Beyond NPK – Soil Amendments and what they do

Beyond NPK soil amendments at the best plant nursery near me.

Written by David S.

NPK are common letters that you find on the front of fertilizer packages. They stand for Nitrogen, Phosphorus, and Potassium. N, P, and K are also the scientific symbols for each of these elements as they appear on the periodic table of elements. In gardening and landscape, each of these elements plays a role in promoting plant health, healthy immune systems, and the successful production of blooms, fruits, and seeds. Well, all are important, fertilizer packages often have more ingredients than just Nitrogen, Phosphorus, and Potassium. This blog is a discussion about soil amendments and fillers and why they are important. 

Nitrogen – The Greening, Growth, and Energy Production

Nitrogen plays an important role within the chlorophyll process and further as part of the photosynthesis cycle. Through that role, it helps plants to remain green – the color of chlorophyll – and to grow strong, vibrant, and healthy leaves. Photosynthesis allows plants to take in sunlight to produce energy. That energy is needed to stimulate and power essential plant processes such as growth, flowering, and fruit or seed production. 

Note: Not all fertilizers contain Nitrogen. Some may only contain Potassium and or Phosphorus. You can tell on the label by the NPK ratio of 10-10-10 is a balanced fertilizer containing all three in equal parts. 0-15-0 only contains phosphorus. 0-0-10 only contains potassium. 0-10-5 is a fertilizer formula that has both potassium and phosphorus. When choosing fertilizer it is important to consider the NPK ratio.

Phosphorus – Growth of Healthy Cells 

Phosphorus is essential in the creation of healthy plant cells. It helps in the photosynthesis cycle with the aid of channeling plant energy into growth and reproduction. Meaning that phosphorus helps plants produce healthy reproductive cells, including flowers, fruit, and seeds. If you are looking for a way to increase the yield of your crops, then phosphorus is one of the best elements to choose. Be sure to test your soil before applying amendments because too much of a good thing can damage or kill your plants. 

Potassium – Nutrient Utilization on a Cellular Level 

Potassium helps plants uptake nutrients and water and transport them at the cellular level. It is important for the development of plant enzymes and throughout the ATP production and cycle (energy conversion.) 

That is the short story on what NPK does for plants. It also helps to explain why some fertilizers may not have all three of these elements. It is important to note that plants may use all three elements during each phase of their growth or only one or two of them at a time. You probably would not want to give plants a nitrogen-heavy fertilizer during the fruiting stage of the plant or crops development as the nitrogen would cause a flux between fruiting and vegetative growth and that would likely decrease the size of fruit and overall yield. 

Soil Amendments vs. Fertilizers 

Soil amendments are a vast category of products. They may or may not contain NPK. Soil amendments are either generalized or specific in what they do. An example would be garden lime, which you use to raise the pH of the soil. If you use the results of a soil test you might find that the soil is balanced in NPK, but it has a pH that is too low for the types of plants you want to grow there. There is no need to add fertilizer as the soil has what it needs. The need is to change or raise the pH to meet the growing requirements of plants. In this case, garden lime is a very specific soil amendment but not a fertilizer. 

As you shop for soil amendments you will come across many terms. One of my favorite brands is Foxfarm, which offers bagged soil products with some groovy names – Cream of the crop, Cultivation Nation, Happy Frog, Strawberry fields, etc. When you start to read the label of their products, you might get a little overwhelmed. 

Cream of the Crop “Cream of the Crop® 50/50 Growing Media is a mixture of buffered coconut coir and perlite, ready to use in your growing environment.” What does that mean? Soil amendments are broken down into several categories – aggregates, nutrients, and soil biota. In the case of Cream of the Crop – this is a soil aggregate. Its purpose is to help break up heavy soil and to help hold water in the soil without preventing excess water drainage. The perlite and coconut coir both help to increase the oxygen exchange in soil, and both will absorb water so that plant roots have what they need during the hottest part of the day. Those two ingredients are all that is in the bag, making these a specific soil amendment aimed at increasing soil airflow and water holding capabilities. 

Happy Frog Potting Soil – “Happy Frog® Potting Soil is a nutrient-rich growing media for use in containers. It is amended with soil microbes that help improve root efficiency and encourage nutrient uptake.” 

From their description, we can tell that Happy Frog Potting Soil is a generalized soil amendment that is good for containers or in-ground beds. It contains a mixture of ingredients including earthworm casting, bat guano, and aged forest nutrients. Earthworm casting and bat guano break down in the soil to become plant nutrients. The living soil biota in this product will help ensure that the decomposition process is smooth and that the nutrient load in the bagged soil will become available in a chemical format for plants to uptake. 

Aged Soil Products – The first layer of soil is the humus layer and in a forest, it is a deep mat of dead and decaying plant matter. It is also rich in soil biota that helps to break down the material so that plants can use it. Happy Frog Potting Soil is rich in the humus layer of soil that you would find in a forest – leaves, twigs, fruit, etc., and all of these materials are in the process of breaking down (aged) so that they are ready to boost soil nutrients. They work in conjunction with the earthworm castings and bat guano. Both the earthworm casting and the bat guano are immediately available for plant use, but the forest floor products will break down slowly so that plants have an even availability of soil nutrients. 

  1. Increase soil nutrient levels 
  2. Increase or introduce beneficial soil organisms 

These two examples show us how vast the differences are between different types of soil amendments. 

Soil Biota 

Soil biota is the living organism in the soil. When you start a container garden or plant a plant in a pot, there may not be any soil biota in the container. You want to choose a product that has active soil organisms so that as you water the soil, these tiny helpers come to life and start to work to support the plants you plant. The role that soil biota plays is important. They do a number of things including converting soil material into usable nutrients that plants can uptake, helping to improve oxygen exchange, so the soil remains healthy, and protecting plants from plant disease and pathogens. 

At Green Thumb Nursery, we understand soil and the role of building healthy soil. We also understand that soil amendments can be confusing. As you shop, be sure to ask our plant experts about soil products and explain the soil problems you want to solve. We are here to help you succeed at all of your growing projects. 

Do you like what you see? Sign up for our weekly newsletter to get content like this every week!