The future framework conditions of a stricter Fertiliser Ordinance demand the highest standards of cultivation systems on cash crop and forage farms. Above all, the efficient use of the environmentally relevant nutrients nitrogen and phosphorus is in the foreground. This requires balanced plant nutrition, which is recommended to farmers by means of fertiliser recommendations derived from established soil analysis methods and many years of field trials.

Christoph Weidemann, Prof. Dr. Karl H. Mühling, Kiel University, and Dr. Lars Biernat, Schleswig-Holstein Chamber of Agriculture

In the past, crop production problems could be effectively corrected by using plant protection products and "repair nitrogen". However, these options are only permitted to a very limited extent under the future targets and must be realised much more with optimised production technology. To this end, agronomic principles must be brought back into focus, including in particular soil analysis, the interpretation of analysis results and the associated fertiliser recommendation. After all, the basis for secure yields and high nutrient availability is an active soil life, an optimal soil pH value and a balanced supply of essential plant nutrients.

In this context, the increasing challenges for agricultural practice are leading to a much more critical view of the soil testing methods recommended by the official advisory service as the basis for regionally typical fertiliser recommendations. Basically, the fertiliser requirements of different crops depend on the nutrient needs and the nutrient assimilation capacity of the cultivated fruits, the respective yield and quality expectations as well as the nutrient supply of the soil, taking into account the prevailing climatic conditions.

Standards in soil testing

In this context, many different methods have been researched and developed worldwide to record the composition and availability of nutrients in soils. However, only calibrated field trials under representative experimental conditions lead to corresponding, assured site-typical fertiliser recommendations. On this basis, the Association of German Agricultural Testing and Research Institutes (VDLUFA) has standardised the soil testing methods used in Germany. Many years of field testing activities led to a quantity concept with classification into content classes (A-E) and fertiliser recommendations derived from this.

In contrast, triggered by various activities of mostly private institutions, the method for determining the cation exchange capacity (CEC) as a basis for fertiliser decisions is currently often perceived as "more modern" or "more advanced". However, its origins date back to the 1940s in the USA. In Germany, too, universities and official advisory organisations carried out intensive research on this in the 1950s and 1960s. The knowledge gained there contributed to the development of today's VDLUFA methods.

Instead of a content class system, the so-called alternative methods define the fertility of soils primarily on the basis of the ratios of the positively charged nutrients (cations) to each other on the exchange particles (clay minerals, organic matter).

However, this does not take into account lime requirements or selective and specific nutrient binding to the various soil exchange particles. Furthermore, regional field trials based on soil and climatic conditions as well as yield conditions are not considered relevant. This completely ignores the interaction of plant and soil. In principle, cation exchange is an important soil property, but when interpreting the analytical methods and the fertiliser recommendations derived from them, the farmer is called upon to critically question the results. The following section is intended to provide assistance in explaining and classifying the essential parameters.

What is the correct way to estimate the pH value?

The pH value reflects the concentration of acidic positive hydrogen ions (W). It can be measured by various methods. In the past, an artificial soil solution with potassium chloride was used for serial tests of agricultural soils. Nowadays, however, calcium chloride is used for this purpose in the recognised standard procedure. The particular advantage of the solution with calcium chloride is based on the constancy of the measurement method in relation to fluctuations in the concentration and volume of the suspension liquid, as well as the lime potential of any given soil. If, on the other hand, only water is used, as in the Kinsey method, for example, the measured values are about 0.3 to 1.0 units higher. Only in soils with pH 7 and above is there no difference between the two methods. Those who do not take this into account are overestimating the pH value on acidic soils. Furthermore, in the standard procedure, target pH values for soils in type and humus content were determined on the basis of numerous field trials, and the necessary lime quantities to achieve them were determined.

What does the cation exchange capacity say?

The cation exchange capacity (CEC) is the ability of the soil to adsorptively bind cations and thus prevent them from being leached out, while at the same time keeping them in a form that is available to plants. The determination of the CAC is primarily used to characterise the site or to assess the soil fertility. A distinction is made between potential and effective cation exchange capacity. The potential CEC determines the maximum possible exchange capacity of a soil at a pH value of 7. Since the optimal pH value of many lighter or organic soils is significantly below pH 7, this capacity cannot be exhausted there or used as a basis for calculating fertiliser recommendations.

There are about eight different methods for determining the potential KAK, each of which provides different results and is not directly comparable. The effective KAK determines the positively charged nutrients that can be exchanged at the current pH value. As the pH in the soil changes, so does the effective CAC, because as the pH increases, so does the effective exchange capacity, and vice versa. The amount of magnesium and potassium that can be attached to the soil particles is directly dependent on the effective CAC. It can thus be calculated via the K and Mg contents determined in a standard investigation. Normally, therefore, there is no need for a time-consuming determination of the CAC. Both the potential and the effective CEC can be estimated with very high accuracy, provided that the clay, silt and humus contents of the soil as well as the pH value are known. (Tab.1).

What is cation exchange?

The soil solution contains cations (nutrient elements with a positive charge) with a single or double positive charge. Each of them, regardless of whether it is ammonium, potassium, magnesium, calcium or sodium, is surrounded by an attached layer of water molecules. In order to attach to an exchanger particle, this water envelope must be stripped off. Since both the number of positive charges (valence) and the size of the water envelope play a role in the extent of this attachment, this process is extremely difficult and can only be calculated in theoretical models. Therefore, it is not possible to arbitrarily change the occupancy of the exchangers by fertilising with potassium, calcium or magnesium by simply calculating the quantity.

If one nevertheless attempts to calculate the necessary nutrient quantities, one quickly realises what exorbitant orders of magnitude result in the conversion to a fertiliser recommendation, in order to theoretically change only one unit (in cmolc/kg soil) of the respective cation in the exchanger occupancy (Tab. 2). Thus, for example, a possible oversupply of potassium cannot lead to the displacement of any number of other cations from the exchanger and, as a consequence, possibly to a deterioration of the soil structure. The causes of poor soil structure often lie in too one-sided cultivation methods and the untimely cultivation or harvesting of arable land and can usually be effectively solved by known agronomic measures (soil cultivation, crop rotation, liming, adapted tyres and air pressure, etc.).

Cation ratios

The cation ratios only provide information about the current ecological condition of a soil, but do not represent growth and earnings factors. They are calculated values that vary within wide limits without any benefit or harm to the plants. There are enough soils that deviate quite considerably from a ratio assumed to be ideal with 60 to 70 percent calcium, 10 to 20 percent magnesium, 2 to 5 percent potassium and 0.5 to 3 percent sodium at the exchanger (Tab. 3) and still produce optimal yields. A wrong understanding of cation exchange can thus lead to inefficient fertiliser use.

Fertiliser forms

While fertiliser recommendations are made from the results of the standard soil test according to VDLUFA in nutrient amounts, some providers of alternative soil tests with included KAK also specify the form of fertiliser. This should be questioned, as crops react differently to nutrient forms (e.g. chloride tolerance) and as nutrient availability and pH value can change after fertilisation. For example, sulphur fertilisation with elemental sulphur leads to severe soil acidification (100 kg of 90% elemental sulphur corresponds to a lime loss of 162 kg CaO), whereas sulphur fertilisation via kieserite is pH-neutral. Moreover, elemental sulphur only becomes plant-available after its conversion into sulphate-sulphur. This conversion process depends on the soil temperature, which means that targeted sulphur nutrition of the crop and efficient nitrogen utilisation are not achieved.

Depending on the ratio of calcium to magnesium and often independent of the pH-value, sometimes considerable amounts of calcium (carbonic acid lime) or magnesium fertilisers (dolomite , kieserite) are suggested. Geologically determined high amounts of Mg in clayey soils (e.g. marsh soils) or also high amounts of Ca in carbonate soils and their effect on soil properties can only be influenced to a very limited extent by fertilisation, if at all. Furthermore, scientific studies show that the Ca-Mg ratio can vary within very wide limits without affecting plant growth and yield. For example, narrower Ca Mg ratios can also be classified as unproblematic. In the case of high pH values and/or high magnesium contents, gypsum fertilisation (CaSO) is often recommended by providers of alternative soil tests. Although this is not pH-effective, it can mean competition for magnesium uptake for the plant due to high amounts of calcium. Likewise, higher amounts of pure potassium can cause potassium-magnesium antagonism, i.e. competition for plant uptake.

A high calcium or potassium fertilisation can therefore cause a magnesium deficiency. A higher soil supply of magnesium, on the other hand, has no direct plant effects, as plants do not selectively take up magnesium, nor do they take it up in excess of their needs. In the meantime, it has also been proven that magnesium has no negative effects on soil structure. Deviating from the site-specific cation ratios, the fertiliser applications required to set ideal nutrient ratios in the soil, according to the ideas of alternative methods, are usually uneconomically high. The aim here is to adjust the nutrients to content class C according to crop-specific fertiliser recommendations via crop rotation - then no negative effects are to be feared.

Instead of carrying out a few expensive investigations that are not representative of all areas, it is much more successful for the optimisation of lime and nutrient supply to specifically sample and investigate the different soil types, especially on different fields. Furthermore, only the CAL method or the DL method used in Schleswig-Holstein is approved for the documentation of the P soil analysis in conformity with the fertiliser ordinance.

The soil testing methods established in Germany according to VDLUFA are based on decades of research, must undergo constant quality checks, and the fertiliser recommendations derived from them are calibrated in numerous regional field trials. Any discrepancies in cation allocation are adequately reflected in the content classifications (A-E) and the resulting fertiliser recommendations. For the determination of fertiliser requirements as a basis for high nutrient efficiency on the basis of good professional practice, the low-cost analytical methods recommended by the official authorities can be recommended without reservation. More in-depth analyses of soil type (sludge analysis), humus content and analyses of micronutrients in the cultivated sites can be very useful. Area-specific nutrient balances as a result of fertiliser supply and nutrient removal can provide valuable information on fertiliser application in line with requirements.