Introduction
IPI's promotional activities started in Central Europe in the late eighties. The first years were spent mainly in getting to know the agricultural structure and situation in the countries and in making contact with scientists and research institutes. The earliest field trials were established in 1993 and sugar beet and white cabbage were the crops tested in the Czech Republic and Poland. Very soon all other Central European countries were also involved in field trial projects, so that today these activities stretch from Poland in the north to Bulgaria in the south (see map). Over the years 248 experiments using 17 different crops have been carried out, by far the most with potassium demanding crops like sugar beet, maize, potato and oilseed rape (Table 1). From all this work the results of many trials are now available demonstrating the positive effect of balanced nutrition with potassium on yield and quality of field crops.

During the last two to three years the focus of research activities has been concentrated more on specialty crops like gooseberry, poppy and cherry. Most of the earlier trials dealt with different application rates of K and the effect on yield, so that a typical field trial with sugar beet had three treatments with 80, 160 and 240 kg/ha K₂O, together with the control without potash application. At the beginning - fifteen years ago - it was difficult to show any effect of potassium, because most of the soils were rich in K as the result of heavy potash applications made in previous years. However, after 1990, because of the hardship brought about by economic changes, farmers in the region stopped applying potassium completely and it did not take long until soil reserves were greatly depleted and field trials were clearly demonstrating the need for potash. Over the years the number of trial using more and more different crops increased as did the selection of sites with varying soil and climatic conditions.

After this first campaign with different rates of application, other topics became more important. These included queries such as the most suitable form of potash for a certain crop and when the application should be made, whether in autumn or spring. It also became of interest as to how the different K rates influence the composition and quality of a crop, e.g. the effect of K on the pectin content of gooseberry or on the morphine content of poppy. The latest research project focuses on the potassium uptake of different rootstocks of cherry trees. All the results from the many trial projects are used in seminars, field days, workshops and conferences to broadcast the message of balanced nutrition with potassium for sustainable agriculture. They are also part of the many publications, which have appeared in the various languages of the Central European countries. A visit to IPI's website www.ipipotash.org will provide an overview of this selection.

Results from experiments
A few examples of the many trial results are discussed below.

Omission plot trial on a crop rotation in Romania

A classical crop rotation with maize, sunflower, spring barley and winter wheat was the basis for this trial. Besides a control without application of nutrients, two levels of full NPK treatment, adjusted to the needs of the crops were compared with omission plots of NP, NK and PK (Fig. 1). The results show that the higher application rate of NPK and the corresponding omission plots did not result in a significant higher yield, yet it improved yields (calculated as cereal units). However, omission of K (the NP treatment) resulted in decline of total cereal units of 8.6-13.2%, greater loss than that with the omission of P (the NK plot). Clearly, omission of N caused severe overall yield reductions. The results also suggest that at higher levels of nutrient application, the omission or short supply of a nutrient may cause higher losses.

Nitrogen-potassium interaction in sugar beet, Poland

First of all it is impressive to see how sugar beet positively responds to potassium fertilization even without any application of nitrogen (N=0; Fig. 2). At both N=90 and N=150, K application increased yields and brought higher returns for the nitrogen applied. Based on the results of this three-year field trial, the optimum fertilizer strategy was shown to be the application of 90 kg/ha N and 240 kg/ha K₂O.

Depot fertilization in maize, Croatia

Amelioration fertilization on very K deficient soils is a practical solution in many cases. At the site in Croatia described here, 500 and 1,250 kg/ha K₂O as MOP were applied in the spring of 2001 to a maize hybrid crop. The response to K application was monitored in 2001, and in 2002-2004, the residual effect of this applied K was observed (Fig. 3).

The yield response in 2001 to the depot K fertilization was more than 9.4% and 14.4% for the 500 and 1,250 kg K₂O/ha, treatments respectively (Fig. 3). In 2002, depot K application was very beneficial, as it improved yields by 16.4 and 23.5% for the K applied. In 2003, poor climatic conditions prevented high yield, nevertheless, the depot K was still affecting yield (+11.9 and +8.5 for the 500 and 1,250 kg K₂O treatments respectively). In 2004, there was virtually no yield response to the K applied in 2001.

It can be concluded that in these soils, a depot application of MOP has a positive effect on maize yields in the year of its application and improved yield for the next two years after application. Potassium effect on potato, Hungary This classical trial project with increasing K application rates took place in Tornyospálca, Eastern Hungary, during 1998-2003. The results demonstrate the efficient transformation of available potassium into yield of potato. Almost 5 mt/ha higher yield could be achieved with 240 kg/ha K₂O compared with the NP fertilized control (Fig. 4). This trial was carried out in the eastern part of Hungary on a sandy soil, where water supply is often a limiting factor. It is assumed that potash application was also serving as a "drought insurance" against dry spells (see also the report on K and drought in e-ifc 12, at http://www.ipipotash.org/e-ifc/2007-12/research3.php)

To summarize the success story of field trial projects in Central Europe, it has been demonstrated on many occasions that potassium is a very limiting nutrient in the region. Not only of importance are the increases in crop yield that are often obtained. More and more the farmers' focus is turning to improvement of quality of crops such as increased sugar or oil content, more colorful apples, better tasting strawberries and higher quality wine. This is to be expected as most of the countries of Central Europe are members of the EU, in which crop quality aspects are likely to play a major role in the development of agriculture.

Our program in the region responds to these expectations: Potassium plays a pivotal role in the quality of agricultural products, and Central Europe Coordination is striving to demonstrate this to the many farmers in the region.

Dr. Thomas Popp was born in Stuttgart, Germany. He joined the Agricultural University of Stuttgart-Hohenheim and finished his diploma in 1978. He got his PhD from the Technical University Munich-Weihenstephan, where he did his thesis at the Institute of Grassland Science based on aerial photos from the years 1917, 1953, 1960 und 1974. Topic of the thesis: "Changes in land use and development of soil erosion in parts of the Bavarian Alps since 1917". In 1983 Dr. Popp joined BASF and became responsible for technical advisory service in Asia and the Pacific. In 1987 he moved to Kuala Lumpur und served the region from there. After returning back to Germany in 1990 Dr. Popp was initially more involved with advisory service for the German market, but in the mid nineties he became responsible for Asia as technical advisor. In 1999 he got another overseas' posting, this time in Singapore, where he spent three years. With his return to Germany in July 2002 he left the nitrogen industry and joined K+S KALI GmbH. Since then he is serving the countries in Central Europe as advisor for potassium and magnesium. Dr. Popp is also acting as IPI Coordinator for this region.