While nutrients are always needed for plant growth, the exact amount and combination of nutrients is based on several factors, like:

• The specific crop species and its physiological needs
• The crop's developmental or phenological stage
• The physical and chemical characteristics of the growing medium (soil-based or soilless)
• The quality and chemical characteristics of the irrigation water, particularly its pH, electrical conductivity (EC), and mineral content.

In horticulture the irrigation recipe describes the desired amount of nutrients in the irrigation water that is being fed to the plants. The table below shows an example of an irrigation recipe.

The nutrients on these charts sometimes vary. For example, instead of being shown as one element, nitrogen may be split up into two compounds, namely ammonium(NH4+) and nitrate (NO3-). While both of these compounds contain nitrogen, each of them has a different effect on plant growth.

You may also see other compounds listed. Sulfur may be listed as Sulphate (SO4) and Phosphorus may be listed as Phosphate (PO4) or Dihydrogen Phosphate (H2PO4). In all these cases, the compound consists of the element we are looking for along with one other element such as hydrogen or oxygen. As we mentioned earlier, hydrogen and oxygen are already in the water so right now we are only focusing on the part of the compound that is supplying the nutrient. 

Sometimes element requirements are shown in ppm (parts per million) and sometimes they are shown in mmol/l (millimoles per liter). You will notice that the concentrations are listed in the table above show both and if you look closely, you will see that there is not a direct correlation between the two. When comparing concentrations in ppm (parts per million) and mmol/L (millimoles per liter), it's important to understand that they measure different things. PPM expresses the mass of a substance in water—specifically, how many milligrams of a substance are present in one liter of water. In contrast, mmol/L measures the number of molecules or ions of a substance in a liter, based on its molar mass (the weight of one mole of that substance). Because each element or compound has a different molar mass, there isn’t a fixed conversion between ppm and mmol/L. For example, nitrogen has a molar mass of 14.01 g/mol, while phosphorus has a molar mass of 30.97 g/mol. This means that 1 ppm of nitrogen in water equals 0.0714 mmol/L, while 1 ppm of phosphorus equals 0.0323 mmol/L.

You may also find that some recipes list the units in milligrams per liter (mg/L) or milliequivalent per liter (mEq/L). Without getting into the details of the chemistry, you can assume that when working with nutrients dissolved in water, 1 mg/L is equivalent to 1 ppm and 1 mEq/L is also equal to 1 ppm.

Finally, you may notice that in a recipe, some of the elements do not have a desired level but rather a maximum level. Sodium (Na) and Chloride (Cl) are not considered essential for crops but they are included in many fertilizer compounds. For instance, you may use Calcium Chloride (CaCl2) to increase the calcium (Ca) levels even though it also adds chloride (Cl) to the recipe. In this case the fertilizer recipe will define a maximum amount above which they would adversely affect the crop.