Balanced fertilization essential for a sustainable agriculture

To achieve its growth cycle, a crop needs a full range of mineral elements.
Six nutrients are required in relatively high amounts. Their concentrations in the plant tissue represent up to a few percent of the fresh plant weight. These are the macronutrients: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Eight other nutrients are required in much smaller amounts, in the magnitude of mg per kg of fresh plant weight. These are called micronutrients: molybdenum, nickel, copper, zinc, manganese, iron, boron, and chlorine. All of these nutrients fulfil specific functions in plants and cannot replace each other. This means that any lack of one single of these nutrients will hinder crop growth even if all other nutrients are fully available. As a consequence, a balanced supply of all these nutrients is essential to grow healthy crops that produce high yields of good quality.

Figure 1: Balanced nutrition gives highest yield

Source: RC Hanninghof 1997, 4b

A balanced fertilization not only guarantees optimal crop production and benefits for the growers, it is also the best solution to minimize the risk of nutrient losses to the environment.  If the balance of nutrients applied is not adequate, the crop won’t be able to grow properly and its overall uptake of nutrients will be limited. The supply of other nutrients will then be of no use, and these will accumulate in the soil leading to potential environmental problems.

In any case, the grower under such conditions will only receive a poor return on his investment in fertilizers.

Figure 2:
A well balanced fertilization optimises the nutrient use efficiency of crops.
The addition of potassium, magnesium and sulphur increased by 50% the nitrogen use efficiency of the wheat crop when compared to the supply of nitrogen and phosphorus alone.

Crop specific requirement
To achieve a balanced nutrition, an efficient fertilization program has to be based on the specific requirements of the crop that is grown. Both the total nutrient requirement and the ”right” nutrient ratio vary from one type of crop to another.

It is common to express the amount of macronutrients, required to achieve a specific yield level, by first fixing the amount of nitrogen necessary and then express the amount of the other macronutrients as a ratio relative to nitrogen (see table 1).

Table 1: Nutrient ratio differs according to crops requirements – (ratios based on whole crop uptake).

Crops also differ in their requirement for micronutrients. For example crops like celery, cabbage, oil palm or apple tree have a relative high boron demand. Any deficiency will lead to growth or quality problems for these crops whereas other crops like maize or onion are rather insensitive to low boron levels.

Supply from the soil
The nutrient ratios give only an indication of the fertilizer requirement of a given crop.
A fertilization program must first take into account the amount of nutrients supplied from the soil or from other sources such as the irrigation water, green or animal manure and residues from precedent crops. Since these sources are rarely sufficient, mineral fertilizer is added to fulfil the remaining nutrient needs of the crop.

Special soil conditions can make soil minerals unavailable for plant uptake, making a balanced supply of nutrients difficult. As an example, micronutrient availability is strongly reduced in soils with high pH levels, even if these minerals are present in the soil. In other cases the uptake of certain nutrients may be reduced due to competition with other nutrient elements that are present in too large quantities in the soil.

The fertilization program must be adapted to the specific growing conditions to alleviate such problems.

Setting the final balanced fertilization program
To finalize his balanced nutrition program, the grower chooses the nutrient sources, the appropriate way to apply fertilizers (e.g. soil broadcasted or foliar application) and when to apply them. This is important since the timing of application of each nutrient may vary for optimal efficiency. While nitrogen is more required during the vegetative stages of plant development to support new shoot growth, calcium is especially important during the early development and potassium during the rapid expansion of fruits and tubers. The same is true for micronutrients where boron- to give an example - is often required for good pollination and fruit setting.

A balanced fertilization strategy is the only way to ensure a sustainable agriculture that can provide the world population with high quality food while minimizing the impact on the environment. All available knowledge about the crop and the environment where it will be grown must be combined to set up the right combination of nutrients to be applied at each step of the fertilization program.

The use of compound NPKs can be a convenient and efficient way of providing balanced nutrition to the crops.