When you are shopping for fertilizer, the main thing to compare is the N-P-K ratings on the front of bag. The N, P, and K are all elemental symbols for the three primary nutrients: N for Nitrogen, P for Phosphorous, and K for Potassium. The numbers represent the total percent of the fertilizer’s weight in those available nutrients. For example, a balanced 10-10-10 fertilizer contains 10% Nitrogen, 10% Phosphorous, and 10% Potassium by weight. Fertilizers have different compositions based on what they are trying to achieve. For example, tomato food is usually 3-4-6 (3% Nitrogen, 4% Phosphorous, and 6% Potassium), while lawn fertilizer is 4-1-2 (4% Nitrogen, 1% Phosphorous, and 2% Potassium). To know whether your plants need more Nitrogen, more Phosphorous, or more Potassium, we have to understand a little about how plants feed.
What Plants Need to Live
Plants are made out of a complex mix of chemicals. To live, grow, and reproduce, they need access to several basic elements. Plants are primarily made up of Carbon, Hydrogen, and Oxygen, but thankfully, they capture plenty of these structural elements through rainwater and the carbon dioxide in the air. Plants also need large amounts of the six macronutrients (Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and Sulfur) and smaller amounts of the eight micronutrients (Boron, Chlorine, Copper, Iron, Manganese, Molybdenum, Zinc, and Nickel). Plants can not truly flourish unless all 14 of these essential nutrients are present in the soil.
The most important macronutrients are Nitrogen, Phosphorus, and Potassium, and this is why they are known as primary macronutrients. A healthy soil generally provides all the micronutrients and secondary macronutrients that plants need, but the three primary macronutrients are consumed rapidly by plants, and must be replenished frequently by fertilizers.
Image of Urea sample by LHcheM.
Nitrogen is the most important nutrient for green growth. Plants use it in nearly every protein they make, so Nitrogen is vital to new plant growth and the creation of chlorophyll. Though Nitrogen is all around us (it makes up the majority of our atmosphere), plants can not capture it on their own, and they rely on Nitrogen-fixing microbes and lightning strikes to fix Nitrogen into the soil where plants’ roots can access it. Nitrogen-based fertilizers are produced by taking a natural gas (CH4+) from the air and using the Haber-Bosch process to transform it into soluble Anhydrous Ammonium Nitrate (NH4NO3) or Urea (CO(NH2)2). If your soil has a Nitrogen deficiency, your plants will not grow vigorously, and the older leaves will turn pale or yellow.
A sample of Phosphate Rock.
Phosphorus is the most important nutrient for root and seed development. Plants use it in the creation of DNA and in the storage of energy, which makes it crucial for seed development and fruit ripening. Phosphorus naturally occurs in soils with a good amount of organic matter. Phosphorus-based fertilizers are produced by taking naturally-occuring Phosphate rock and adding Sulfuric Acid or extreme heat to render the phosphate into a water-soluble form. If your soil has a Phosphorus deficiency, your plants will not grow strong root systems, their development will be delayed, and the leaf tips will look scorched, with the older leaves near the bottom of the plant having a darker green or red/purple tint to them.
Potash sample.
Potassium is the most important nutrient for a plant’s overall health and disease resistance. Plants use it in the creation of quality fruit and seeds, and it is also vital to photosynthesis. Potassium-based fertilizers are produced by mining Potash ore from subterranean deposits and mechanically processing it into a soluble form. If your soil has a Potassium deficiency, growth will be puny, leaves may distort and curl or develop blotchy chlorosis late in the season, fruit can be bland, and whole branches may even die.
Why to Fertilize
A lot of gardeners simply use high amounts of balanced fertilizer all year long. This is not recommended because overfertilizing can often cause scorch, especially when using salt-heavy fertilizers. Overfertilizing also often has a negative effect on plants’ root growth and on existing populations of decomposing organisms. For the healthiest garden, you should be more strategic and think about why you are adding fertilizer.
Test your soil’s Nitrogen, Phosphorous, and Potassium levels, plus the pH.
If your soil is deficient in one or more nutrients, the first thing you should do is add fertilizer to restore the soil. If you notice the telltale symptoms of a nutrient deficiency in your plants, you should immediately start adding a fertilizer high in that nutrient. But by the time your plants are showing symptoms, you have already lost precious growing time. A Soil Test Kit can tell you what quantities of nutrients your soil contains so you can get your soil in shape early.
Generally a balanced N-P-K ratio is healthy for plants, but that isn’t what we want all the time. A nutrient imbalance in the soil will have an interaction effect on plants, where the plant “learns” to take advantage of the most prevalent nutrient and absorb that one at the expense of the scarce nutrients. A Nitrogen-heavy soil will encourage plants to produce more leafy growth, a Phosphorus-heavy soil will encourage plants to grow roots and flower buds, while a Potassium-heavy soil will encourage buds to bloom and fruit to ripen.
We can take advantage of this tendency in plants to help them flourish in the growing season, flower or fruit earlier, and produce more flowers and fruit throughout the season. But to do this effectively we must be sure and apply the right fertilizer at the right time.
When to Fertilize
Note: Fertilize on a calm day, so the wind and rain don’t take the nutrients away. Add fertilizer to established plants, not seedlings—wait for the plant to toughen up first so that it does not suffer fertilizer burn.
When to Fertilize Infographic.
1. First, amend the soil with compost to condition the soil, create a base of available nutrients, and develop beneficial microorganism populations. A balanced organic fertilizer (NPK 3-4-4) is another great way to restore nutrients and develop beneficial microorganisms. Because compost and organic fertilizers take time to break down, do this step well in advance of the growing season, either in late fall or early spring. Organic fertilizers often have low N-P-K numbers because that is the ratio of nutrients that are immediately available. More Nitrogen, Phosphorus, and Potassium become available as the fertilizer breaks down. Chemical fertilizers, by contrast, usually have higher N-P-K numbers and are good for delivering nutrients to plants quickly, but do not apply them until they are needed, because if they sit in the soil un-used they tend to leach out quickly.
2. Once growing is underway (NOT before), add a fertilizer that is highest in Nitrogen and low in Phosphorous and Potassium, like Blood Meal (NPK 12-0-0). This will feed all that green leafy growth that is developing. Fast-acting fertilizers should be re-applied every 2 weeks throughout the growing season, while slow-release fertilizers work slower and last longer.
3. When it is time for your plant to start producing flowers, switch to a fertilizer that is lower in Nitrogen and high in both Phosphorous and Potassium, like Bud & Bloom (NPK 10-30-20). This induces flowering while maintaining root health and good disease resistance.
4. For fruit plants, when it is time for fruit to develop, switch to a lower Nitrogen, higher Potassium fertilizer, like Tomato-Tone (NPK 3-4-6). This will help induce fruiting and ensure that your fruit grows large and flavorful. Note that this formulation is still relatively balanced, because flavorful fruit still requires some Phosphorous, and healthy greenery still requires some Nitrogen.
5. Once the flowers and fruit are done for the season, stop fertilizing and let your plants go into dormancy. Keep building your store of compost, and then in late fall/early spring, begin amending the soil again (per step 1).
