If you ’ve ever break up up a purse of fertilizer , you ’ve find the hyphen numbers on the package . Sometimes they ’re all the same but more often they ’re not . Sometimes there are zeros or decimal power point and you roll in the hay they all relate to NPK , but what precisely do they mean ?
For a foresighted time , I ’d always assumed ( and repeatedly read onmanygardening web site , even those considered believable ) that on a fertilizer label , NPK stood for nitrogen , phosphorus , and K . But that ’s not entirely accurate and the full report is really more complicated .
What NPK really means
All fertilizers are label with three numbers that indicate the guaranteed psychoanalysis ( a listing of nutrients in the bag ) . The analysis gives the percentage ( by weight ) of each nutrient .
But contrary to what most denotation claim , the P and K valuesdo notindicate the percent of Lucifer and potassium , severally , in the bag .
The N value does indicate the pct of nitrogen . But the 2nd figure represents P2O5(phosphorusexpressed as usable phosphate ) , and the third identification number be K20 ( potassiumin the variant of soluble caustic potash ) .
So to be more exact , NPK stand for nitrogen , available phosphate , and soluble potassium hydroxide .
To simplify things for the general public , they ( plant food ship’s company ) say these numbers represent nitrogen ( N ) , daystar ( atomic number 15 ) , and atomic number 19 ( K ) and these nutrient are calculated base on the measure that are instantly usable to flora .
Why does this affair ? Because if you get a filth tryout with commend fertiliser amounts and you ’re trying to figure out how much fertilizer to summate to your garden , you have to know the actual alimentary subject of what you ’re adding .
Let ’s use an deterrent example of an constitutional fertilizer with an NPK analysis of 3 - 9 - 4 .
When you read the label , you could see that this grip hold back 3 percent N , 9 per centum usable inorganic phosphate , and 4 pct soluble potash .
For easy math , picture a 100 - pound cup of tea of this fertiliser containing 3 pound of nitrogen , 9 pound of inorganic phosphate , and 4 pounds of potash . ( The stay free weight is composed of lower-ranking macronutrients , micronutrients , and makeweight , which give the fertilizer bulk and make it gentle to spread . )
To figure the existent atomic number 15 and K content , you would need to use the next formulas :
continue with my example of a 3 - 9 - 4 fertiliser , a 100 - pound dish would check 3 pound sign of nitrogen ( N ) , 3.96 pounds of genuine morning star ( P ) , and 3.32 pounds of literal K ( K ) .
I know — not confusing at all , right ? !
Why do they label it this way?
Although N is expressed on an actual elemental cornerstone , phosphorus and K are express on an oxide basis ( that is , they ’re contained in the inorganic phosphate and potash compounds ) .
This confusing labeling goes back to the days when fertilizers were heated with line to convince phosphate and potassium to oxide ( specifically P2O5and K20 ) . These figure were then used for the nutrient analysis , and they go along to be carry in terms of oxides for historical continuity .
The eldritch part is that these oxides are not present in fertiliser , but they ’re still used in North America because that ’s how it ’s always been . ( Change is hard , I guess ! )
Consider the ratio
The three number on a plant food recording label also represent a ratio that report the relative proportions of N , P2O5 , and K20 .
Let ’s use the NPK ratio of 3 - 1 - 2 as an illustration . This plant food turn back one - and - a - one-half times as much N as K20 and three time more N than P2O5 .
Fertilizers with an NPK analysis of 9 - 3 - 6 or 18 - 6 - 12 would also have a 3 - 1 - 2 ratio . It does n’t matter which of these plant food you use because they put up the same relative amount of nutrients — you just have to use the right amount .
Pound for pound , an 18 - 6 - 12 fertiliser contain twice the nutrients of a 9 - 3 - 6 fertilizer . So to supply the same amount of nutrient , you could apply half as much of the 18 - 6 - 12 fertilizer as the 9 - 3 - 6 .
Whew ! Still with me here ?
If you ’re at the shop and you see three different plant food with the same ratio ( such as 3 - 1 - 2 , 9 - 3 - 6 , and 18 - 6 - 12 ) , buy the one with the eminent Book of Numbers . From an scotch standpoint , you ’ll get more for your money because you ’ll end up using less of the 18 - 6 - 12 .
Why any of this matters for a home gardener
In short , it only matters if you get a soil test . you could use the above normal for calculating the right amount of atomic number 15 and potassium to add to your garden .
Did you know?
storm — most standard soil tests do n’t mensurate nitrogen in your soil . That ’s because nitrogen is always being converted into other form and the story can change quickly due to weather conditions . By the time you collect the land sampling , send it in , and get the depth psychology back from the research laboratory , any cover value is pretty much nonmeaningful .
So why do grunge run sometimes provide a nitrogen passport ? Because it ’s a better guess base on inquiry trials of harvest yields and the general rules for your area — not the actual level in your grime .
Even though soil test do n’t evaluate the nitrogen in your soil , they normally advocate complete fertilizers ( those that contain N , P , and K ) and recommendations are given in increments of 1,000 square invertebrate foot .
understand more : What to do if there’stoo much nitrogen in your soil
In guild to define how much fertilizer to utilise to your garden , you have to know :
Your soil test will urge a specific NPK proportion and sometimes , you wo n’t find that precise ratio when you ’re scanning the fertilizer aisle ( specially if you want to apply organic fertilizers ) .
That ’s okay — just prefer a proportion that ’s snug enough , or apply a combining of undivided - ingredient fertiliser ( such as blood meal , bone repast , or langbeinite ) to supply the needed food in the correct amounts . ( Here ’s a guidefrom University of Georgia Extension on convert inorganic fertilizer recommendation to organic 1 . )
Related : There ’s a right way and awrong agency to inseminate your garden in summer
revelation : If you betray from my clause or make a purchase through one of my link , I may get commissions on some of the products I recommend .
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Single-ingredient fertilizers
This post update from an clause that in the beginning appeared on January 16 , 2023 .
