How To Use Water Soluble Fertilizers

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Like any living organism, plants need food to grow into its fullest potential. What most people don’t realize is that feeding plants effectively is more complex than just following the general instructions on the packaging. Different plants need different fertilizers in various quantities according to its growth stages. In this article we will run down the various applications of the different types of Peters WSF (Water Soluble Fertilizers). There are many other water soluble fertilizers available on the market and the principles remain the same, but for ease of illustration in this article we’ll focus on Peters WSF.


We need to understand what a plant needs to grow to complete its lifecycle. Plants need macronutrients and micronutrients. There are 6 macronutrients required by plants: Nitrogen, Phosphorus, Potassium, Carbon, Oxygen, and Hydrogen. Some argue that Calcium should also be a macronutrient. While macronutrients are required in larger amounts, the micronutrients are, as the name suggests, required by plants in smaller levels than macronutrients.


Water soluble fertilizers are often characterized by three numbers in the front of the packaging. The numbers indicate the Nitrogen (N), Phosphorus (P), and Potassium (K) contents, which are the primary macronutrients in the nutrient formula. It’s important to remember that the N-P-K numbers represent a percentage of content per net weight. So for example, on a 100g package of 20-20-20 NPK: 20g is N, 20g is P, and 20g is K. The rest are composed of various other salts and minerals which compose the micronutrients. This helps in calculating the total input of each element, let’s say, Nitrogen (N) in total weight during the course of the plant’s lifecycle.

Advanced growers play with different formulas according to different ratios of nutrients. Ratios refer to the relative values of each element in an application. For example, for leafy vegetable growers, it’s not unusual to use a 3:1:1 ratio in their formula (which means 3x more Nitrogen than Phosphorus and Potassium). Peters WSF Hi-Nitro (30-10-10) is an example of this where there’s 3 times more N than P, and K.


Plant development can be classified into different growth stages. For fruiting plants such as tomato, cucumber, and chili peppers, a plant’s development is mainly characterized into two stages: vegetative (growth stage) and flowering (reproductive stage). This can be further expanded into the ripening phase, first-harvest phase, secondary-harvest phase, etc. For leafy vegetables, the two stages generally apply too but instead of fruits, the plant will develop flowers to set the pollination stage for the production of seeds.

Therefore, understanding the stages of a plant is essential to know which water soluble fertilizers to use. Obviously, a tomato plant that is still early in its growth stage will not have a need for heavy dosage of potassium, which is more needed at the flowering and ripening stage of the fruits to make tomatoes big and sweet. Knowing what a plant needs at the right time of its growth stage will help maximize the total performance of the plant.

As a quick guideline:

  • Plants in the vegetative stage need more N for increased foliage production and P for healthy root production.
  • Plants approaching flowering stage need more P for flower and fruit production.
  • Plants with fruits need more K to improve size and flavor.




Peters General Purpose 20-20-20

For the most balanced nutrition, Peters WSF 20-20-20 offers equal dosage of N, P, and K.

Buy 100g here

Peters Hi-Nitro 30-10-10

For plants that need a boost of N, usually green leaf vegetables such as lettuce or petchay.

Buy 100g here

Peters Yield Booster 15-10-30

For fruiting plants to increase size and improve flavor of fruits.

Buy 100g here

Peters Blossom Booster 9-45-15

For plants to increase flowering and fruiting.

Buy 100g here


To know how much to feed a plant, consider that feeding plants is much like feeding ourselves. If we’re already full, we won’t eat anymore. If we don’t eat enough, we’ll get hungry. Same applies to plants. Avoid overfeeding (or underfeeding) plants as it’s wasteful of valuable nutrients. The right way to know how much to feed your plants is to use a TDS/PPM meter. Set your feeding targets on a weekly basis and monitor every day for consumption and water evaporation. If your PPM reading is low, make sure to adjust to a higher level to make up for the consumption of nutrients. As a plant grows bigger, it will consume more nutrients, therefore when you set your feeding targets, always consider the total life-cycle of the plant to accurately assess how much to feed and during what stage.

We’ve created a simple video on how to mix and measure nutrients. Have fun and happy planting!