## 4 Discussion  
Measuring corn plants fertilized with organic fertilizers using field spectroscopy is much more difficult than other plants fertilized with mineral fertilizers due to the release of soil-borne N in combination with environmental circumstances such as the soil temperature and water supply through rainfall [52]. Nevertheless, it was possible to detect differences in the development of the plants. Similar to the findings of [53], in this experiment the sensor showed these differences in different values of the REIP vegetation index. At the first measurement date at the early growth stage of BBCH 13 (Figure 3), the development of the above-ground biomass was very low. The visible soil had a slight effect on the measurement results taken at that time [54; 55]. Similar to the findings of [56] for winter wheat, the results showed clear differences between the various organic fertilizer variants. It is known from previous studies that corn reacts to a differentiated supply of N at this vegetation stage [57; 58; 59; 60]. This can be measured with multispectral sensors and the REIP vegetation index [61]. Moreover, at this early stage the fresh composts showed the highest REIP values of all fertilizer variants. Unlike matured compost, for fresh compost the composting time is much shorter. Fresh composts are generally not yet as stable as matured composts [62; 63; 64]. Both organic waste composts, as well as green composts, contain a larger proportion of easily decomposable organic matter that promotes soil life and mineralization within the soil [65]. This can explain the faster fertilization effect in corn and, therefore, the higher values of the REIP vegetation index. At growth stage BBCH 14 (Figure 4), these differences became significant. Except for the unfertilized control variant, the fertilization with biogas fermentation residues, as well as fresh green and fresh organic waste composts, at an application rate of 120 kg N/ha and the matured green compost at an application rate of 240 kg N/ha were statistically clearly distinguishable. On the other hand, the highest REIP values were found for the variants fertilized with biogas fermentation residues and fresh organic and fresh green waste composts at an application rate of 240 kg N/ha. Except for the control variant, these compost variants were not statistically clearly differentiated. At growth stage BBCH 32 (Figure 5), other values of the vegetation index REIP could be observed. The highest values were recorded in the variants fertilized with matured organic waste compost with an application rate of 240 kg N/ha and with microbial carbonized compost. Fresh composts and biogas fermentation residues had decreased. A possible explanation could be that between the very early growth stages and stem elongation at growth stage BBCH 32, more organically bound N was released  
Figure 7: Measurement results of the soil-nitrate at growth stage BBCH 32.  
due to higher soil temperatures and water supply [52]. These differences became clearer at the next growth stage of BBCH 33 and were significant (Figure 6). The highest REIP values were observed for the variant fertilized with matured green compost at an application rate of 240 kg N/ha (Figure 6). The second highest values of the REIP vegetation index showed the variant fertilized with 240 kg N/ha of the microbial carbonized composts. The variants with the organic waste compost and the microbial carbonized compost could be statistically significantly differentiated from the variant fertilized with green compost. All three of them had an application rate of 240 kg N/ha (Figure 6). A first result was that the differences between variants fertilized with 120 kg N/ha and 240 kg N/ha were not very high at later growth stages. This result was supported by the values of the REIP vegetation index.