Each of these three major nutrients plays a significant role in plant health and also in promoting particular elements of growth. Nitrogen facilitates general leaf growth, phosphorus helps to create strong roots and potassium promotes flowering and fruiting.
In addition to these nutrients, which arc needed in fairly large quantities, there are a number of trace elements that arc needed in lesser amounts but are still important to plant health. The principal ones are magnesium (Mg), zinc (Zn), sulfur (S), manganese (Mn), molybdenum (Mo), and boron (B). It is possible to make a detailed study of the nutrient requirements of your chosen plant, since plants vary in their needs. However, for most amateur gardeners with a mixture of ornamental and edible plants, general-purpose fertilizers (in which the different nutrients have been combined in particular formulations) will be quite sufficient. The exact quantities of different nutrients are obviously ol far greater importance if you are growing plants to show, or if you run a farmer’s market and your livelihood depends on the size and quality of your crops. II you arc growing crops for the table, you may to concentrate slightly harder on the condition of the soil than if you are growing plants that arc purely decorative. You may well discover that your soil is lacking in certain nutrients or trace elements when common symptoms appear on your plants .
Replacing soil nutrients
There is a range of organic prod¬ucts available that contain varying amounts ol the major nutrients: nitrogen, potassium, and phos¬phorus (see table, right, for common sources ol these nutrients and the percentages found in them). Nitrogen is the most important and is best applied in the form of bulky manure. The. more concentrated the nitrogen, the less of that substance you need to apply For example, generally 1 lb. 2 ox. of cow manure will supply enough nitrogen for 1 ft2. of soil; however, with dried poultry manure, which is far richer in nitrogen, you would only need one-filth of this amount. Hoof and horn, dried blood, fish meal, and soot all include generous quantities of nitrogen in their makeup.
Heady-mixed fertilizers in various forms can be bought from garden centers and applied according to the manufacturer’s instructions. If you are mixing your own general-purpose fertilizer, then a good formula for a once-a-season application might include 1 part dried blood, 2 parts bone meal, 3 parts wood ash, and 4 parts leaf mold. To increase the nitrogen content of this mix, increase the blood and bone meal component. You can dilute the mix with garden compost or sand for topping up throughout the season.