(Early) Season Nutrient Management – Why Apply What When?

When it comes to fertility management, and fertilizer application in field horticulture situations, there tends to be a great deal of focus on the pre- and early-season time periods. You might (or might not) ask yourself why? Why apply nutrients at such and such a time? Does it really matter? Well, there ARE some actual reasons for applying fertilizers and soil nutrient amendments at certain times (especially in springtime) for certain crops. Let me flesh out the general principles surrounding some of them.

Manures:

There are lots of things to know and understand about manure, but the main parts are that it is an organic (meaning carbon-based) nutrient source, providing low levels of available nutrients for plants to use. It is essentially nature’s slow release fertilizer. It takes time and it takes a large quantity.

First off, in case you did not know, fresh or raw manure (meaning non-composted or not rotted) should not be applied to crops directly. There are several reasons for this, but the main ones are as follows:

• There is a risk of introducing human health pathogens to actively growing plants (a.k.a. food safety No-No)
• The manure will be too rich or too “hot” for plants, resulting in a toxic situation and a high likelihood of plant death
• Manure is often full of weed seeds, as well as potential plant health pathogens, which need to be reduced in number prior to use

So, it is important to properly compost manure before it is used. That is an entirely different topic, for another day, so I will not spend time on it. But let us assume that we are working with a composted manure for our discussion.

Manures are typically applied in the fall, or in years prior to when they will provide the direct nutrient benefits. This is because you must apply quite a lot of this low nutrient product and incorporate it thoroughly. And the overall main benefit of manure comes from the gradual release of nutrients. It takes time for it to start to break down, so you need to have it in place long in advance. Therefore, we apply it at least the season before (usually fall or early winter), with the expectation that it will provide some nutrients throughout the growing season. The nutrient contribution does not come in a big burst, but rather as a mostly steady release.

Green Manures and Cover Crops:

These crops are much like manures in that they are grown and incorporated in the year(s) prior to when the nutrients that they supply will be needed. They need time to break down, so that they can release their stored nutrients.

Fertilizers:

Most of the time, in conventional agriculture, when we want to make quick nutrient adjustments, we rely on inorganic and/or synthetic fertilizers. This allows us to precisely increase the levels of certain micro- and macronutrients as required, in some cases in high levels, to match plant demand. In most situations, these products are applied prior to the time when the plants will use them, to allow time for them to move through the soil and be taken up by plant roots. However, due to the mostly readily available nature of the nutrients in these products, the timelines do not have to be massively stretched out.

Occasionally, we will use foliar applied nutrients, which are taken up through the leaves for more immediate use.

Timing:

For many of the fruit crops in Alberta, most nutrients are applied just as the crops wake up and start growing actively. Most of the nutrients being applied are mobile forms of nitrogen, which is needed for plant growth.

For crops that have 1-2 year growth patterns, such as raspberries, nutrients are applied just as the snow melts and in mid-spring, to supply the new vegetative growth (primocanes) and allow the nutrients to move up into the plant as it starts to draw in water and nutrients. Nutrients are not generally applied later, as sudden changes in nutrient balances can shift plants’ focus back to a vegetative state, rather than fruiting. Late applications can also cause the plant to gear down for winter, potentially resulting in a greater risk of winter injury.

For woody bush crops, such as Saskatoons and black currants, nutrients are applied in mid-May and late June, to provide nutrients at a time when the plants are waking up and drawing in nutrients, when buds are breaking, and when flowering is completed and early fruit is set. Nutrients are not supplied earlier than emergence from dormancy, to avoid loss of the nutrients. Fertilizer applications are focused on providing an adequate supply for the season, but entirely front-load the system/plant all at once. Fertilizers are not applied later, to avoid mixing up plant signals for shutting down prior to winter, improving winter survival.

For non-woody perennial fruit crops, such as strawberries, applications are early, just as the plants are starting to wake up, after they are uncovered. The nutrients are pushed to the plant, to carry it through regrowth and the establishment of a foliar canopy for the season, and to supply adequate nutrients for flowering, fruiting, etc. Later in the season, after harvest, nutrients are supplied to top things back up, ensuring that the plants have adequate nutrients to establish branch crowns for next year’s flowering/fruiting.

For more annual-type perennials, such as day neutral strawberries, nutrients are supplied throughout the entire growing season, to support the ongoing, continuous growth and development of the crop. Due to the shallow root systems of these plants, and to supply enough but not too many nutrients, fertilizer is applied in small amounts. This also helps to avoid overloading and cause excess growth; however, the application rates are sufficient to match the rate of growth set by the plant. These nutrients are often supplied in water, or with water, to ensure that the nutrients get to the roots and into the plants, without delay. They may be applied as foliar applications, but that depends on your preference.

For annual vegetable crops, nutrients are generally applied prior to planting, incorporated into the soil, being then available for use by the crop throughout the season. Some water-soluble fertilizers may be applied at the same time as when transplants are planted, to ensure that the new plants have ready access to nutrients as they grow out from the growing medium. Some crops may receive additional fertilizers throughout the growing season, to top up their nutrients as they rapidly grow. For example, sweet corn often receives a top/side dressing (surface broadcast band application) of nitrogen in early-mid summer as growth really gets going. Some crops may benefit from specific foliar applications of certain nutrients, to meet changing needs and potential deficiencies.

Conclusion:

Nutrients are generally applied shortly before they are needed unless they are supplied in a form or way that delivers them directly to the plant without a high potential for loss. Overapplication or premature application can result in wasted nutrients or potential issues with plant growth. It is complicated, but the correct application of nutrients will result in good, healthy growth and good yields.