Nitrogen Changes Potting Mix pH

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October 1, 2016

Container gardeners might want to pay closer attention to the type of nitrogen in their fertilizer – it can actually change your potting mix pH. Things like minerals in your tap water, or the amount of limestone in the growing media in a given pot can affect pH levels in any growing media, but nitrogen? Yes. Both the fertilizer you’re adding, and any that might come in the bagged potting soil you buy.

Nitrogen Forms Change Potting Mix pH DifferentlyYou need to know more about the kind you’re using than how much of it is in the NPK analysis, and whether it’s instantly available or slow-release. Those big numbers tell you at a quick glance if a fertilizer is formulated to feed leaves and structure, or flowers and fruit, or roots. But the only other information offered by them is what percent of 50 pounds of the product each macro nutrient represents. Double digit numbers may translate to better growth to the unsuspecting, but the plant won’t use most of it. Which, unfortunately, is the reason for fertilizer runoff. But back to the N thing…

There are 3 different kinds of nitrogen used in making fertilizers. They all provide foliage and stems nutrition, but plants process each one a bit differently, which does something to the pH level in the potting mix. The fertilizer label on the right just happens to guarantee that all the different types of nitrogen are present.

  • Ammoniacal Nitrogen
  • Nitrate Nitrogen
  • Urea Nitrogen

All fertilizer labels break this down below the total percentage of Nitrogen. If you’ve ever wondered why it’s displayed this way, now you see how it can be beneficial to have this information available for as long as you are using the product. You’ll find the same information on potting mixes that contain fertilizer, and bagged composts too.

Most garden fertilizers contain only nitrate or ammonium as nitrogen. Some have both. There are a few that list urea, but it’s more common in farm and lawn fertilizer. The only deviations from identifying the form of nitrogen I’ve found is with organic fertilizers and compost. Some have a break down, while others do not. However, plants react differently to organic nutrients than they do to synthetic nutrients.

Ammoniacal Nitrogen (NA4+)

Once taken up, a plant’s root system converts this form of nitrogen to ammonia (NH3). This causes the release of positively charged hydrogen through the roots. As a result of this chemical process, the potting mix pH drops. How much will it fall? That depends on how heavy the nitrogen content is. Another time when bigger isn’t always better, unless you’re growing something that needs low pH.

Microbes in your soilless container growing can also make ammoniacal nitrogen available to your plants. This happens through nitrification, when the microbes release the hydrogen that cause potting mix pH to drop. But cool conditions slows or stops their activity, so nitrification is inefficient when media is at temperatures below 50°F.

It’s doubtful that the container vegetable gardener would be doing much growing in temps below 50°F, especially in an indoor garden. However, there are food crops we can grow in cooler weather, and if you’ve got the grow bug, your gardening might spill over into strictly ornamental plants. So, it’s good to know what happens with ammoniacal nitrogen in spring and fall weather conditions.

You don’t want to use fertilizers with high concentrations of this form of nitrogen beyond the warm season, or without a heat source. Without the microbes breaking down the ammoniacal nitrogen, ammonia toxicity starts showing up with chlorosis and necrotic spots on older leaves.

Nitrate Nitrogen (NO3)

The chemical reaction here is the opposite. Plants’ roots release a negatively charged OH- or HCO3- anion after taking up nitrate nitrogen. Naturally, this has the opposite effect on your potting mix pH – the level raises instead of dropping. The higher the concentration of nitrate nitrogen, the farther your pH will climb.

Urea Nitrogen ((NH2)2CO)

Urea: Cheap, Unstable Form of NitrogenThe reason it is most commonly seen in ag and lawn fertilizers? Urea is a cheap source of nitrogen. When a cropland needs only nitrogen enrichment, like an N hungry cornfield, it’s the perfect solution for a farmer. Very economical. It also grows the part of a lawn a homeowner is most interested in really fast – the emerald green leaves. Lawn grasses are also heavy nitrogen consumers. The low cost of urea nitrogen is probably why it is the bulk of N nutrition is Miracle Gro Plant Food, and why the label recommends you should apply it as often as every week. It’s a here today, gone tomorrow treatment unless incorporated into the soil or medium. Urea converts to nitrogen gas in a matter of days if topically applied.

Conventional fertilizers that contain urea use a synthetic form that is classified as an organic chemical, though it is far from organic. However, you might find it listed in the analysis of natural or organic fertilizers and soilless mixes. And that’s why it was included in the analysis image earlier in this post, it’s from a natural fertilizer. The reason for this is that urea was originally the name for the nitrogen content in urine, and all composted poultry or animal manures contain urine. Totally different than it’s manufactured facsimile.

Urea nitrogen carries no charge, it is neutral. Therefore, it has no effect on your potting mix pH when taken up by plants. However, microbes rapidly break this form of nitrogen down, converting it to ammonium and carbon dioxide. As a result of microbial activity, the plants take it up as ammoniacal nitrogen, triggering hydrogen emissions. Those in turn cause pH levels in your media to increase.

Organic Nitrogen

Microbes must always break down any source of natural nitrogen to ammonium or nitrate before it’s available for plants to take up. Consequently, the effect on your potting mix pH is not so cut and dried. Whether it causes an increase or decrease in PH depends on how the microbes break down the organic component. The only way to know for sure is testing the growing media’s pH.

Other Factors

Besides through nitrification, the pH of your potting mix is only affected when the plant takes up the nitrogen. So, when you have immature plants, or they are dormant, very little nutrient is being used. As a result, the amount of hydrogen being emitted will also be greatly reduced compared to actively growing or larger plants in the same container size with the same fertilizer.

To better optimize your gardening results, pH testing the potting mix is best – especially with crops that are super pH sensitive. Do it regularly. That will allow you to keep a good grip on the availability of all macro and micronutrients. It’s a lot easier to do a slight pH up or pH down adjustment periodically, than to identify and correct a nutrient imbalance. A lot more crop-friendly too. Plants produce more willingly when they aren’t suffering from a dietary deficiency.

Tammy Clayton

Tammy Clayton

Contributing Writer at Garden Culture Magazine
Tammy has been immersed in the world of plants and growing since her first job as an assistant weeder at the tender age of 8. Heavily influenced by a former life as a landscape designer and nursery owner, she swears good looking plants follow her home.
Tammy Clayton

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