Nitrogen fertilization in ornamental plant production based on in-season demands through proximal sensing and soil modelling

The ornamental plant production sector produces high quality products, but faces challenges in times of increasing environmental constraints.

Growth patterns and nitrogen uptake dynamics of five ornamental crops were determined during three growing seasons (2016-2018). This resulted in nitrogen uptake quantities sufficient for optimum plant quality.

Open field production of herbaceous and woody ornamentals is characterized by a very broad range of species and cultivars, differing in growth dynamics, biomass accumulation as well as cultivation method. The Flemish ornamental plant sector produces high-value plants but faces plenty of challenges to maintain this high quality in times of increasing environmental constraints. For open field production, Flanders imposes nitrogen fertilization standards in order to comply with the European Nitrates Directive (91/676/EEC). To keep ornamental plant growers competitive on the national and international markets and to comply with these environmental restrictions, the sector needs a differentiated nitrogen (N) recommendation system, based on in-season demands. Indeed, high quantities of residual nitrate can be measured in the soil profile after the growing season, which is partly due to fertilizer gifts that all too often exceed the crop demand. In periods of high precipitation, this nitrate is susceptible to leaching through the soil profile, which subsequently leads to ground and surface water contamination.

Limited information was available about the nitrogen requirements of open field ornamental plants, despite this being an essential input variable of fertilizer recommendation systems. Therefore, N uptake dynamics of five ornamental plants species was assessed during three growing seasons (2016-2018). This was done through regular measuring and sampling on trial fields with different fertilization levels and nurseries where business-as-usual practices were applied. Chrysanthemum morifolium (potted chrysanthemum, cultivars ‘Maya’ and ‘Orlando’), Acer pseudoplatanus (sycamore, deciduous tree, forest seedling production), Ligustrum ovalifolium (Korean privet, semi-evergreen shrub), Prunus laurocerasus ‘Rotundifolia’ (cherry laurel, evergreen shrub) and Tilia cordata (small-leaved lime, deciduous avenue tree) were selected as representative for the open field ornamental plant production. This resulted in N uptake ranges that may be considered as adequate for a good plant quality. Because different plant ages and management practices were considered, we repeatedly demonstrated that N uptake was influenced more by age, planting density, transplanting of perennials, etc. than by the species itself. Furthermore, gathering data over a period of three growing seasons revealed that the N uptake is strongly affected by year-to-year varying climatic conditions, stressing the importance of multi-year nitrogen uptake datasets.

These findings highlight the importance of split-applications and in-season determination of plant N demand and soil N supply. With regard to the latter, based on our results, we would propose a soil analysis early in July and early in August for the woody ornamentals (A. pseudoplatanus, L. ovalifolium, P. laurocerasus ‘Rotundifolia’ and T. cordata) and C. morifolium, respectively. To determine in-season N status of the plants on the other hand, the potential of proximal leaf and canopy sensors was explored in this thesis. To date, these techniques are underexploited for (woody) ornamentals, yet they offer countless opportunities to support N fertilization.

At leaf level, we found that epidermal polyphenolic compounds measured with the Dualex sensor were highly correlated with foliar N% in C. morifolium. For the woody species, chlorophyll measured with both a SPAD meter and a Dualex sensor were a reliable proxy for foliar N%, but only when the leaf mass per area was considered. These results prove that proximal leaf sensors can be valuable decision-support tools to assess the in-season nitrogen status of both C. morifolium and different woody ornamental species. Nevertheless, some caution is recommended when different plant species are used, especially if leaf characteristics differ (e.g. presence of a wax layer). A relative approach, where a saturation index was calculated using a nitrogen-rich reference field, was considered to improve the usability because there are no absolute reference values available for an optimal (foliar) N% for the many different species and cultivars grown in nurseries.

At canopy level, the GreenSeeker was used for three consecutive growing seasons on the one hand. This widely used sensor measures red and infrared light reflected by the plant canopy and subsequently calculates the NDVI, a vegetation index related with photosynthetically active biomass. The obtained correlations between NDVI and biomass and N uptake were generally high, but species-specific. Furthermore, we also encountered saturation problems when the canopy closed. It was concluded that NDVI measurements can be useful to identify the need for additional fertilization and potential in-field variability, but do not serve as a proxy for plant N%. Here as well, a relative approach can help to rule out other parameters except an N deficiency, as long as the reference plot is representative for the rest of the field and a threshold value for action can be set.

On the other hand, the potential of a novel hyperspectral spectroradiometer was assessed in 2018. This device measures reflectance continuously in the visual and near infrared part of the electromagnetic spectrum (340-820 nm) and allows the comparison of multiple vegetation indices or the exploitation of multiple wavelengths through multivariate statistics. Although other vegetation indices besides NDVI were highly correlated with biomass and N uptake (especially for C. morifolium), mainly partial least squared regression (PLRS) was considered promising to provide site-specific diagnostics of crop performance. Here, for plant N% as well, high correlations were obtained on species-level. For both C. morifolium as for the woody ornamental species, end-of-season specific difficulties, including flower bud initiation and the relocation of N towards perennial storage organs (roots/stem), interfered with the correlation; however, as additional fertilization is often unfavorable around that period, these dates could be excluded during further research. More data confirming the robustness of the correlations are needed to develop a decision support system based on the prediction models.

Because plant growth is generally determined by interactions with the soil in a bottom-up manner, an important chapter in this thesis is devoted to soil N release, with emphasis on the influence of mechanical weed control and the opportunities of catch crops. For the case of avenue tree production, characterized by a low planting density, a low N uptake and repeated mechanical weed control, a field experiment was set up at two nurseries. During three growing seasons following the application of farmyard manure, soil mineral N was monitored by means of regular soil sampling. At one of the nurseries, we showed that at least 30% of the applied N was released in the second year. Partly due to the heterogeneity of animal manure, results were not always significant or consistent over time, nevertheless, the N release tended to continue the second year after application at the other nursery as well.

Attempts were made to validate the results obtained by the field experiment through a model-based approach in order to be able to quantify nitrogen release and leaching. As both processes strongly depend on water availability and flow, a soil water balance was used to simulate daily water movement. Furthermore, the mineralization speed of the soil and the farmyard manure were calculated through an incubation experiment in laboratory conditions, however, due the inhomogeneous nature of the manure (and perhaps other unknown interactions), results could not be translated into field conditions. Therefore, the model approach needed more fine-tuning than expected. Nevertheless, the nitrogen balance models were useful as an exploratory tool to quantify N release and leaching after management practices for the first time.

At both nurseries, there was a considerable mismatch between nitrogen fertilization and uptake by the trees, which could have been almost entirely covered by the N release after soil cultivation (if its timing would be synchronized with plant demand). N release succeeding a soil disturbance event was depending on time of the year, number of events and model decisions, but resulted in an additional N release of minimum 109 kg N ha-1 during the three experimental years at both fields. Average estimated N release after soil disturbance varied between 8 and 71 kg N ha-1; the more soil cultivation events were carried out, the less N tended to be released each time. In-field differences in nitrate content could be mostly attributed to the different weed control management, which shows that its impact should not be neglected. For example, an autumn rotovation of the soil following chemical weed control resulted in an exceptionally high amount of residual nitrogen and a doubling of modelled nitrate leaching during winter (± 108 NO3--N ha-1 compared with ± 54 kg NO3--N ha-1 in the other years). Lastly, we were able to associate catch crops, even when sown late, with a reduced N leaching of 33.5% on average. We therefore suggest delaying the control period for obligatory residual soil sampling for perennial crop growers in Flanders. Based on our results, we would recommend advancing the first soil cultivation, and thus (cold hardy) catch crop incorporation, to increase N availability at the start of the growing season instead of applying a mineral fertilizer.