Why do fertilizers for the same plant type have significantly different ratios?

Question

We have a beautiful, large citrus tree that hubby grew from a seed 25 years ago. It lives in a pot due to cold winters here, but spends as much time as possible outside - better for the tree and us. Now that the days are getting warmer, the tree seems "hungry" and the growth phase should kick in.

Obviously potted plants need extra nutrition and so we went to our local garden center to get a new fertilizer. And promptly started wondering:

• Brand A
  3-1-5 NPK plus 0.8% S (sulfur), 1.5% Na (sodium), 0.3% Fe (iron)
• Brand B
  3.5-6 NK plus 0.01 Fe
• Brand C
  7-3-6 NPK plus 3% S (2% soluble S), 1.0% Na, 0.1% Cloride

All are labelled as fertilizers for citrus plants. And while I see some similarity between A and B, C is in a totally different range.

So what should we do? And why do I have that nagging feeling that I'm missing an essential part of the big picture here?

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Answer 1

I'm with Giacomo in that specific NPK ratios for certain plants seems to be mostly marketing but there may be other micronutrients and elements included that benefit certain types of plants. Iron, magnesium, sulfur, calcium, pH adjusting additives, etc.

The plant doesn't care what the NPK ratio is on the bag. It only cares about whether there are enough accessible nutrients in the soil.

One popular approach is the "sufficiency level" approach. If your soil has enough potassium for example, you don't need to add more potassium. Nitrogen gets used up pretty quickly and leaches out of the soil with water so the addition of that is usually required.

Soil testing by a good lab will tell you exactly what's in the soil and provide recommendations on how much N, P and K to add based on what you tell them you're growing. You're better off having your soil tested than trying to guess which bag's NPK formula is best for you.

Check your local university's cooperative extension office if you're in the US. You can use out-of-state labs as long as your state is not on their exclusion list. I found the prices at UConn soil testing lab reasonable.

How much nitrogen, phosphorous and potassium the plant actually winds up using, and how much washes away is going to vary based on different growing conditions. I believe fertilizer manufacturers formulate their NPK ratios based on generalizations they think will work best.

If you really want to provide optimum nutrition to your plants it's worth doing a soil test every 1-3 years, add the recommended lbs of NPK based on the results and keep track of what you add and how the soil changes over time to see if the plant is getting everything it needs or if you're adding too much of something and it's building up in the soil. You may notice that phosphorous keeps increasing so you can cut down on the amount of phosphorous you add.

If you don't want to do soil testing you can just pick a bag of a fertilizer you think is good and see how the plant responds. Various nutrient deficiencies (or sometimes excesses) will cause physical changes in the plant or different results that you can see. Make adjustments based on that.

Answer 2

There is no correct ratio, so manufacturers label suitable fertilizers as useful for citrus plants.

Additionally the ratio has also not a real meaning alone. Plants care about nutrient substance, so you need to multiply the ratio with the quantity you supply to the plant.

I usually don't care about Sulfur, Natrium and Cloride (the other part of molecules containing N, P or K, these are chosen for slow or fast release and acidity).

Mg, Fe and B are other essential elements, but plants uses them in a small quantity, so ground and a fertilizers every few years should be enough. (But it depends on the plant and fruits).

The ratio are calculated from what a plant will give away every year, so the extra annual wood, the foliage (but on professional cultures, the foliage is not accounted: it is assumed that it will remain on ground, so on long term it will balance) and harvest.

So foliage and harvest have significance, but these depends a lot on variety and how it is cultivated.

Answer 3

3-1-2 is a general "all-purpose" NPK ratio, and it's a proper ratio for container citrus, afaik.

I've never seen 3-1-5 recommended for Citrus - I'm not a citrus grower but I'm familiar with them.

I make my own plant food nowadays, but for this guy, fert-wise, I'd recommend all-purpose miracle-gro shake and feed. But don't shake it - dig a 1"-1.5" trench (looking like a moat) in the soil about halfway between the trunk and the pot, all the way around, and apply the CRF there. Then cover the CRF. I'd also add a pinch of epsom salt into the trench.

I reco the MG because it's got a good supply of micronutrients. Lime is an even better micronutrient amendment (use lawn lime - garden lime is slightly more concentrated and 5x the cost, so buy lawn lime and use a tiny bit more).

I'd also look into how much woody growth you should be keeping every year. Like I said I'm not a citrus grower, but for typical container trees that's far too much wood. It's too much green competition for an apple tree as well, so there may be sunlight discrepancies with some of the growth.

Okay, I'm starting to write a novel. Breakdown:

• Use Miracle-Grow All-Purpose (12-4-8) Shake and Feed
• Apply it trench style with a pinch of Epsom
• Micronutrients don't displace a plant's uptake of NPK unless ridiculously overused, so if the soil is old you can throw some lime on.
• Look into how much woody growth to prune yearly - the trunk is mighty and good-looking but there may just be too much wood
• Check some youtube videos on pruning citrus (seems too competitive to me)
• Make sure your growing medium is properly amended for aeration and drainage
• Look into proper citrus root pruning for the next time you repot