The verification of the concept “Facing the critical value challenge” has been performed on 9 arable fields of an agricultural farm (52 ha) located in the village Kujawki (Golancz District, Wielkopolska Region, Poland). In total, 99 soil samples were collected at the depth 0-20 cm from 9 fields. Basic properties were determined, i.e., soil particle distribution, organic carbon, pH (in 0.01 moles CaCl2 dm–3), cation exchange capacity (CEC). Moreover, Cu, Zn, Fe and Mn were extracted by 6 moles HCl dm–3 (pseudo total forms) and 0.005 moles DTPA, pH 7.3 (active and potentially mobile forms). Plant material (99 samples) was collected from winter triticale, winter wheat, winter barley, grass mixtures, winter oilseed rape and sugar beet at respective growth stages, dried, ground and analysed for Zn. The elaboration of soil (Zn-DTPA) and plant (Zn-Plant) critical values proceeded in two steps, i) tabular interpretation of data through adjustment to critical values in literature, ii) graphical readaptation of the C-Shaped, i.e. Piper-Steenbjerg curve. The results revealed that the amounts of Zn-DTPA varied in a wide range, from 0.80 to 4.30 mg kg–1, but its overall share in total Zn fluctuated from 2.0 to 7.9%. This implies that the geoavailability of zinc compounds seems to be relatively high. The relationship established for the pairs Zn-Total versus Zn-DTPA (YZn-DTPA = 0.092Zn-Total – 2.00; R2 = 0.63) yielded a significantly high coefficient of determination as a proof of the importance of Zn-Total in controlling the Zn-DTPA pool. The critical Zn-DTPA range varied from 0.80 to 1.43 mg kg–1 with a mean value of ca 1.08 mg kg–1, reflecting 67% of all investigated sites. The readapted C-Shaped, i.e. Piper-Steenbjerg curve (Zn-Plant versus Zn-DTPA) allowed establishing a critical Zn-Plant content at 15.3-39.7 mg kg–1 for the investigated crop plants. The mean critical value reached 33.3 mg kg–1 and divided plants into two groups, i) experiencing deficiency: winter wheat, winter oilseed rape, sugar beet and grass mixtures and ii) not experiencing deficiency: winter triticale, winter barley and winter wheat. These findings give a new insight into the urgent need for elaborating critical values for a wide range of crop plants in use in Poland.
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