Apply fundamental principles of fertilization - even in tough times

Published: September 6, 2022 in SA Grain (Translated from Afrikaans to English)

Written By:

Dr. John Tolmay,

Kynoch Agriculturist, Central Region

Renier Bothma,

Kynoch Agriculturist, Central Region

Sunflower and soybean fertilization is often neglected when fertilization planning is done on the farm, but the anticipated shortages of oilseeds in the world and current good product prices place a renewed focus on these crops as good and alternative crop rotations. However, due to world events, fertilizer prices have skyrocketed and producers are trying to make sense of the situation. This article attempts to revisit the basic principles of fertilizing these crops within the current context.

The fundamental point of departure in fertilizing any crop is to provide sufficient nutrients for the production of the crop on the land and at the same time maintain essential levels of nutrients in the soil and build up where necessary. While fertilization planning is always based on an expected yield target, in any season there may be less or more fertilization required by the crop than was applied during the season. There can therefore be a surplus or a shortage of specific nutrients in any given season. If the crop requires less nutrition than was applied (a surplus situation), nutrients can in many cases be transferred to the following season. However, if a shortage arises, according to Liebig's Law, the yield will be limited to the lowest spar in the barrel (which represents the limiting factor).

It is therefore extremely important that the fertilization of any crop is considered holistically with soil fertility as the starting point of fertilization planning. Inherent soil fertility acts as a buffer in the good seasons when the plant's need exceeds the nutrition from the bag. Building up soil fertility must always be part of long-term planning to ensure sustainable and profitable production.

There is a strong focus these days on grid and precision analysis to improve soil fertility. With this, the liming of the soil is varied according to the lime requirement on different sections. So not only does the pH get attention, but also the availability of essential elements such as calcium (Ca), magnesium (Mg) and potassium (K). In fact, the correction of soil fertility plays an important role in the production of sunflowers and soybeans because both of these broadleaf crops are sensitive to soil acidity and have limited root systems.

The basic principle of good fertility management is therefore just as or even more important for these crops than for maize. The tendency in the past to plant these crops on shallow, infertile or marginal lands and grow maize on the more fertile lands never allowed the production of the crops to come into their own and helped to fully develop their production potential.

Withdrawal of nutrients

The extraction of nutrients according to the planning yield and the availability of nutrients in the soil (with recent soil analysis) remains the basis of any good fertilization proposal.

Since corn is mostly considered the main crop and is normally well fertilized, the misperception can arise that sunflowers and soybeans are light feeders that do not require much fertilization. However, if one compares the extraction rates of sunflower and soybeans with maize (Table 1) at the same yield levels, it is clear that these crops are actually heavy feeders that extract large amounts of certain elements.

A soybean yield of 2 t/ha removes the same amount of potassium as a 15 t/ha maize yield! If these crops are fertilized poorly (or not at all), as is sometimes the case with soybeans, this means that the soil is "mined" and that the reserves that will be needed in the future become depleted. The land may be able to provide a part of the need temporarily, but in the long term this can have serious consequences, especially if reserves are needed in a subsequent season to produce in a good year.

Fertilization of sunflowers

Although sunflower has the lowest requirement for nitrogen (N), phosphate (P) and potassium at the normal planned yield levels, the adequate supply of these macronutrients (as well as sulfur [S]) is essential for the successful cultivation of the crop. Sunflower usually responds very well to supplementary nutrition such as foliar sprays in combination with growth stimulants and regulators. The challenge, however, is that the crop suddenly grows tall and that it is difficult and/or expensive to apply further fertilization or even foliar feeding after the rosette stage. In the case of sunflower, it is therefore important that the necessary nutrients are available early in the season.

While sunflower, like all plants, requires the full range of micronutrients, the supply of boron (B) remains extremely important with this crop. Boron moves with difficulty in the plant and a boron deficiency can lead to serious losses if flowers start to fall. Although there are fertilizer mixes available that contain boron, their effective supply to the plant with fertilizer (through the soil and the roots) is not guaranteed and the spraying of boron and other microelements must still be continued throughout the season.

Fertilization of soybeans

Fertilizing soybeans is essential because the crop removes large amounts of phosphate and especially potassium during the season. Unfortunately, the perception has arisen that soybeans can only be planted on acidic, infertile soils and that they do not respond well to "fresh" fertilizer either. However, practice has proven otherwise and good producers know that soybean production on fertile and well-developed soil produces the best results. Since the band farm of fertilizer does pose a risk that can harm the inoculation, producers often prefer to spread the fertilizer in advance and then plant without fertilizer close to the seed. It can only be effective on well-developed soils where the phosphate status is sufficient to have enough phosphate available for the young seedling.

Phosphate that is then given for the season replaces the phosphate that will be used during the season. Furthermore, it remains desirable to provide additional potassium, even in soils that normally test high in this element, because all potassium in the soil analysis is not necessarily available. Like sunflower, soybeans respond very well to growth stimulants, the addition of microelements and biological supplements. Most soybeans produced are genetically modified to be glyphosate resistant (GM). However, this does not mean that the crop is not held back by the application of glyphosate and additional stimulation and strengthening before spraying usually helps to reduce its effect.

Furthermore, the rhizobium associated with soybeans binds nitrogen and the plant uses extremely large amounts of it (similar to a 10 tonne maize yield) with a 2 t/ha soybean crop. At particularly high yield levels in high-potential areas, it has become practice to apply additional nitrogen and sulfur at the R4-R5 growth stages to help increase yield levels. At these growth stages, the ability of the rhizobium to bind nitrogen is no longer impaired.

Strategies for the upcoming season

In these times of price instability and high input costs, planning for the coming season becomes increasingly difficult. There is a great temptation to adjust fertilization programs downwards and thereby try to cut costs. However, if one takes into account the needs of the crops, it does not really make sense to cut down on fertilization on a large scale, as this can have unforeseen and serious negative implications in the long term. Instead, stick with what works in the specific environment and cut out marginal lands that carry high risk. Go for the best possible production on the good sections and use the time to improve marginal sections for the next season. That way you give yourself the best chance to survive the tough times.

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