*3.2. Harvest Index (HI)*

The HI of the modern varieties of the intensively-cultivated grain crops is expected to fall within the range of 0.4 to 0.6 (40–60%) [37–39]. Considerably lower HI values were observed in our experiment for both crops: 33.1 to 44.0% for common wheat and 28.4 to 36.4% for spelt (Figure A1 in the Appendix A). In agreemen<sup>t</sup> with White and Wilson [38], N-fertilisation significantly increased the common wheat harvest index. The *t*-test resulted in a *p* = 0.03 probability value when HI of the N0 and N40 levels were compared to the HI values of the N80 and N120 levels. In contrary, HI of spelt was the highest in the control treatment every year. The difference in HI was significant between the N0 and N40 fertilisation levels (*p* = 0.039) and even between the N80 and N120 levels (*p* = 0.0001). On average every 10 kg ha−<sup>1</sup> increase in the N fertiliser dose decreased the HI of common wheat with 0.3%. This result was in good agreemen<sup>t</sup> with previous findings that spelt is significantly more vigorous in tillering than standard bread wheat cultivars [13]. LAImax data (see Section 3.3) also confirms it.
