Promoting Root Growth: Choosing the Right Fertilizer

By: Ken Wilson

Selecting the correct fertilizer can be a tiresome (and headache-inducing) experience. While most store-bought fertilizers specify the N-P-K ratios, the information is often sketchy on what that does, and choosing the right fertilizer for promoting root growth is not always clear. To help clear up the uncertainty, we’ll examine what encourages plants to put out strong roots.

While phosphorus and potassium promote plant root growth, phosphorus is the greater driver. Plants require balanced nitrogen, potassium, and phosphorus for optimal growth; each macronutrient is vital. Various factors, like soil and plant type, influence how much fertilizer plants need.

Now that you know you need a phosphorus-centric fertilizer, how much do you need? How does phosphorus fertilizer promote root growth? Does it improve other aspects of plant growth? And are there organic substitutes?

What Fertilizer Promotes Root Growth?

Plants require balanced amounts of nitrogen (N), Phosphorus (P), and Potassium (K) to thrive (and many other “micronutrients” in smaller amounts). However, each macronutrient influences the plant slightly differently.

Nitrogen is essential for all “green” growth (stems and leaves), and potassium is critical for fruit and flowers (it also promotes root growth). Phosphorus facilitates nutrient and water transportation in the plant while subsequently facilitating root growth/development.

Most store-bought fertilizers display a “N-P-K” ratio, which describes how much of each macronutrient there is in relation to the others. Although plants need P to improve root growth, without a healthy balance of the other nutrients, plants won’t benefit.

Consequently, when adding fertilizers to your soil, you’re attempting to correct a deficiency because when there is too little of a particular nutrient, a plant’s growth is stunted.

How to Choose the Correct Phosphorus Fertilizer

A systematic approach is essential to choosing the correct fertilizer from the overwhelming number on the market.

Test Your Soil For Deficiencies

Before purchasing a fertilizer, you must test your soil to determine any deficiencies. While there are home testing kits, taking a soil sample to a nursery or the lab they use might be worthwhile to get a better result.

The energy and money you spend on soil testing depends on your goal. A proper test is essential if you’re starting a small-scale farming project. If you’re planting a garden, a home test should be sufficient.

Select the Correct Fertilizer Ratio

Once you’ve identified the deficiency, selecting the correct N-P-K ratios is the next step. You’ll want a fertilizer with a 1-2-1 ratio (N-P-K) for root development. Alternatively, a 1-2-2 mixture focuses on fruiting and flowering but will also help root development.

Before purchasing a fertilizer, take the soil test results and enquire with professionals in the field. They may suggest an all-purpose/general fertilizer with a ratio of 1-1-1 as your soil could have sufficient P and K but requires an environmental change.

The most commonly used phosphorus fertilizers include:

  • Double superphosphate (17.5% P) – phosphoric acid produces better results than sulfuric acid.
  • Monoammonium phosphate (21% P) – mixing ammonia and phosphoric acid.
  • Superphosphate (9% P) – rock phosphate is mixed with sulfuric acid to release the nutrients.
  • Triple superphosphate (20% P) – A more refined P source also produced with phosphoric acid.
  • Ammonium polyphosphate – this inorganic salt comes from mixing ammonia and polyphosphoric acid.
  • Diammonium phosphate (20% P) – a different solution/process of mixing ammonia and phosphoric acid.

Bury Your Fertilizer During Application

Burying P fertilizer roughly 6” underground encourages roots to “reach” toward it, promoting quicker growth. By burying the nutrients, you’ll also delay the plants from flowering/fruiting, which results in more nutrients available for root and other growth/development.

How Does High-Phosphorus Fertilizer Promote Root Growth?

Macronutrients influence plants at a cellular level, and the various plant cells require different nutrients to fulfill their essential functions.

Phosphorus is involved in all forms of nutrient transportation in the plant. I.e., without phosphorus, there is no movement of nutrients between the plant’s roots and the rest of the plant.

Plants absorb P from the soil through root hairs. Once inside the plant, P will stay in the roots or move into the stem, leaves, and flowering parts as required.

Once inside, P (and K) encourage plants to push out new roots (from the growth tips) and help to thicken existing roots to better stabilize the plant.

What Other Benefits Does Root Promoting Fertilizer Provide?

Although the macronutrients benefit certain plant parts, the wonderful news is they influence many more areas than only the one targeted.

Aside from root growth, plant cells use phosphorus for several purposes, including:

  • Cellular division (phosphorus becomes part of the DNA and RNA)
  • Converting sugar and starch
  • Energy transfer and nutrient movement/transportation
  • Fruit and flower production
  • Photosynthesis

Thanks to its role in transferring nutrients through the plant, phosphorus is essential for all plant parts. Without sufficient P, plant growth becomes stunted.

Do Other Fertilizers also Help the Roots?

It’s critical to remember that macro (and micro) nutrients function together in the plant to promote growth. Although P is “better” for root development, plant roots won’t grow without N and K.

Nitrogen is paramount to practically all of a plant’s functions. It is in chlorophyll (for photosynthesis), forms part of plant cells (and is used for the growth of all plant parts), and in the roots, nitrogen forms proteins and enzymes that absorb nutrients from the soil.

Potassium influences the transportation of carbohydrates and nutrients, moving water in the plant, and activating enzymes, which influence other processes.

Other Factors Which Influence Root Growth and Fertilizers

Macronutrients provided by fertilizers (and those already in the soil) are essential to plants, but simply adding more does not always produce the desired results.

Several additional factors, including the following, significantly influence plants’ uptake of particular nutrients.

The Plant Species and Its Nutrient Requirements

Which plant you’re growing determines what its nutrient requirements are. Although you can play the “guessing game” with plants, most plants come with a list of their requirements on the packaging.

If there are none, asking at a nursery (or online) is a reliable method to find out. We often make assumptions about our plants’ needs, only to discover it was a small part of the picture.

The Soil and What’s Inside of It

After figuring out what your plant needs, you’ll need to know what’s in the soil (nutrient composition) and the soil type, as these influence how much fertilizer and in what form the plants need.

Macronutrients occur naturally in the soil; however, because of their importance, there are often deficiencies. Different soils (clay, sand, and loam) hold varying amounts of nutrients for plants. The soil also determines how easily plant roots grow.

Sandy soil is often softer, and roots move easily through it, while clay soil is much “harder,” and roots take time to grow. Loam is the happy median. Additionally, P binds to iron and aluminum molecules in the soil, which makes it unavailable to plants, so understanding what other compounds are present is essential when applying soil amendments.

Water Availability Influences Nutrient Uptake

How much water stays in the soil relates to the soil type (sand has the most drainage, while clay retains the most), watering schedule, climate, etc. This water influences how much fertilizer plants absorb.

Plant roots absorb nutrients dissolved in water molecules, and too little water means they won’t absorb enough fertilizer to promote root (and other) growth.

When applying fertilizer, it’s essential to sufficiently water your plants; otherwise, there is a risk of chemical burns (where the fertilizer salts draw water out of the plant’s roots. Too much water is also bad as it washes the available nutrients away. Providing the correct amount of water is essential.

Are there Organic Fertilizers for Root Growth?

There is a considerable movement from chemically-heavy fertilizers to organic sources, particularly in home gardening. Organic nutrient sources are often “slow-release,” perfect for long-term soil enrichment. These fertilizers are less likely to burn plants and over-nutrify the soil.

Rock phosphate, compost, manure, and bone meal are fantastic sources of phosphorus (P). At the same time, potassium (K) comes from wood ash (particularly hardwoods), kelp meal, and greensand (but manure and compost usually contain some K as well).

It is usually more beneficial to use compost, manure, and other organic sources when providing nutrients to the soil instead of harsh chemical fertilizers. The organic counterparts improve soil structure, help to hold water, and encourage soil biota.

Final Thoughts

Plants require a balance of macronutrients to flourish; however, phosphorus and potassium (to a lesser extent) are essential for promoting root development. While chemical fertilizers are widely used for crop production, supplementing P deficiencies in the soil with organic-based products like compost and manure is more beneficial to plants and soil organisms in the long run.

About the author 

Ken Wilson

Long time career in the home services industry from remodeling to patio construction. Currently residing to in SWFL and active contributor to multiple home & garden publications.