Remember the 4-Rs when applying nutrients to the soil

Tuesday, February 5, 2013

Applying the Right nutrient source at the Right rate, Right time and Right place will ensure the nutrients needed for optimum growth are available to plants when and where they are needed

by KEITH REID

If you have read about fertilizer in any of the farm papers, you may have seen reference to the 4-R Nutrient Stewardship Initiative. This industry-led initiative encourages farmers to use nutrients properly by following the 4-Rs – apply the Right nutrient source at the Right rate, Right time and Right place.

Following these guidelines will ensure that the nutrients needed for optimum growth are available to plants when and where they are needed, and are not wasted. It is only when more nutrients are applied than the plants can use, or applied where or when the plants can't reach them, that they can become pollutants. They will also ensure that farmers are gaining the greatest benefit from their nutrient applications.

But what do these 4-Rs mean in your fields? How do you decide what is right for you?  I will try to answer these questions and share my perspective on what you should think about.

This column will focus on source and time, followed in later columns with rate and place.  Keep in mind, however, that all four Rs interact, so what is the right rate, place and time for one material could be quite different for another.

Right Source – what does it mean? Don't worry. I'm not going to tell you to change your fertilizer. This may occasionally be the case, but most of the time it is much simpler than that. I'm just going to suggest the best possible use of the nutrients you already have on hand around the farm.

If you have residual nitrogen in the soil from a previous legume crop or livestock manure that you will be spreading on your fields, then those are the right sources to be using first.  

If you still need to apply mineral fertilizer after accounting for on-farm sources of nutrients, there are only specific cases where you would need to move away from the standard materials available from most fertilizer outlets (usually urea, MAP or DAP, and muriate of potash). Some considerations for switching fertilizer sources include the potential for losses with the chosen application timings, potential for fertilizer burn with the chosen placement, or the need for additional nutrients beyond what is included in the "standard package."

Urea can lose a significant portion of its nitrogen to the air if it is left on the soil surface during warm, moist conditions. You may want to switch to ammonium nitrate or ammonium sulfate if you are surface-applying nitrogen later in the season.  

The choice of materials is limited for seed-placed fertilizers, so you should stay away from urea, DAP or potash in this application. These materials can be used safely in a 2X2 band, but only up to a point. If you need high rates to meet crop requirements, then you should either choose a different material, like MAP, or split the application between banded and broadcast methods.

The need for secondary or micronutrients will also dictate the use of non-conventional materials. If you need sulfur, substituting ammonium sulfate for urea, or potassium sulfate for muriate of potash, will help to overcome deficiencies of this nutrient.

There are numerous micronutrient fertilizer materials that can be added to a fertilizer blend to meet particular requirements. You should generally use the material with the highest solubility, but be aware that some of the soluble materials can absorb water out of the air and cause the fertilizer to cake if it is not used soon after blending.

Right Time – what does it mean? The timing of nutrient application is generally more important for nitrogen than for the other nutrients.  Phosphorus or potassium will stick to the soil particles, while nitrogen ends up in a form that moves with water in the soil and so can be lost over winter. The general rule is to apply soluble forms of nitrogen as close to the time that plants will be using it as possible.

For short season crops, like spring cereals or canola, nitrogen applied immediately before planting is usually available as top-dressed nitrogen, but applying more than a day or two ahead of planting should be avoided. Longer season crops, like corn, show a distinct benefit from splitting the nitrogen between a starter application at planting and then a larger side-dress application when the corn is about to start its rapid growth phase.

In our Ontario climate, fall applications of nitrogen should be avoided. Our soils don't stay cold enough over winter to stop biological activity and the risk of nitrogen losses through leaching or denitrification is significant. One exception is the application of solid manure. There is some evidence that the organic nitrogen in this material is more available to the crop from fall-applied manure, as it has had more time to mineralize.

There is also one situation where application timing is critical for phosphorus and that is for broadcast phosphorus without incorporation. Do not apply P fertilizer and leave it on the surface over winter as the risks of losses with runoff water are extreme. If you must broadcast P in the fall, be sure it is incorporated soon after application to lock it into the soil. BF

Keith Reid is manager (eastern Canada), Soil Nutrient and GHG Management Agriculture and Agri-Food Canada, Guelph.