Organic Fertilizer and NPK what it is and how to calculate it

Organic Fertilizers and NPK

Hi I am Stephen with Alberta Urban Garden Dot CA.

On todays episode we are going to talk about Organic Fertilizers and what and how to calculate NPK.

According to Colorado State University an organic fertilizer refers to a soil amendment derived from natural sources that guarantees, at least, the minimum percentages of nitrogen, phosphate, and potash.  Examples include plant and animal by-products, rock powders, seaweed, inoculants, and conditioners.

By definition these products have to guarantee at least a known amount of Nitrogen Phosphorus and Potassium or NPK

So what is NPK? NPK refers to the percent weight of that Nitrogen Phosphorus and Potassium makeup of the material in question. Generally the industry standard is to consider include the entire weight of the organic molecule as a part of this weight. Using Nitrate as an example the % weight includes the Nitrogen and three Oxygen atoms that make up N03

Nitrogen is available in three forms. Ammonia, Nitrate and Nitrite. NH3 NO3 and NO2
Phosphorus is most commonly found in P2O5 and Potassium in K2O.

So if you have results that represent the elemental numbers such as the lab results we have been presenting recently you will have to convert them to generate your NPK numbers so that you can compare then.

The first step to do this is to figure what percent the N P and K make up of their organic forms.

This is done using the atomic weight of the element in questions. The atomic weight generally represents the weight of the individual of each atom. Each atom on the periodic table has a different weight.

Lets use Potassium as an example

In the case of potassium the atomic weight is 39.10
Oxygens weight is 16.00

Meaning for K2O there is one oxygen and two potassium atoms to total weight of K2O is:

Total weight: (39.10*2) + 16 = 94.2

In order to figure out what % the nitrogen is simply divide potassiums combine atomic weight by the total combine weight.
Present weight of Potassium (78.2/94.2)*100 = 83.01%

The root equation here is K = 0.8301 x K2O

So to convert out K number to get the K2O we are seeking you divide our result by 0.8301

K = 0.8301 x K2O

K2O = K / 0.8301

And then divide by 10,000 to convert mg/kg (ppm) to percent (parts per 100)

Mg/kg / 10,000 = %

our final conversion of potassium:

(20000/.8301)/10000= 2.41

This represents our K number in NPK.

So as many of you know one of my goals this year is to test my own garden practices to see if the science holds true. In order to do this I have already presented the analysis of fall leaves and I have sent samples of coffee grounds, coffee, and comfrey to Maxxam analytics for assessment.

Over the next few weeks Ill be presenting these results for coffee grounds, coffee and comfrey to see if the results support my garden practices.

Ill be putting all of the results video in the Testing Garden Assumptions Playlist.

Colorado State University:

About Stephen

The Alberta Urban Garden Channel hopes to promote organic gardening that is simple, sustainable and does not have to cost a lot. We do this by investigating the Science behind gardening, methods, practices and products to make sure that you will have the best chance of successfully growing your own food at home.