When fertilizing, achieving the best possible efficiency is right at the top of the agenda. The effects on the transformation processes in the soil must also be considered – including the “priming effect”. What does this involve?
The mineralisation of nutrients is affected by a number of factors. The priming effect stands for the following phenomenon: More nutrients are mobilised than were applied with the fertilization – where do they come from? From the soil pool and the organic fertilizers. This occurs as a result of processes in the soil. The micro-organisms that are required for the transformation processes in the soil also need nitrogen and sulphur.
The amendment of the Fertilizer Ordinance will further increase the demand for fertilizer management. It will thus become even more important to optimize the production potential. The choice of fertilizer can also contribute to this.
Several factors are important for selecting the appropriate fertilizer: The soil type and pH value, the crop and its stage of development, also the nutrient content of the soil – and their relationship to each other.
Release of nutrients for plants and soil
An adequate supply of nutrients increases the microbial activity of the soil fauna in the rhizosphere, leading to increased mineralization – in other words, the release of nutrients: The organic matter of the soil is not initially available to plants as a nutrient pool for growth. It acts as a nutrient store, as the nutrients it contains must first be tapped by micro-organisms and enzymatic activity and made available to plants. Using mineral fertilizers not only feeds the crop plants, but also nourishes soil life, thereby encouraging its activity. This in turn promotes the release of nutrients. This effect works best in combination with organic fertilizers.
Last year, higher N-min values were often recorded than were achieved through fertilization in the previous year. Soils can demonstrate a nitrogen content of 4000-7000 kg/ha. However, this quantity is mainly organically bonded;1-3 % of this is mineralized each year and is thus accessible to plants. As a result of the effect described, this pool can be more strongly exploited. Particularly with activation within the vegetation period by using mineral fertilizers at the right moment – namely when the nutritional requirement of the plants is at its highest. Intensive soil tillage can also release a great deal of nutrient potential – although mobile nutrients in particular are lost unused if subsequent planting is inadequate.
Mineralization with the correct C:N ratio
In addition to the factors already mentioned, the quality of the organic substance and the soil structure is also crucial for mineralization. For example, nutrients are often not accessible in compacted soils. A further factor influencing mineralization is the C:N ratio. The lower this is, the faster mineralization occurs. An example of organic fertilizer with a wide C:N ratio is manure. When spread shortly before sowing, this can even fix nitrogen, and the crop will suffer from lack of nitrogen unless this is supplied subsequently.
To be able to apply mineral fertilizers as efficiently as possible, it is essential to know the nutrient content of the soil and of the organic fertilizers. In the “red areas”, an annual analysis of the slurry has been mandatory since the last amendment. But annual in-house sampling offers only a rough overview. Using near-infrared spectroscopy (NIR) during application allows the entire amount of slurry to be spread in a controlled manner in real-time. Depending on the status of homogenisation, nitrogen values can vary by up to 2 kg/m³. The finite resource phosphorus should also be used more effectively. In liquid manure, the majority is found predominantly in the sediment, thus undersupplied areas could be specifically targeted, while the liquid slurry could be specifically spread close to the farm, to save on transport costs.
The most reactive, concentrated solid fertilizers have the greatest priming effect.
Particularly with higher pH values, acidic mineral fertilizers should be used in a targeted way. The effect is pronounced in soils with a high clay content, which start slowly in spring and only warm up slowly. When using ammonium sulphate, there are various processes in the soil which lead to an increased release of nutrients; fertilizers with a high ammonium content promote micro-organisms.
Micro-nutrients available to plants due to acidification
A further process is the release of fixed nutrients by direct and physiological acidity. Due to the positive charge of ammonium (NH4+), the root emits H+ ions to balance the charge and has a physiologically acidic effect. The sulphur content has a direct effect; due to the negative charge of sulphate, this can bind with positively charged cations. Consequently, the pH value in the rhizosphere decreases and fixed nutrient potential is released. Calcium phosphate and micro nutrients in particular become available to plants in this way.
Sulphur as protein booster
For many years, little attention was paid to sulphur fertilizers, especially in cereal production. But due especially to reduced nitrogen fertilizing, crude protein contents will decline. To make full use of up to 15 kg nitrogen, at least 1 kg sulphur is required per hectare. It is an essential component of protein compounds and sulphur-containing amino acids. As the quality assessment of baking wheat will in future be more focused on baking quality, sulphur is particularly important here with regard to high molecular weight proteins, to achieve high baked volumes.
But which form of sulphur fertilizer is the right one? Sulphur is best calculated in sulphate form, as this is directly available to plants. Since the take-up of sulphur runs parallel to the take-up of nitrogen, a targeted combination of nitrogen and sulphur fertilizer in spring is recommended. Each fertilization measure, and all further actions on the soil, have effects on the interaction of macro and micro-nutrients. As a general rule: Soils with low clay content react faster and more sensitively to imbalances. In addition to nitrogen fertilization, balancing with lime suited to the location is a must, especially for soils with a lower proportion of carbonate-bearing parent rock.
The availability of macro-nutrient elements is strongly dependent upon the levels of exchangers. All factors must therefore be considered to make best use of the legally limited nutrients nitrogen and phosphorus.