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If you decide that your plants need a health boost and you decide to fertilize, it’s important to make sure that your fertilizer is organic.
Organic fertilizer feeds the microbes that build relationships with plants to deliver nutrients to them. We want to foster this relationship!
On the other hand, synthetic fertilizer decreases the need for plants to form beneficial relationships with microbes - they don’t need their nutrients anymore (teaming with microbes).
In fact, organic fertilizer actually requires soil bacteria and fungi to make the nutrients bioavailable to the plants (source).
So organic fertilzer boosts soil health and microbial communities while synthetic fertilizer disrupts the soil food web.
There are other reasons as to why we want to only apply synthetic fertilizers to our crops.
Synthetic fertilizers have a high carbon footprint because they are typically made from petrochemicals (source).
Synthetic nitrogen fertilizers alone account for 2% of global GHG emissions. To put this into perspective, that’s the same amount released from commercial aircraft globally.
Moreover, synthetic nitrogen fertilizers emit nitrous oxide, which is 265 more potent than carbon dioxide (source).
NPK stands for nitrogen, phosphorus, and potassium, which are three essential nutrients for plant growth. They are often referred to as primary macronutrients. In a fertilizer's NPK rating, the numbers indicate the relative proportion of each nutrient in the product.
An essential nutrient for plant growth, especially the green leafy growth of plants. For a fertilizer's NPK rating, the N is the proportion of nitrogen within the product. Signs of nitrogen deficiency are pale to yellowing leaves and stunted growth.
Nitrogen is in the air, and some plants have a symbiotic relationship with organisms in the soil that help them to transform atmospheric nitrogen into a usable nutrient for the plant.
An essential nutrient for plant growth, especially stem, root, and tuber development of plants. In a fertilizer's NPK rating, P is the proportion of phosphorus within the product.
Signs of phosphorus deficiency are small and misshapen tubers or roots of root crops or purple-tinged leaves (PennState + GlobalNet).
Potassium helps with the fruiting and rooting of plants, supports water uptake, and enhances photosynthesis efficiency. In the NPK rating, K is the proportion of potassium within the product.
Signs of potassium deficiency are small and misshapen fruits or stressed plants (PDA).
The numbers are how many pounds of each nutrient would be in a 100 pound bag. For example, a 4-4-4 blend has 4 pounds of nitrogen, 4 pounds of phosphorus and 4 pounds of potassium in 100 lb bag of that particular fertilizer (OSU).
In addition to nitrogen, phosphorus, and potassium, there are several other important nutrients for plant growth.
As you see in the chart above, nutrients support the plant with leaf growth, fruiting, photosynthesis, root growth, nutrient update, and seed development.
Let’s take a closer look at these secondary nutrients.
Signs of calcium deficiency include blossom end rot, and browning tips of leaves (source).
Signs of magnesium deficiency might present yellow discoloration between the viens of leaves, while the veins themselves stay green. Sometimes, the leaves can even become pink and curl.
Signs of sulfur deficiency included stunted growth on young plants, and yellowing pale leaves (source).
You'll notice in this chart that sometimes symptoms that you see in plants need to be examined more closely before you start your fertilizer application to remedy it.
Sometimes, the symptoms are unmistakable.
For example, early maturity is a telltale symptom of nitrogen deficiency.
But other symptoms need to be looked at more closely.
For example, stunted growth could be a sign of a deficiency in phosphorus, nitrogen, potassium, sulfur, copper, molybdenum, zinc, calcium, or boron deficiency.
The general recommendation for stunted growth could be a nice balanced quick acting NPK fertilizer, say a 10-10-10.
You could see if the plant growth improves to test out if it was indeed one of those macronutrients. If plant growth doesn’t come on, then you could know it’s one of the secondary macronutrients or micronutrients.
But there is actually some nuance to it that can help us narrow down what nutrient our plant may need.
The key is to look at our plants closely and notice if there are any other symptoms happening.
Stunted growth from a nitrogen deficiency would be paired with yellowing on older leaves, usually starting from the top of the leaf. If the stunted growth is seen with early maturity - for example, small stunted zucchini plants flowering, then a lack of nitrogen is likely the cause.
If the stunted growth is paired with darkened, blue tinted leaves, or purple-tinted leaves, it it likely due to a phosphorus deficiency.
Stunted growth showing up in potassium-deficient plants would also likely exhibit chlorosis on the outline of the older leaves.
Stunted growth from a boron deficiency affects the roots, and is paired with plant leaf and root misshapen. Stunted growth with a boron deficiency also will show up on younger leaves.
If the slow growth is due to a copper deficiency, it may also be paired with new leaves having a malformation. Leaves may also curl in a way that they form a concave shape.
Molybdenum-lacking plants may have stunted growth paired with yellow spots on the edges of the plant, or the yellow spots may show up between the veins of the leaves.
A zinc deficiency is likely to show up through stunted growth with plants in the fabaceae family.
Yellowing spots on leaves, or chlorosis, is something that could be treated with a balanced NPK fertilizer. This might solve the problem, but we might take a closer look at the leaves to get a better understanding of how the yellowing is presenting itself.
Uchida points out the following distinctions for yellowing leaves:
If the yellowing is appearing at the top of mature leaves, with leaves eventually falling off, it’s likely a nitrogen deficiency.
If the yellowing is around the perimeter of the mature leaves, it is likely that a lack of phosphorus is the culprit.
If the veins of the leaves are still green in appearance, and the yellowing is occuring on the more mature growth, it’s likely a case of magnesium deficiency.
In contrast, if the yellowing occurs on the youngest leaves with green veins, it could be due to a lack of iron in the soil.
If the yellowing is showing up as tiny spots throughout the leaves, adding manganese to your soil could help.
The plant forming in a funky way could be either a boron deficiency or a calcium deficiency.
A boron deficiency is usually seen in root vegetables, where the root is elongated, thin, and in general misshapen.
A calcium deficiency would present itself in curled, concave, or other difference from the norm happening in the youngest leaves.
Leaves that are so pale, they almost look white could be a sign of either a iron or magnesium deficiency.
Uchida describes the following distinctions:
If the whitened leaves are presenting themselves on new growth with otherwise normal leaf formation, it’s likely that iron is the issue.
If the whitened leaves are showing up deformed, or plants are stunted that would point to a lack of copper as the root cause (source).
As a homesteader and gardener, I understand the importance of using high-quality organic fertilizers to maintain soil health and promote thriving plants.
That's why I've personally worked with each of the following products and can confidently recommend them to anyone looking to enhance their gardening practices.
These fertilizers provide a host of benefits, from boosting soil health and microbial communities to avoiding the GHG emissions and pollution from synthetics. All of them are OMRI listed for organic agriculture.
If you're looking to take your gardening to the next level, consider trying one of my top picks for organic fertilizer in 2025.
Nutri-Rich 4-3-2 Fertilizer Pellets (50 lb)
Down to Earth Vegetable Garden Fertilizer 4-4-4 (25 lb)
Peaceful Valley Organics Premium Earthworm Castings
Numerous studies have shown that vermicompost produces end yield and plant health results similar to chemical fertilizers, and sometimes even surpasses them (source). It also helps to enhance soil microbiology.
A study in 2020 found that vermicompost and vermicompost tea can effectively substitute synthetic fertilizers while also decreasing pest and disease, leading to an additional benefit of a reduced need for herbicides and pesticides (source).
Vermicompost is also a great way to keep green waste out of the landfill, as it’s what the worms will eat to make their castings.
By choosing organic fertilizers over synthetic ones, you're not only helping to maintain healthy soil, but you're also reducing your carbon footprint and contributing to a healthier planet. Understanding the role of NPK and other essential nutrients for plant growth is a crucial step in selecting the right fertilizer for your garden. Our top picks for organic fertilizers in 2024 offer diverse benefits, from fast-acting formulas to slow-release options that promote long-term soil health. With these organic fertilizers, you can boost your garden's fertility while supporting a more sustainable future.
