Limiting Factors

2016-04-13 13.27.34

Spring has finally arrived here at 7,100 feet, and I’ve been feverishly planting—move the mulch, dig a hole, dump the perennial out of its  pot and stick it into the ground. Fill in any gaps with leftover dirt, replace the mulch. Rinse, repeat.

As I work around the lawn, adding flowers everywhere I can, I’ve noticed how abysmal my dirt is. Since we added compost, I assume that eventually it will qualify as soil, but right now I’m dealing with lumps of bentonite clay embedded in a deep layer of gritty, coarse sand. The clay was supposed to be seven feet down, but in the process of digging a basement, it got mixed  with the surface layers.

Since I highly doubt that there are any nutrients in this cold, wet dirt, I have been adding a time-release fertilizer. Yes, I know, I should take my own advice and get a soil test, but as there weren’t even any weeds growing here last summer, I have a pretty good idea of what I’m dealing with.

This is why I’ve been reading up on fertilizers. My previous garden, planted into soil I spent 22 years improving, had plenty of phosphorus and potassium, so all I ever concerned myself with was nitrogen. Now, however, I’m starting from scratch. And since this is far, far removed from topsoil, for this year, at least, I’m adding a balanced fertilizer—10:10:10. That means 10 parts nitrogen (N) to 10 parts phosphorus (P) to 10 parts potassium (K). (It’s traditional to list them as NPK, in that order; the fertilizer illustrated below contains twice as much phosphorus as nitrogen and potassium.) Next spring I’ll just add more nitrogen; P and K persist in the soil for years.

You may have read articles telling you that fertilizers high in potassium “promote root growth,” while those high in phosphorus “increase flowering.” Here’s a typical excerpt:

Phosphorus and potassium are the two main nutrients that support root growth in plants. …  This means that fertilizers high in phosphorus and potassium are especially helpful during the active growing season. But before you dose your garden with these nutrients, keep in mind that phosphorus also stimulates flower and fruit production—a bonus if you’re hoping for a colorful garden.

The problem with this advice is that it’s wrong. Fertilizers do not stimulate growth! Shocking, I know. Growth is actually triggered by the release of specific plant hormones. There is a very clever feedback loop that helps a plant keep its top growth and root growth in balance. (More about that in a future post.)

Once the plant decides to grow (roots, leaves, flowers, whatever), it will do so—provided it has the proper conditions and the resources it needs. “Proper conditions” might include sunlight, warm temperatures, and porous soil (so the roots can extend through it). “Resources” include water, oxygen, carbon dioxide, nitrogen, phosphorus, potassium, iron, and other micronutrients.

Anything on this list that is missing, that stops the plant from growing (or growing well), is called a limiting factor. Nutrients may be limiting factors, but they’re not the only ones. For example, a plant that is suffering from drought won’t grow well, no matter how much fertilizer it has.

miracle grow fertilizer highlight cropThe purpose of fertilizer is simply to correct any nutrient deficiencies in the soil so they are not limiting factors. While it’s true that plants need P and K for root growth, once they have enough, adding more won’t result in more roots.

On the other hand, you can have too much of a good thing. Excess use of fertilizers can bring nutrient levels to a toxic level. In some places (especially in the eastern US), homeowners have added so much potassium and phosphorus to their lawns that the grass is dying from an overdose!

Colorado soils usually have plenty P and K, along with calcium, sulfur, and other nutrients. (We have a lot of iron, too, but it’s often not available to the plants. Read more here.) Moreover, these elements stay put, persisting from year to year. That’s why I’m only using a balanced fertilizer once. Nitrogen, however, tends to disappear over time, and needs yearly replenishment.

So the next time you read an ad or receive some advice to “add potassium to stimulate root growth!” or “Use our high-phosphorus fertilizer for more flowers!” you can save your money. Even better, those savings mean that you can afford more plants!

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