The aim of our experimental work of developing the fertilisation consultancy system of silo sorghum was to examine the effect of N, P and K supply levels on the yield and quality of silo sorghum at well separable nutrient supply levels in a long-term fertilisation experiment. The long-term fertilisation experiment was established in 1989 on deeply calcareous chernozem meadow soil at 4 different N, P and K supply levels each, applying 64 treatments, i.e., the whole treatment combination. This study presents the results of P fertilisation with respect to the main quality characteristics of silo sorghum based on the examinations performed between 2000 and 2002, the results of which are summarised below:

1. Averaged over the years of experiment, the raw protein content of silo sorghum without P fertilisation (120–138 mg kg-1 AL-P2O5) was 9.15%, which showed a trendlike increase as a result of higher P supply level (156–339 mg kg-1 AL-P2O5) and no significant increase in raw protein content could be shown as a consequence of better P supply level. Crop year affected raw protein content more significantly than P supply.

2. No significant P impacts were observed in the 120–339 mg kg-1 AL-P2O5 range of the soil in the raw fat and raw fibre content of the silo sorghum. Crop year affected raw fat content to a higher extent than the level of P supply.

3. The increase of P supply did not significantly affect the raw protein content of silo sorghum, but resulted in the change of a few amino-acids expressed in the percentage of dry matter in certain experimental years. These changes are not consequent as they do not affect the same amino-acids and they show decrease in dry crop years and increase in wet crop years depending on the water supply during the growing season.

4. The level of P supply caused a significant change in the amino-acid composition of raw protein. In dry crop years, the proportion of the majority of P supply decreased within raw proteins, while it increased in wet crop years as a result of improving P supply. P supply did not caused any significant change in the proportion of essential and non-essential amino-acids within raw proteins.

Keywords: P supply, silo sorghum, yield quality, long-term experiment