Fertilizer…by the numbers

What is a plant fertilizer?

Let’s start at the beginning. By definition, a plant fertilizer is either a chemical or a natural material added to the soil to increase its fertility and aid in plant growth. Your mother may have drenched her plants with a blue, water-soluble chemical fertilizer every week and maybe you think you should do the same. (I will use these on occasion to kickstart but buy the generic its the same stuff probably blended in the same facility with different packaging). But, there’s been a major shift in thinking over the past decade when it comes to how to fertilize plants. We’ve moved away from the idea of “feeding plants” and toward the idea of “feeding soil.” The plants know what to do, but they need soil that contains the building blocks.

When you use naturally-derived fertilizers instead of those derived from chemical salts, your plants are provided with a much more balanced nutrient source. Natural fertilizers provide nutrition for growing plants by feeding the soil’s living organisms (example, mycorrhizae). In turn, these microscopic critters (most of which are fungi and bacteria) process these fertilizers, breaking them down into the nutrients plants use to grow. When we feed the soil, our plants reap the benefits.

Compost is one of the best plant fertilizers and soil amendments because it contains a broad diversity of nutrients essential for plant growth and acts as a great food source for soil microbes. Encouraging healthy, biologically active soil by compost, manures or other decomposed organic matter to the garden every year is the best way to promote optimum plant growth.

However, there are times when our plants need more nutrition, such as when the nutrients contained in the compost are depleted or unavailable. Or, when our back or our budget can’t afford to spread wheelbarrow after wheelbarrow of compost. For those times, there are a number of easy-to-use natural fertilizers on the market that also do an excellent job of feeding the soil. These fertilizers can be added to your garden throughout the growing season. However, it’s important to understand what’s in each bag or bottle of fertilizer and how these products influence plant growth. Doing so enables gardeners to get the most bang for their buck while keeping excessive nutrients from potentially harming our plants and creating imbalances in soil chemistry.

What do fertilizer numbers mean ?

Plants are mostly water like we are. If broken down, most plants are around 90 % water and 10 % dry matter.

Then 10 % of dry plant material is made up of the “organic” components of carbon (C), hydrogen (H), oxygen (O) which are mostly derived from air and water, and a number of “inorganic” ions such as nitrogen (N), phosphorous (P) and potassium (K) which are derived from the soil.

If we take this 10 % of the dry plant material and remove the carbon, hydrogen and oxygen, only about 1/10 remains as inorganic ash. Therefore, plant nutrition using inorganic fertilizers directly affect only 1 % of the plant by weight (but that 1 % is critical to plant growth).

When shopping for fertilizers, spend some time reading the labels. It’s important to understand the fertilizer numbers you see on the front of the package, as well as the ingredients that are used to make the fertilizer.

The three numbers you see on the label of every bagged or bottled fertilizer represent the product’s N-P-K ratio. These are identified as the most prevalent macronutrients. The (N) in the ratio stands for Nitrogen, the (P) stands for Phosphorous, and the (K) stands for Potassium. The fertilizer numbers found on the package represent the percentage (by weight) of these three macro-nutrients as they are found in that package. Though plants use many different nutrients to fuel their growth, these three are used in the largest amounts. For example, a bag of fertilizer with the fertilizer numbers 10-5-10 on the label, holds 10 % N, 5 % P, and 10 % K. The remaining 75% of the bag’s weight is carrier product.

Calcium (Ca), Magnesium (Mg) and Sulfur (S) are the other 3 macronutrients for a total of 6. These may be identified on fertilizer product labels.

The LITTLE SIX micronutrients are found at much lower concentrations in plant tissue than the BIG 6 macronutrients and include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B) and molybdenum (Mo).

These twelve elements or nutrients are commonly provided by various fertilizer sources applied to crops and should be referenced on product labels or data sheets.

Six additional elements or nutrients that are considered essential or beneficial for normal plant growth and include sodium (Na), chloride (Cl), Silicon (Si), Nickel (Ni), cobalt (Co) and aluminum (Al). These elements are often found as contaminants in fertilizer mixes, substrate or soil or irrigation water and it is not normally necessary to add these as supplements.

Note: Many gardeners struggle with powdery mildew commonly found with zinnias, roses, squash and cucumbers. Silicon has been discovered to provide benefits by improving plants tolerance to stress and pathogens. Silicon can be applied as foliar or soil drenches with potassium or sodium silicate. research by USDA-ARS found a relationship between increased silicon uptake and resistance to powdery mildew when grown in root substrates with rice hulls or miscanthus grass (which contain silicon) compared to peat moss or pine bark based substrates (which are the majority of the bagged soil products found at retail outlets. Yes, they use composted pine bark. I worked in the industry and all of these products are basically the same thing with a different name and they leave a lot to be desired. I personally avoid them.

Essential Plant Nutrients, % in Dry Plant Material and Descriptions

MACRONUTRIENTS

Nitrogen 4.0 %

Component of chlorophyll, nucleic acids, proteins and enzymes

Phosphorous 0.5 %

Required to store and transport energy

Potassium 4.0 %

Osmotic regulator in water absorption and retention

Calcium 1.0 %

Cell structure and secondary plant hormone

Magnesium 0.5 %

Central ion in chlorophyll molecule

Sulfur 0.5 %

Component of nucleic acids and proteins

MICRONUTRIENTS

Iron 200 ppm (parts per million)

Necessary for chlorophyll synthesis and energy pathways

Manganese 200 ppm

Also necessary for chlorophyll synthesis and energy pathways

Zinc 30 ppm

Activates enzymes

Copper 10 ppm

Involved in respiration and oxidation/reduction reactions

Boron 60 ppm

Essential for cell division and diffentiation of young plant tissue

Molybdenum 1 ppm

Involved in nitrogen metabolism

BENEFICIALS

Sodium 500 ppm

Osmotic Regulator

Chloride 0.1 % (1000 ppm)

Required for photosynthesis

Silicon 0.05-0.15 %

Pathogen defense, drought and heat tolerance

Whew, my brain hurts now. Maybe too much boron.

For example, a bag of fertilizer with the fertilizer numbers 10-5-10 on the label, holds 10 % N, 5 % P, and 10 % K. The remaining 75% of the bag’s weight is other minerals and carrier products.

What plants do with the N, P, and K in the fertilizer.

It’s very important to understand how plants use these three nutrients.

Nitrogen

Nitrogen is a component of the chlorophyll molecule, and it promotes optimum shoot and leaf growth. Adding a fertilizer high in nitrogen (such as 6-2-1 or 10-5-5) to a fruiting or flowering plant, like a tomato or a petunia, can result in excessive green growth often at the expense of flower and fruit production. But adding it to a green, leafy vegetable plant, such as spinach or lettuce, makes much more sense.

Phosphorus

Phosphorous, on the other hand, is used for cell division and to generate new plant tissue. It promotes good root growth and is used to encourage fruit and flower production. Phosphorous is particularly important for root crops, like beets, carrots, and onions, as well as for encouraging flower and fruit production. That’s why fertilizers that contain bonemeal and gypsum (rock phosphate) are often recommended for use on root crops; both are rich in phosphorous. Choose a fertilizer that’s higher in phosphorous for plants that produce edible flower buds (like broccoli or cauliflower ), fruits (like tomatoes and cucumbers), or edible roots (like carrots and radishes).

Potassium

This plant nutrient helps trigger certain plant enzymes and regulates a plant’s carbon dioxide uptake by controlling the pores on a leaf’s surface, called stomata, through which gasses pass. Potassium levels influence a plant’s heartiness and vigor.

Now that you know what the fertilizer numbers stand for, let’s look at what ingredients you might find in the bag or bottle.

What’s in a package of fertilizer?

While synthetic chemical-based fertilizers are made from salts synthesized in a factory, along with lots of filler ingredients that you won’t find on the label, natural fertilizers are made from blends of naturally occurring materials. Natural fertilizers have four main ingredient sources.

1. Plant materials
These are fertilizer ingredients derived from plants. A few examples include alfalfa meal, kelp meal, and cottonseed or soybean meal.

2. Manure materials
You may also see pelletized poultry manure, dehydrated cow manure, cricket manure, bat guano, and worm castings on the label of a natural fertilizer.

Natural fertilizers for plants may also include mined minerals, such as greensand, rock phosphate, crushed limestone, and sulfate of potash.

Using fertilizers containing a combination of these ingredients is a terrific way to feed your soil when nutrients become depleted and adding more compost isn’t an option.

The fertilizer numbers on the bag, combined with the ingredient list, tell you all you need to know about a fertilizer.

Which fertilizer numbers should you choose?

To make your decision easier, you have two basic choices when it comes to natural fertilizers for your garden. Using a balanced general purpose fertilizer like 10-10-10 or 13-13-13 is fine for general crop production. There are many available resources especially on the internet as you learn as a gardener and refine your approach to producing different crops.

Know Before You Broadcast or Sidedress

Before you start tossing fertilizer onto your garden, make sure it’s actually needed. A simple soil test will tell you where your soil stands in regards to its nutrient content. You can purchase a soil test kit from an independent laboratory, or from a nearby university extension service (https://extension.missouri.edu/programs/soil-and-plant-testing-laboratory) if you live in Missouri. Once the existing nutrient levels are determined, calculate what’s needed by using the fertilizer numbers on the bag. Testing is a necessary practice to determine the true state of your soil. Do this every few years. A soil test also tells you about another critical aspect of soil health: the pH. Read more about the importance of optimum soil pH here.

Though starting with a soil test is always the best way to determine how much fertilizer to add, at the bare minimum, follow the application rates listed on the label of the fertilizer.

Now that you know all about fertilizer numbers and how to use them to grow a better garden, you’re sure to have your best growing season ever!