I had the good luck of talking with a knowledgeable feed store employee. The subject was the big 3: nitrogen, phosphorus, and potash. My concern was the green beans would not get enough phosphorus and potash. They do not need nitrogen since they are a legume and produce their own. His response was: do not worry about it.
Phosphorus and potash are very insoluble and move very little in soil. They are not affected by weather conditions. Nitrogen is leached out of the soil by rainwater and usage by roots so it must be replenished periodically.
Nitrogen
Nitrogen gives plants their dark green color and increases leaf and stem growth. The crispness and quality of leafy vegetables such as lettuce and spinach is influenced by nitrogen levels. Plants deficient in nitrogen have light green to yellow leaves and appear stunted.
Phosphorus
Phosphorus encourages root growth and establishment. Phosphorus is also crucial for plant flowering and fruiting, especially seed production. Most of the internal plant chemical reactions are dependent on phosphorus. Poor flowering and fruiting may be signs of the lack of phosphorus. Some plants, including corn and tomatoes, may exhibit red or purple leaves. Cold soils can prevent phosphorus uptake, even though the element is present.
Potash
Potassium or potash increases the plant’s vigor, winter hardiness and resistance to diseases. Stems and stalks are stiffer. Plant seed or fruit yield is improved. Reduced vigor, susceptibility to diseases and thin skinned or small fruit may be signs of potassium deficiency.
Fertilizer Contents
A complete fertilizer contains all three nutrients: nitrogen, phosphorus, and potash (potassium). The fertilizer bag has three numbers on it: like 12-10-8. The first number is nitrogen, second is phosphorus, and the third is potash. Each number is the percent weight of the element. For example a 50 pound bag of 12-10-8 has 6 pounds of nitrogen, 5 pounds phosphorus and 4 pounds of potash. In our example there is 15 pounds of nutrients and 35 pounds of filler: ground corn cobs, sawdust, vermiculite, and other which makes it easier to apply the fertilizer.
Application
I use a balanced fertilizer, like 12-12-12 or 10-10-10, on other vegetables like corn and potatoes. I admit I do not have the soil tested but I do rotate crops to mitigate the spread of disease. Application is easy: run a line of fertilizer on each side of the row of plants about 6 inches from the stem. Work the fertilizer in the soil with a hoe. Water immediately to start the fertilizer process or wait for rain. Do not let the fertilizer touch the plant or else it will burn it.
Using time-release fertilizer is not a good idea since it releases nutrients based on environmental conditions rather than plant needs.
Soil Testing
If you cannot measure it you cannot control it. A soil testing kit
Test the first 6 inches of soil for phosphorus and potassium. Nitrogen is tested in the first 24 inches of soil.
Doug