University of Maryland Extension

What is a "balanced diet" for plants?

Author: 
Neith Little
Farmer and consultant meet in a field of strawberries

(Photo credit: Image # K9186-1 by Scott Bauer, USDA-ARS)

Recently a farmer asked me the following question: “I read an article that suggested using a ‘balanced’ fertilizer. What does that mean? Does ‘balanced’ mean the N, P, and K should be the same?” 

This is confusing, because different people use the word “balance” differently when writing about nutrient management. I have seen at least one article that suggested using a “balanced” fertilizer that has equal amounts of the three macronutrients: nitrogen, phosphate (P2O5), and potash (K2O). But in most cases, I don’t think that’s good advice. 

To me, a “balanced diet” for crops means a fertilizer or combination of fertilizers and soil amendments that provides nitrogen, phosphate, and potash in the ratios that the crop needs. There are very few situations in which a crop will need equal amounts of nitrogen, phosphate, and potash. 

For example, imagine your soil test comes back recommending you fertilize your tomato crop at the start of the season by applying 45 lbs of nitrogen per acre, 100 lbs of phosphate, and 200 lbs of potash, and then side-dressing later in the season with 45 more lbs of nitrogen per acre and no more phosphate or potash. If you tried to use a fertilizer with an N-P2O5-K2O ratio* of 10-10-10, you would end up either over-applying or under-applying two out of the three macronutrients. For example, if you applied enough 10-10-10 to apply the recommended 45 lbs of N per acre, that would also only supply 45 lbs of P2O5 and 45 lbs of K2O per acre. 

This is why fertilizers that supply only one of the big three macronutrients are so popular: because you can apply them together to reach custom ratios of nitrogen to phosphate to potash. 

Similarly, this is one of the reasons using compost or manure as your primary fertility source can be tricky. In the long-term, they do tend to build your soil’s ability to supply nitrogen, phosphate, and potash. But in the short term, they might not supply those macronutrients in the ratio that your crop needs them. Say you have a composted manure with an N-P2O5-K2O ratio of 0.7-0.3-0.4. If you tried to use that compost to apply the recommended fertility for your tomatoes, again you would end up either over-applying or under-applying two out of the three. 

So in this situation, if you wanted to apply the recommended nitrogen, phosphate, and potash as a combination of urea (46-0-0), triple superphosphate (0-45-0), and potassium chloride (0-0-60) you could calculate how much to use as follows (and you thought you’d never use algebra):

 

I used the single-nutrient sources as an example because they are the simplest to calculate, not out of any bias against compost or other organic fertility sources. Figuring out how much composted manure to use requires a couple more steps, so I won’t subject you to more math right now, but fear not, it is possible. And we at UMD Extension are here to help. 

Thank you for your patience, and if you’d like to get into compost calculations next time, let me know! 

Neith Little, Urban Agriculture Extension Educator

nglittle@umd.edu

 

*Note: As a source of further confusion, in the US most fertilizer labels and recommendations are calculated as available nitrogen (N), phosphate (P2O5), and potash (K2O) instead of elemental nitrogen (N), phosphorus (P), and potassium (K). But for pronounceability’s sake the N-P2O5-K2O ratio is often referred to as the N-P-K ratio. Because we pretty much all use the same system, you shouldn’t ever have to convert between available phosphate (P2O5) and elemental phosphorus (P), so you don’t really need to worry about it. N-P2O5-K2O and N-P-K should mean the same thing in most situations where you will encounter them. Unless you go to Europe, or into a research lab. Then all bets are off. 

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