If a little is good, a lot is NOT better! As Blaine Rogers used to say when he taught botany at Columbia College, “Life exists within a very narrow range (pH, temperature, nutrient levels) on the surface of a membrane.” For example, very little exposure of our skin to sunshine is a good thing. It allows our bodies to create Vitamin D, uptake calcium and store it to make stronger bones. If our skin is exposed to too much sunshine, however, it is not good. Damage results, ranging from sunburn to damaged DNA and, eventually, skin cancer.
Our recent drought has clearly demonstrated the tree mortality that comes with too little water. On the other extreme, most of us have experienced watching a plant die with all the symptoms of wilting while we continued to give it water, only learning too late that the plant had received too much water and its roots had rotted away.
So, too, it is with fertilizer. Many people believe that if a little fertilizer is a good thing, a lot more will create healthier plants. Not so. Fertilizer should never be added “just because.”
Many people are aware that our foothill soils are considered to be nitrogen poor. The intuitive response might be to make sure that all plants have plenty of nitrogen. However, dosing plants with a “miracle”-type fertilizer containing more nitrogen than the plant can utilize will have detrimental effects. In one Chinese study (Yang 2004), “Vegetable yields were not increased continuously with N rate, and an excess input of N fertilizer more or less reduced plant growth, leading to yield decline for the earlier harvests.”
Nitrogen can move through the soil in response to rainfall or irrigation run-off. Nitrogen not utilized by the plant is free to move away. It will eventually arrive in a local water body – stream, pond, lake or ocean. In the water, nitrogen is available to organisms that also need nitrogen to grow. And grow they will, creating “pond scum,” algae “blooms,” and eutrophic water bodies (meaning too rich in nutrients). Eutrophication causes dense plant growth. Excessive plant growth (and subsequent death and decomposition) deprives the water of oxygen, resulting in the death of animal life (U.S. Geological Survey http://toxics.usgs.gov/definitions/eutrophication.html).
Phosphorus (the “P” in the N-P-K formula on the fertilizer box or bag) also contributes to eutrophication. Standard application of a “balanced” fertilizer like 12-12-12 or 16-16-16 can provide a higher concentration of nutrients than your garden plants need or can use. Once again, phosphorus (in the form of phosphates) can move through the soil, creating long-term consequences beyond the boundaries of your garden.
A simple test kit from the local garden center can provide you with a means to determine if your soil needs fertilizer at all. And, if you need a more in-depth professional soil analysis, there are labs located in the Modesto area (http://www.al-labs-west.com/). Consider not adding synthetic fertilizers to your garden, but substituting a layer of compost or an organic fertilizer instead. Compost, though low in nutrient concentrations, seems to provide time-release access as compounds in the compost are broken down into forms that plants can utilize.
According to the Sunset Western Garden website, http://www.sunset.com/garden/garden-basics/crash-course-fertilizers, organic fertilizers are derived from the remains of living organisms while synthetic fertilizers are derived from chemical sources. Because these (organic) fertilizers act slowly, it’s almost impossible to kill lawns or plants by applying too much (overdosing with synthetics, in contrast, can have potentially fatal results).
So, remember: be judicious in your application of fertilizers: if a little is good, a lot is definitely not better.
Rebecca Miller-Cripps is a University of California Cooperative Extension Master Gardener of Tuolumne County who uses home-made compost to nourish the soil in her garden.