Living SoilHave you been told that one bag of synthetic fertilizer will kill the life in your soil? Or have you been told the opposite, that it makes no difference what you feed with, as long as the plant gets its nitrogen? If you are trying to build living soil, which one do you believe?
It matters, because the wrong answer either costs you a harvest or quietly costs you the soil. Here is the surprising part: the extension sources themselves do not agree. So rather than pick a side for you, this article shows both, and then the ground they share.
The claim: chemical fertilizer kills soil life
A master gardener handout puts it plainly: chemical fertilizers negatively impact the soil food web by killing off entire portions of it. Its advice is to avoid additives with high NPK numbers, along with rototilling and walking on your beds.
The other side: fertilizer feeds soil life through the plant
An extension article on soil microbes calls this a common misconception. It agrees that fertilizers may harm microbes directly exposed to granules or to anhydrous ammonia. But it says the overall effect of fertilizer is to markedly increase microbial numbers and activity in the soil, through increased plant growth. It adds that many studies of turfgrass, in sports fields, golf courses and home lawns, have shown soil microbial populations are not compromised by normal management practices.
The middle line
A third source, a gardener handbook from extension, takes a position between the two. Synthetic fertilizers, it says, may not support beneficial microbial populations to the same extent as natural fertilizers. Another extension page says organic fertilizers support beneficial soil microbes and improve workability.
The big idea: the sources argue about whether fertilizer harms soil life. None of them argue about too much fertilizer. That is where the damage everyone agrees on happens.
Synthetic vs organic fertilizer: what really differs
Synthetic fertilizers are more water soluble, so their nutrients are available to plants more quickly. The cost of that speed is that it is easier to overapply them and burn plants. Their nutrients can also wash away quickly, so they may need applying more than once a season.
Organic fertilizers that need processing by soil organisms work on the soil's schedule: they are more available when the soil is warm and moist, and less available when it is cool or dry. But organic does not mean harmless. Even natural sources of nutrients can be overapplied and damage plants, and fresh manure can carry excess salts, weed seeds or even human pathogens.
Over fertilizing: the salt problem
Most fertilizer materials are highly soluble salts. Too much of them pulls water out of the plant, which is why fertilizers carry filler to make burning less likely. In dry weather the risk is higher, because fertilizer salts can burn roots when there is not enough moisture, so feeding should be reduced or delayed.
Salts build up in soil when fertilizers are applied excessively or irrigation water is poor. High salt levels dry roots out, and plants can wilt even when water is plentiful. Once you have a salt problem, the only way to get rid of it is to leach the salt out of the root zone. For how, see fertilizer burn and how to flush soil.
Fertilizer runoff: where the extra goes
Even the source most relaxed about fertilizer and microbes warns that excessive applications will likely lead to more runoff and leaching. The nitrate form of nitrogen is highly mobile: it moves through soil after rain or irrigation and can contaminate drinking water. Phosphorus holds tightly to soil but reaches water through runoff and erosion. Excess nitrogen and phosphorus are associated with algal blooms, low oxygen and fish kills in lakes, bays and still water.
The fixes are simple. Apply only what a soil test recommends. Use slow release fertilizers where you can, and work them into the soil. Avoid excess nitrogen and phosphorus. Turn off the water when the soil stops absorbing it.
What soil life does need
Whichever side of the argument you lean toward, the research on feeding soil life is not in dispute. Microbes need a constant supply of organic matter, or their numbers decline. To promote soil organisms, incorporate organic matter, till as little as possible, avoid compaction, keep pH and fertility in range, and use organic mulch on the surface. Fertilizer, of any kind, is one part of that list, not a replacement for it.
Where to start
Get a soil test before you feed, and feed only what it calls for. Hold off feeding in dry spells, and turn off the water when the soil stops absorbing it. Then put your effort into organic matter and mulch, the care of soil life that no source disputes. To see what that web is and does, read what the soil food web is.