Fertilizers or plant foods, also referred to as nutrients, are materials added to the soil to stimulate plant growth and produce flowers. Many "good" soils are already rich in the necessary elements, but when soil is used over and over again and constant production is expected, soil must be continuously amended. For best results three major elements-nitrogen, phosphorus, and potassium must always be present in soil in correct proportions.
Fertilizer manufacturers list these three main elements on packages and cans in a numerical ratio in the order given above, as 4-12-4 or 5-10-5. Thus, a 4-12-4 fertilizer contains 4 per cent nitrogen, 12 per cent phosphorus, and 4 per cent potassium. (Sometimes there is a statement as to how much of the nitrogen is from organic sources.) Inert or filler material makes up the amount to 100 per cent.
Nitrogen stimulates growth, strengthens stems, and promotes healthy green foliage. Weak, stunted or spindly growth and yellow-green leaf color usually indicate a lack of nitrogen. An oversupply of nitrogen results in soft lush leaves, few flowers, and susceptibility to disease.
Phosphorus is also a stimulant; it promotes root growth and encourages flower and seed production. If too little phosphorus is available, plants incline to shed lower leaves and yellow edges develop on those that remain. Plants get sappy and require more staking than usual when soil lacks phosphorus.
Potassium or potash builds up resistance to disease. It stabilizes growth, improves plant and flower color, and increases productivity. A dull-looking plant usually lacks potassium, but too much potassium causes weak growth that cannot support itself. Mottled foliage also indicates lack of potassium lower leaves are affected first, and yellow edges progress to brown until the whole leaf is affected.
Then there are the various trace elements iron, boron, manganese, and others which in minute quantities are important to healthy plant life. The commercial "all-purpose" plant foods contain the necessary trace elements, so unless you want to go into the matter rather thoroughly, just rely on them, changing brands occasionally to take advantage of the different emphasis of elements in the different formulas, and do follow the manufacturer's directions for mixing and applying.