Plant nutrition and health cannot be understood by focusing either entirely on fertilization or entirely on soil microbes and life. Chemical fertilizers affect a number of soil aspects that, in turn, affect soil microbes that can adversely affect soil health and plant growth in future years. Gardening or farming with chemical fertilizers involves balancing plant nutritional needs with soil health requirements.
Changes in pH
In addition to adding nutrition that helps plants and some microbes, chemical fertilizers can sometimes cause soil to become more acidic. In other cases, they can cause the soil to become more alkaline. One of the reasons many fertilizer manufacturers recommend having your soil tested is to understand how the chemical fertilizer will affect soil pH. By testing soil, you can use a chemical fertilizer to intentionally adjust soil pH to a more healthy level or avoid unintentional changes in soil acidity.
Effects on Microbes
Certain enzymes can only live at a particular pH. By changing soil pH, chemical fertilizers can cause certain enzymes to disappear. Because some microbes feed exclusively on particular enzymes, a change in soil pH can result in a drop in some soil microbe life. In some cases, fertilizers like ammonium sulphate can have a negative impact on nitrogen-fixing bacteria, beneficial nematodes and earthworms.
However, some chemical fertilizers can also have a beneficial effect on microbes. Phosphorus and calcium in proper proportions can help many soil microbes. Those microbes, in turn, can help transfer nutrients to plants and help to break down existing soil nutrients. The key to keeping soil healthy is to avoid over-fertilization.
Effects on Other Organics
If a soil is over-fertilized and microbes are adversely affected, natural organic nutrients in the soil may not have the microbes necessary to turn them into usable plant nutrients. A natural reaction to this situation might be to add more chemical fertilizers, which would only result in more soil damage. After a number of planting seasons, more and more chemical fertilizers are needed as the health of the soil decreases.
Crop rotation and a periodic augmentation of soils with organic material can help to avoid this cycle of increasing fertilizer needs and decreasing soil health. This helps to increase trace elements and amino acids that encourage healthy soil microbes.