Phosphorus: What’s the Tie-Up?

Phosphorus: What’s the Tie-Up?

May 1, 2020

What’s the Deal With Phosphorus?

Phosphorus is a nutrient that is greatly needed for high yields in crop production. It stimulates root development, increases stalk and stem strength, improves flower formation and seed production, improves crop quality, and supports development throughout the entire life cycle of the plant.

Phosphorous has been receiving a lot of attention lately in the agriculture industry, with an emphasis being on using 4R Nutrient Stewardship’s best management practices to reduce or eliminate phosphorus moving into the Great Lakes. Many growers within the province currently still do not soil sample properly. If a farm is not soil sampled correctly, understanding how much phosphorous is currently in your soil is simply a guess. Having large quantities of phosphorus in your soil doesn’t mean it will all be available to a crop.

There are 3 different ways that a plant will take in phosphorus from the soil: interception, mass flow, and diffusion. The majority typically is taken up through diffusion, which is the movement of ions from areas of high concentration to areas of low concentration. Phosphorus is very insoluble, so movement is very slow. Correct placement and maintaining good soil levels of phosphorus are essential. An example of this is if you have loam soil; phosphorus must be less than ¼’’ from the root to be taken up. Phosphorus does not readily leach out of the root zone; the loss of phosphorus mainly stems from erosion and run-off of fertilizer or soil-adsorbed phosphorus.

Many growers do not know that phosphorus can easily become tied up and unavailable in their soil. Two important things to remember about phosphorus are:

  • High pH levels (anything above 7.3) has calcium that can combine chemically with phosphorus, creating a compound that is insoluble and that crops can’t access.
  • Lower soil pH (below 6.3) can have iron and aluminum and manganese begin to tie up the phosphorus, also making it unavailable to the plant.

Phosphorus can exist in both organic and inorganic forms. In organic materials, the phosphorus is released by mineralization which occurs when microorganisms break down the soil’s organic matter. For inorganic phosphorus, when phosphorus reacts with iron, aluminum and calcium, it will create a product that is not very soluble, and this is considered to be fixed or tied up.

How Can I Protect My Crop’s Phosphorus?

There are several ways to protect and manage your phosphorus to ensure it’s not becoming tied up in your soils:

  • Band your phosphorus. This keeps areas concentrated in the soil, allowing less to become tied up than if it was just broadcasted on top or broadcasted and then incorporated.
  • Take advantage of products that protect your phosphorus from getting tied up. Using a product such as ***** from Sylvite can be a great option as it reduces fixation of phosphorus in the soil.
  • Cold Soil and Starters. If organic matter is a source of P, then it will release slowly if the soil is cool and wet. Roots have a hard time absorbing P when the soil is cold; that’s why starters with P close to the seed is so important.
  • pH Counts. We know that phosphorus is most available between a pH of 6.5 and 6.8, but it can be hard to keep our soils exactly within that range. Accurate, proper, and timely soil sampling can make sure lime is being applied to correct the pH, or acidifying products are used to lower the pH.
  • Mycorrhizae to the rescue. These are fungi that live in the soil and invade plant roots. They can provide supplementary P to the plant. These fungi move out over the soil and absorb many minerals, such as phosphorus, and pass that on to the plant

Myths about Phosphorus

There are some misconceptions out there about phosphorus. Here are a few:

Myth: Orthophosphate is better than polyphosphate because it is in the plant available form.

  • Polyphosphate ions are readily converted to orthophosphate ions in the presence of soil moisture. If the soil temperatures are normal the conversion can be completed in days with normal soil temp. Trials done show that fertilizer applied in the different forms clearly show that a pound of P is a pound of P.

Myth: Liquid Phosphate is more mobile in the soil.

  • Phosphate will not move very far at all in the soil regardless of fertilizer form (liquid or granular.)

Myth: Liquid Phosphate is available throughout the entire growing season.

  • Phosphate fertilizer in any form can get tied up quickly. Only 10-30% of applied phosphate fertilizer is available during the season it was applied in. It is important to remember that seed placed P fertilizer will always be more efficiently available compared to any broadcast that is spread on the field.

Mainly a Myth: Elemental Sulfur acidifies the soil and frees up phosphate for the plant.

  • While it is true that elemental sulfur can acidify the soil or acidify a part of the soil locally, it has not been proven to be effective for improving phosphate availability.

Availability Factors for Phosphorus

Several factors come into play when you are dealing with phosphorus and making it available to the plant.

  • Aeration and Compaction. If water levels are low, absorption by roots is low.
  • Moisture. Diffusion needs moisture, excess limits water.
  • Phosphorus level of the soil. More P concentration to move around.
  • Amount and Type of Clay. The more clay that is in the soil, the more fixation that can occur.
  • Time and Method of Application. The longer the time between when he P is applied and the crop is planted, the more chance it has to bind with the soil. The method of application determines how far the P will be placed from the roots.
  • Other Nutrients. The levels of other nutrients can affect uptake.
  • Soil pH. Most available at 6.5 – 6.8.
  • Temperature. Temperatures influence plant growth and P conversion.
  • Crop. Deep rooting crops have more areas to pull from.