Most home gardeners waste a great deal of money on garden fertilizers, primarily because they either apply a lot more product than is needed or they apply the wrong products.
Fertilizer should be applied to garden soil based on:
- The soil’s natural fertility,
- Quantity of organic matter that is present in the soil,
- The type of fertilizer being used,
- The vegetable or fruit being grown.
There is only one way to determine the current condition of the soil and that is to have the soil tested by a laboratory. When you have received the test results, you will be able to determine what areas of the soils structure need to be improved and hence, choose the correct formulation and type of fertilizer.
Soil testing is available from:
- Most universities and colleges that offer agriculture programs,
- State Agriculture Departments,
- Private laboratories,
- Garden Centers,
- On-line garden facilities.
Fertilizers, by law in the US and Canada, must provide an analysis of the three main elements of their fertilizer’s formulation, commonly known as N-P-K, which we mistakenly refer to as nitrogen, phosphorus and potassium when in fact the N stands for nitrogen, P for phosphate (P205), and the K is for potash (K2O).
The numbers given, on the bag or box, are based on each elements amount by a percentage of weight. As an example, a 50-pound package of fertilizer labeled 10-10-10 would have 5 pound of N, 5 pound of P205 and 5 pound of K2O. The conversion is as follows:
P205 (phosphate) to phosphorus multiply the weight by 0.43.
K2O (potash) to potassium multiply the weight by 0.83.
In our example of the 10-pound package of fertilizer, you would have the following elements:
Nitrogen = 5 pounds.
Phosphorus = 2.15 pounds.
Potassium = 4.15 pounds.
Fertilizers and pH:
The pH of the soil is as critical as the elements available for plant nutrition as pH extremes can stop the nutrients from being available to the plants. Your soil analysis could show that the elements are in fact in the soil but with a pH around the 8.0 level elements such as phosphate, iron, and magnesium will not be absorbed by the plant roots. If the pH is 4.5 or lower, elements such as calcium, magnesium, and phosphorus will not be readily available for the plant to absorb. In some cases, having the wrong pH can increase the availability of certain elements to a level whereby they are toxic to the plant. Soil that contains aluminum at a low pH will have a toxicity problem. The pH can be raised by the addition of lime. Vegetables do best with a pH between 6.0 and 7.0.
Believe it or not, a prosperous vegetable garden requires 16 elements that are essential for healthy plants. The air and water provide three of the elements - hydrogen, oxygen, and carbon. Plants also require three elements in large quantities; termed macronutrients, these are the elements that are contained within commercial fertilizers - nitrogen, phosphorous and potassium. Three other elements that are essential and also considered to be macronutrients are calcium, magnesium, and sulfur.
Most home gardeners do not have to concern themselves with these three elements as they are generally present in most soils and are added by the use of materials such as lime.
The seven micronutrients are required in trace amounts and do not present a challenge to most home gardens as they are normally present within the soil and are enhanced with proper soil preparation techniques.
It is important to use both synthetic and organic fertilizers. The trace elements that are needed for a healthy garden are present in the organic fertilizers. As synthetic fertilizers are chemically formulated, they are a relatively pure product without additional trace elements within them. The best organic fertilizers are manure, mulch, and compost.
It is impossible to know what supplemental fertilization is necessary in your vegetable garden beds, to produce abundant and healthy plants, without soil analysis. It would be like going to the pharmacy and buying an assortment of pills, because you don’t feel well, without having a doctor determine what is wrong with you first. Once you know what is lacking, you can purchase the correct fertilizer and use it in proper amounts.