This is a basic introduction to the complicated topic of fertilizers.
Plants need certain elements in order to flourish, in addition to the obvious water. Carbon dioxide is needed to build carbohydrates and cellulose in the process of photosynthesis. Nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron and other trace elements contribute to growth.
These elements all have an impact on growth, health and appearance, and deficiencies or excess of nutrients can lead to contorted or discolored leaves and stems. In a natural ecosystem, nutrients are released from soil particles, taken into the plant, and then are rejected back into the soil as organic waste decomposes.
Native plants are adapted to the particular mix of nutrients naturally found in the soil of a given area.
In a garden, the soil has been disturbed and possibly topsoil is depleted, and plant parts are harvested or thrown out instead of being cycled back. Plants from many different origins are gathered together, and their individual requirements may differ widely, bringing a need for soil additives.
It is good to feed the soil at planting time, and then to top dress regularly, paying careful attention to the directions on the bag of fertilizer chosen. More is emphatically not better. The manufacturer has designed the product to fulfill certain needs, and to exceed these directions will not improve performance and may cause lasting damage.
On a bag of commercial fertilizer, you will see three numbers, such as 5-10-15. These numbers tell the percentages of nitrogen (5 percent, in this example), phosphorus (10 percent) and potassium (15 percent) in the product.
If the three numbers are the same, it is a balanced fertilizer suited for all purposes. Some brands will contain other trace elements, and this always will be listed.
Iron (Fe), magnesium (Mg) and sulfur (S) frequently are found.
Nitrogen is always the first number and is important for the growth of leaves and stems. A nitrogen deficiency will show up in stunted stems and yellowish leaves, while too much will turn top growth dark green and delay ripening. It is usually necessary to provide nitrogen at the beginning of the growing season.
In addition to its pride of place in purchased non-organic fertilizers, nitrogen occurs naturally in organic fertilizers such as compost, manure, bone meal and other long-lasting forms.
Phosphorus is necessary for good root growth and for the production of good fruit and seed production. Plants short of this element may have leaves that are smaller or darker than desired, and development will be under expectations.
Rock phosphate frequently is used by organic gardeners and is a good source of phosphorus if the soil already is rich in humus.
Potassium encourages overall growth and strength, brings some immunity to disease and makes plants more tolerant of excessive heat or cold. The lack of potassium may show up as poor fruiting and browning of leaf edges. It is found naturally in compost and manure and in ground natural mineral sources.
Animal manures are excellent plant nutrients, but vary widely according to their source. Cow manure should be covered and stored until well rotted. Poultry manure is rich, and is best combined with some organic substance such as straw before storage and use.
The animal manure sold in bags is useful, but has to be used in large quantities to equal the effectiveness of the product straight from the farm.
Compost, either homemade or commercial, has values lower than animal manure, but provides excellent humus, is readily available and can be the mainstay of the home garden.
This whole subject is very complex, and I do not pretend to be an expert. A soil test will tell you if you need soil additives and the lab where you get this done will generally advise on amounts needed.