Many growers stand in front of fertilizer bags confused: 20-20-20, 13-40-13 or 12-12-36? The numbers summarize what is inside the bag, and anyone who understands them picks a fertilizer that fits the crop and its stage instead of trial and error. NPK combines the three essential nutrients every plant needs: nitrogen for vegetative growth, phosphorus for roots and flowering, and potassium for fruit and quality. In this article we explain what the numbers mean, the common formulas and when to use each, the right application methods, and how to avoid common fertilizing mistakes in the soils and water we farm with.
In this article we explain what the numbers mean, the most common formulas, when to use each one, and how to apply the fertilizer correctly.
What the numbers mean: three numbers representing the percentage by weight of nitrogen (N), then phosphorus (P2O5), then potassium (K2O) in the fertilizer.
Example: a 20-20-20 fertilizer contains 20% of each nutrient, a balanced formula that suits most crops at different growth stages.
High-nitrogen formulas such as 30-10-10 suit the start of the season and the vegetative growth stage.
High-potassium formulas such as 10-10-40 suit fruit set, fruit development and coloring.
Note: higher numbers are not always better; what matters is that the formula matches the crop's needs and stage.
How do you read the N-P-K numbers on the bag?
The numbers are always written in the same order: nitrogen first, then phosphorus, then potassium. If you see a fourth number, it is usually a secondary nutrient such as magnesium or sulfur.
- 20-20-20: balanced, suitable for general feeding.
- 19-19-19: balanced and widely used in drip irrigation and foliar spraying.
- 13-40-13: rich in phosphorus to strengthen roots after transplanting.
- 12-12-36: rich in potassium to improve fruit size and quality.
When should you use each formula?
After transplanting and at the start of growth
The plant needs strong roots, so it suits a formula higher in phosphorus with moderate nitrogen.
Vegetative growth stage
Raise the nitrogen share to encourage leaves and stems, while keeping the other nutrients at sufficient levels.
Flowering, fruit set and coloring stage
Lower the nitrogen and raise the potassium to improve fruit set, fruit size, color and taste.
Ways to apply NPK fertilizer
- With irrigation water: best for drip irrigation, and make sure the fertilizer is fully dissolved.
- Broadcast on the soil: for granular types, then irrigate the field right away.
- Foliar spray: at a low concentration to correct a quick deficiency or support the plant under stress.
You can browse agricultural fertilizers and soil and choose the formula that suits your crop, and see our humic acid guide to improve nutrient uptake.
Mistakes to avoid
- Applying a large dose at once instead of splitting it.
- Using one formula all season without considering the growth stage.
- Mixing the fertilizer with incompatible products without a small test first.
- Fertilizing in the hottest hours of the day when spraying on the leaves.
How to Choose the Formula by Crop
Leafy vegetables like lettuce and spinach benefit from higher nitrogen because dense foliage is the goal. Tomato, cucumber and pepper need moderate nitrogen at the start, then shift to higher potassium at fruit set and coloring. Fruit trees and date palms need a program spread over the year, including a potassium round before fruit ripening. Make your first question: which stage is the crop in now? The answer sets the formula before any other consideration.
The Effect of Water and Soil Here
Much of the water in our regions is relatively saline, and sandy or calcareous soils have high alkalinity, which changes the plant's ability to absorb nutrients. Soluble fertilizers through drip irrigation suit sandy soils best because they are applied evenly with little loss. In calcareous soil phosphorus may be locked up, so humic acid or mild acids help improve uptake. A soil and water analysis once a season saves a lot of waste.
Deficiency and Excess Symptoms
Yellowing of lower leaves usually points to nitrogen deficiency, reddish or purple leaf color may indicate phosphorus deficiency, and scorched leaf edges with weak fruit relate to potassium deficiency. Conversely, excessive vegetative growth with poor fruiting means too much nitrogen, and root burn with wilting despite irrigation means salinity from over-fertilizing. See the foliar fertilizer guide for quick correction of deficiencies.
Storage and Handling Tips
Store fertilizers in a dry, shaded place away from moisture because they cake and lose quality, close the bag well after each use, and keep them away from children and animals. Do not mix different fertilizers in one tank without a small compatibility test, and flush drip lines after fertilizing to prevent deposits.
Frequently asked questions
What is the difference between granular and soluble NPK fertilizer?
Granular fertilizer is broadcast on the soil and releases gradually, while soluble fertilizer dissolves completely in water and suits drip irrigation and foliar spraying because it is absorbed quickly.
Is 20-20-20 suitable for all crops?
It works as general feeding for most crops, but it is better to adjust the formula to the growth stage and the crop's needs.
What happens with too much nitrogen?
It increases vegetative growth at the expense of fruiting, and makes the plant more prone to insects such as aphids and to fungal diseases.
Can NPK be mixed with pesticides?
Some products are compatible and some are not, so read each product's leaflet and test on a small amount before mixing fully.
How often should I apply NPK fertilizer?
The recommended amount is split into several applications during the season, and the interval varies with the crop and the irrigation system.