General Purpose 4-3-9 Fertilizer

Calculating Nutrition

All fertilizer labels have three numbers. The first number is the amount of nitrogen (N), the second number is the amount of phosphate (P₂O₅) and the third number is the amount of potash (K₂O).

These three numbers represent the primary nutrients (nitrogen(N) – phosphorus(P) – potassium(K))

10-10-10 = 10% nitrogen, 10% phosphate and 10% potash

Additional numbers with abbreviations are sometimes used at the end of the 3 digit code to represent other significant nutrients such as sulphur and calcium.

Calculating Nutrient Content

To calculate the nitrogen in an 8kg bag of 4-3-9 fertilizer (Orgunique General Purpose granular), multiply 8kg by 4% (0.04) which equals 0.32kg (320g). You can do the same for phosphate and potash by multiplying by their percentages. In this example we see for each bag contains 1.28kg of nutrients; 0.32kg N, 0.24kg P, 0.72kg K.

Calculating Your Needs

Each soil is different and it is best to do a soil test analysis to understand what nutrition is available in the soil. Even though some tests may show you are sufficient in a nutrient, other soil conditions such as pH may make the nutrients unavailable to your plants. This can be the opposite when soil nutrients test deficient but are actually abundant and tied up in the soil organic matter that takes time to release into a plant-available form.

Typically crops will have a recommended fertility rate. For example Garlic’s recommended rates are 55kg(120lbs) nitrogen, 73kg(160) phosphorous, 68kg(150lbs) potassium and 14kg(30lbs) sulphur per acre.  While most home gardeners are not growing an acre of garlic, these numbers can be translated into kg/m² (lbs/sqft) by “kg/acre of fertilizer / 4047m²“. So for 1m2 of garlic, there should be 13.6g N, 18.0g P, 16.8g K and 3.5g sulphur.

Based on your soil testing, you can add (to-up) to the proper amount of nutrition needed for your crop by using different fertilizer products to meet the needs of the crop. You will rarely find a fertilizer blend that is an exact match to your needs, but by blending a few different products you can get close. For this garlic example, I would start with meeting the needs of the sulphur first because it is the smallest requirement. Sometimes using a product to meet the lowest requirement will contain other nutrients that can deduct from the total balance of each nutrient.

You can then calculate the amount of fertilizer needed for your space based on the amount of nutrients needed and the amount of nutrients supplied by each fertilizer.

Still confused? There is a lot to learn with soil fertility management and there are other resources online that can assist you. Contact us as our staff can help you understand the numbers and basic calculations.

We also offer paid consultation to create detailed fertilizer plans based on your cropping situations.