 fertilizer may be a good place to begin. An established lawn may benefit in the spring from a high nitrogen fertilizer such as 29–2–4 to encourage green growth. Roses about to bloom may benefit from a higher phosphorus fertilizer such as 5–10–5.

Organic Fertilizers

Experts often define the term "organic fertilizer" as meaning that the nutrients are derived from living or once-living organisms but that definition doesn't make sense when applied to synthetic organic fertilizers like urea. Organic chemistry is a branch of chemistry that studies chemical compounds consisting primarily of carbon and hydrogen but may also contain other elements. Perhaps the term organic fertilizer would be better defined as a fertilizer in which the nutrients are derived from chemical compounds that consisted primarily of carbon and hydrogen. Perhaps we need another term. In the meantime, I'll assume that when we speak of organic fertilizers, it's with the understanding that most organic fertilizers come from sources that were once living.

Of the three primary nutrients, nitrogen is the most easily lost and can be taken up by organic materials such as mulches so nitrogen replacement is commonly needed more frequently than phosphorus or potassium.

Organic fertilizers are generally slow release by nature because they must be broken down by microorganisms in the soil to be made available to the plants. This gives them an advantage is many uses because microorganisms tend to be less active during the winter when it's usually best to cut back on fertilizing to prevent frost damage. For uses that require continued high availability of nutrients during cold weather, this can be a disadvantage. Most organic fertilizers also increase bacterial activity and help improve the structure of the soil by increasing permeability for water and roots of clay soils and increasing water retention of sandy soils.

Some organic fertilizers such as blood meal also contain other important nutrients such as iron that provide additional benefits. Blood meal is made from dried blood from slaughterhouses making it a poor choice for vegans.

Because cottonseed meal tends to raise soil pH (more acidic), it can be a good choice for acid-loving plants such as azaleas. Cottonseed meal is typically high in nitrogen which supports green growth.

Fish emulsion (made from fish) provides some trace minerals. It can smell bad for a few days after the initial application. [I suspect that fish emulsion typically has a lot of salts but have not verified this.]

Knowing what type of fertilizer to use, how much to use, and when to use it depends on a variety of factors. Sandy soil requires fertilization more often than clay soil because nutrients are washed away more easily in sandy soil. Plants that are grown for foliage generally need nitrogen more than plants grown for fruit. In fact, low nitrogen levels may encourage fruiting plants to bloom and produce more fruit as a survival adaptation. Plants that grow slowly need fertilizer less often than plants that grow rapidly. When plants are transplanted, they may need extra phosphorus to make up for phosphorus that is not yet available in newly fertilized soil. Extra potassium may help plants that are susceptible to frost damage. Fertilizing young Mesquite trees can make them grow top-heavy and blow over in heavy winds. Fertilization plans need to be adapted to the individual situation.

When possible, fertilizer should be placed under a layer of soil below seeds to prevent damage to tender roots which will grow downward to find the fertilizer. Fertilizer should be placed away from the base of established plants but just close enough that the roots can still reach the fertilizer. Foliar feeding (spraying nutrients directly onto foliage) provides the plants with nutrients quickly without the nutrients becoming sequestered in the soil. (The soil still needs nutrients.)

Applying Fertilizers

Some fertilizers are made to be very soluble in water and available to the plants immediate. Others are less soluble in water, must be broken down by microorganisms before they can be used, or are encapsulated to release the nutrients over time. The former are helpful to the plants right away and the latter are longer lasting. Avoid applying slow release fertilizers late in the growing season because their release during the winter might encourage new growth which could be easily damaged by frost.

Fertilizers made for acid-loving plants such as azaleas and rhododendrons may have incredients such as sulfur intended to lower the pH (acidify) the soil. Because our soil tends to be high in calcium carbonate, it's difficult to lower the pH so extra sulfur may be necessary.

Fertilizers contain salts and some fertilizers are particularly high in salts, so the soil should be watered well immediately before application to wash away old salts and after application to ensure the salts in the new fertilizer won't dry out the roots.

More fertilizer is not necessarily better. Applying an excess of nitrogen, for example, will not help the plants but may encourage growth of weeds.