Leaf expansion,photosynthesis, and water relations of sunflower plants grown on compacted soil

The potentially mineralisable nitrogen extracted from 10 soils by two methods involving boiling with dilute KCl were compared with the actual uptake of soil N by spring barley in the field, as determined with the use of ¹⁵N-labelled fertiliser. Generally good correlations were found for those soils which had previously been cropped with cereals (defined in Great Britain as either N-Index zero, or low nitrogen status soils, for fertiliser recommendations), with the results obtained by the more severe of the two methods being somewhat better than by the other method. When organic matter content was also taken into account, correlations were improved. Mineral nitrogen in the soil at sowing was highly correlated with potentially mineralisable nitrogen, and with uptake, but this relationship did not hold for soil samples taken in January, well before the likely sowing date. This suggested that early measurement of soil mineral nitrogen (when decisions on cropping are normally made) was not a practicable method for determinining spring fertiliser applications, and that the measure of potentially mineralisable nitrogen appeared more promising in this regard.