Applying the right water soluble fertilizers can work wonders on field crops, in home gardens, and even for houseplants. Just as the fruits, flowers, and vegetables you grow need adequate light and water to thrive, they need essential nutrients, too. Providing macronutrients and micronutrients that plants can quickly and easily access can help you net better results overall. Water soluble fertilizers can boost both crop growth and the quality of produce. In fact, which nutrients you provide—and how and when you provide them—can affect everything from the health and size of cut-flower blossoms to the thickness of your lawn and the taste of fruit and veggies.
What are water soluble fertilizers?
To understand what water-soluble fertilizers are and how they work, you first need to understand how plants access nutrients. Plants acquire nitrogen, phosphorus, potassium, and other essential soil-based elements through their roots. But it isn’t until you fully water plants—or they get a good, soaking rain—that soil-based elements become accessible to your plants’ roots. Once watered, your plants’ roots draw in much-needed moisture and the nutrients present in the resulting soil solution.
Typically available in pellets or granules, water-insoluble fertilizers don’t readily dissolve in water. Instead, these “slow-release” dry fertilizers offer nutrients very gradually. Slow-release formulas are worked into the soil near the plants they’re intended to feed. When it rains or when you water your plants, some of the dry fertilizer’s nutrients reach your plants’ roots.
Of course, the nutrient sources in fertilizer products vary widely. Some ingredients come from natural, organic sources. Others come from artificial, inorganic sources. Although such chemical-based liquid fertilizers often contain higher percentages of essential nutrients, it’s possible to have too much of a good thing.
Why avoid chemical-based liquid fertilizers?
From a plant’s vantage point, nitrogen synthesized from a nitrate form like ammoniacal nitrogen or calcium nitrate is just as useful as nitrogen derived from natural ingredients like bat guano or blood meal. The same goes for a potassium-containing potash (which is produced from potassium chloride) and potassium derived from natural sources like sea kelp. Nevertheless it’s best to avoid chemical-based liquid fertilizers.
Synthesized from chemical salts, inorganic fertilizers can negatively impact soil health and structure. With their continued use, accumulating sodium residues contribute to higher acid pH levels. This, in turn, repels worms and other beneficial soil-dwellers and can “lock up” your plants’ ability to uptake nutrients. Excess fertilizer salts also draw water away from plant roots—the cause of fertilizer “burn” injuries. In container gardens, excess fertilizer salts can lead to scale formation on the outside of the pots or the top of the soil. Eventually, the soil’s capacity to retain water also declines. Excess nutrients, such as soluble phosphates, can leach into area waterways, contributing to algae blooms and other harmful environmental effects.
Why natural liquid fertilizers are better
Liquid fertilizers derived from natural sources are better for plants and the soil. They generally have a low salt index, meaning they’re less likely to cause fertilizer burn injury, alter soil pH, or disrupt soil microbial activity. Because they’re derived form natural rather than inorganic ingredients, they may also be chloride-free and include bioactive extras like amino acids, enzymes, and beneficial bacteria. These work to feed and support the soil itself.
What plants can you use a water soluble fertilizer on?
You can use water soluble fertilizers on new seedlings, in well-established gardens, and everywhere in between. Want to kick-start very young plants? Start with a relatively weak nutrient solution. Want to induce early flowering or early fruit formation? Incorporate phosphorus and micronutrients like zinc and manganese into the fertilizer you apply. As all of your plants grow larger, you can provide a stronger, all-purpose mixture every couple of weeks. Ditto for routine feedings of houseplants and container gardens.
Water soluble fertilizers have many benefits—and some drawbacks. Their uniformity of application is one of their biggest strengths. Plants take in dry, slow-release fertilizers only when water is present in the soil. Unless this fertilizer remains evenly distributed, you may end up with some nutrient-rich pockets and other nutrient-poor areas. Plants near heavier concentrations of fertilizer salts are at greater risk of burning.
Conversely, water soluble nutrients are immediately available to plants wherever they’ve been applied. They are fast-acting but also short-lived. As a result, water soluble fertilizers are less likely to injure your plants, but they must be reapplied more frequently. Also, some of these cost a little more than dry, slow-release products. Still, being able to deliver specific nutrients exactly where and when you want to just might be worth it.
What nutrients do liquid fertilizers add?
You can tell how much of each of the “big three” essential nutrients a liquid fertilizer contains by checking its label for three numbers, separated by hyphens. This is known as the NPK ratio. (Nitrogen, phosphorus, and potassium are represented as N, P, and K, respectively.) Say a product label shows a ratio of 3-2-6. That means the product contains 3% nitrogen, 2% phosphorus, and 6% potassium by weight. Just what do nitrogen, phosphorus, and potassium do for plants?
- Nitrogen (N)—important for green, leafy growth and new shoot development
- Phosphorus (P)—stimulates blossoming and fruiting; aids in new root development and prompts root growth
- Potassium (K)—essential for critical functions like plant rooting and cell wall formation
Other important nutrients include but aren’t limited to:
- Calcium (Ca)—plant building block bolsters strength of cell wall; neutralizes certain plant acids; aids in protein manufacture
- Magnesium (Mg)—important chlorophyll component; helps make plant fats, starches, and more
- Zinc (Zn)—needed for chlorophyll production as well as certain plant enzymes and hormones; helps plants set seed
- Boron (B)—regulates cell growth and metabolic processes
- Molybdenum (Mo)—essential for plants’ nitrogen uptake and use; helps plants make proteins
- Manganese (Mn)—another chlorophyll component; assists with uptake of other nutrients
Liquid fertilizers may contain different combinations of these additional elements. Check your product’s ingredients list for references to secondary nutrients, micronutrients, or trace elements.
You can buy organic water soluble fertilizers in either dry or liquid forms. Often available as a powder or in pellets, dry preparations are intended to be measured out, mixed with water, and then applied to plants. Similarly, the concentrated liquid formulas also require some measuring, dilution into water, and mixing before use. Depending on the age and size of your plants, you may want to adjust your fertilizer-to-water ratio. Carefully read the manufacturer’s recommendation on the product label before you mix. Some commonly used organic nutrient sources follow.
Liquid kelp/seaweed
Liquid kelp and seaweed preparations can really pack a punch since they often contain some nitrogen, potassium, and many of the harder-to-find trace elements all in one place. Because it prompts roots to grow, some gardeners use very diluted liquid kelp/seaweed solutions as pre-germination seed soaks. Whether you’re growing cherries or cherry tomatoes, the nutrients found in most liquid kelp preparations also can improve the quality of the produce. When applied during fruit development stages, liquid kelp/seaweed may increase fruit size and boost sugar content.
Fish-based water soluble fertilizers
Most fish-based fertilizers are derived from whole fish or fish parts which have been ground up and liquified. These products are usually richest in phosphorus and nitrogen and also may contain some trace elements, beneficial enzymes, and amino acids. Because they may be lacking, some manufacturers add extra sources of potassium to their fish-based preparations.
Compost or earthworm castings tea
Although it’s possible to make your own fertilizer teas from finished compost or vermicompost, you won’t necessarily know which nutrients and microorganisms are in your DIY concoctions. Commercial manufacturers offer at least some specifics about the myriad ingredients in liquid compost/earthworm castings tea. These products often contain important trace elements, beneficial bacteria, and fungi which can improve soil structure and health.
Combination liquid fertilizers
Created from a mixture of different nutrient-rich ingredients, combination liquid fertilizers usually contain staples like fish or bone meal, animal manures, and seaweed or kelp extracts. Sometimes marketed as fertilizer supplements, there’s a combination liquid fertilizer for nearly every use case. For example, calcium deficiencies contribute to flower drop, fruit dropping, and tomato blossom end rot, and calcium-rich combination liquid fertilizers exist to mitigate (or, better still, to prevent!) these issues.
How to use water soluble fertilizers
You can apply water soluble nutrients with a simple watering can or even an elaborate irrigation system. Want to use water soluble fertilizers with drip irrigation? Just be sure that your chosen fertilizer is thoroughly mixed first. (You might also want to strain it to remove any potentially clogging particulates.)
Besides directing liquid nutrients to your plants’ root zones, you can also apply them to plant leaves as a foliar spray. For this use, check your product label for foliar application instructions and dilute water soluble fertilizers accordingly. Then, decant your mixture into a clean spray bottle. Foliar feeding is particularly helpful if you need to take quick, corrective action. (To avoid burning your plants, lightly mist foliage when the ambient heat and humidity are low—early mornings or evenings are best.)
Get growing!
Water soluble fertilizers provide a greater degree of control in terms of the strength of the nutrient solution you provide and the frequency with which you provide it. What’s more, because organic water soluble fertilizers also contain many essential micronutrients and even beneficial microorganisms, they feed your plants and the soil. That puts you in a better position to combat common issues like insect pests, plant pathogens, and adverse weather events like record heat and drought. Best of all, no matter what you grow, you can choose liquid formulas containing combinations of essential nutrients to meet any special requirement of crops.