Quantification of the nutrient flows

For the evaluation of the system, the following flows were considered:

 Nutrient solution: Incoming solution for irrigation.

 Leachates: Outgoing solution, excess of water drained from the substrate.

 Fruits: Production of the culture (tomatoes).

 Biomass: Leaves and stem of tomato plants.

 Perlite: Substrate of the crop.

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For each flow considered in the assessment, two different parameters were measured experimentally. On the one hand, the total amount of the flow, such as the volume of nutrient solution or the kg of produce. On the other hand, the concentration of the nutrients in the flow, for instance, the ppms of N in the nutrient solution or the g of N per kg of produce. The product of these two parameters provided the quantity of the flow, for example, the total g of N in the nutrient solution supplied along the crop or contained in the produce collected.

The study of the consecutive crops has been an iterative process. Although the measurement of the flows was similar for the three crops, the procedures and techniques for the analysis of the concentration of nutrients were modified in the second and third crops according to the experience of the previous ones. Table 7.2 shows the analytical techniques employed to analyse the concentration of nutrients in each of the crops.

Table 7.2 Measurement techniques used to analyse the concentration of nutrients in the different flows considered.

Production ICP-OES EA ICP-OES

Perlite EA* ICP-OES EA ICP-OES

S1=Summer crop 1, W=winter crop, S2=Summer crop 2, IC=Ion chromatography, ICP-OES=Inductively Coupled Plasma Optical Emission Spectroscopy, EA=elemental analysis,

*spectrophotometry was used for S1

The following sections explain in detail the methodology implemented for each flow and each crop.

7.2.3.1. Nutrient solution and leachates

The volume of nutrient solution used in each crop was measured with a conventional flow meter at the entrance of the irrigation head circuit. For the leachates, two water collection trays (three for crop S2) were placed under substrate bags in different parts of the crop to collect the leachates and to measure their volume. This value was used to estimate the total volume of leachates leaving the system.

Regarding the concentration of nutrients in these flows, samples of the nutrient solution were analysed once per week whereas the leachates were analysed three times per week during crops S1 and W. For the third crop (S2), a more accurate procedure was used to increase the reliability of the quantification. A sample was collected daily for each of the flows, and a representative sample for each week was prepared taking the proportional volume of the daily samples according to the water used for irrigation and the leachates generated.

As shown in Table 7.2, the analysis of the samples was made using ion chromatography to obtain the concentrations of nitrite, nitrate, phosphate, potassium, sulfur, magnesium and calcium for the three crops. In crop S2, additional micronutrients were measured:

sodium, iron, zinc and manganese. These micronutrients were measured using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).

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The product of the volume and the concentration of nutrients in the flow provided the total flow of nutrients supplied and leached.

7.2.3.2. Fruits, biomass and perlite

Regarding the fruits (tomatoes), all the production from the three crops was weighed to obtain the total amount of produce. Two samples of tomatoes were collected for crop S1 and six for W and S2 to assess the concentration of nutrients in the produce. Each sample was prepared collecting six representative tomatoes (average sized, normal shaped and non-perimeter) from one line of plants, which were dried at 343 K and mixed.

The resulting sample was analysed for nitrogen, phosphorus, potassium, sulfur, magnesium, calcium, sodium, iron, zinc and manganese using ICP-OES (Table 7.2). For each crop, average nutrient concentrations from the samples analysed were considered.

The product of these average concentrations and the total produce of the crop provided the total nutrient flows.

A similar methodology was implemented for biomass. During the crops, all the biomass from pruning was weighed, as well as the remaining biomass from the plants at the end of the crop. Regarding the concentration of nutrients in the biomass, four representative plants (three for S2) were cut from different lines at the final stage of the crop (to minimise the influence on production). For each plant, leaves, stems and the remaining fruits were separated, and the leaves and steams were analysed following the same process and techniques used for tomatoes (for the same nutrients). As for tomatoes, the average concentration of the nutrients in the samples analysed was considered for each crop and the product of this value and the total biomass from each crop provided the nutrient flow.

For the retention of nutrients in perlite, the total amount of perlite was measured weighing a dry bag (unused), multiplying its weight by 57 (bags in the crop) to obtain the perlite in the crop. The concentration of the nutrients was measured at different points during the three crops conducted to obtain the accumulation of nutrients in this perlite.

Firstly, a preliminary assessment was performed for nitrogen analysing a sample of perlite after S1 and a blank (unused perlite) using an autospectrophotometer (Bran+Luebbe AutoAnalyser3). After this previous assessment, samples of perlite that had been used for different periods of time were analysed. At the end of the second crop (W), two perlite bags were replaced by new ones and kept. Therefore, at the end of the experiment the bags removed had only been used for the first and second crops (S1, W), the replaced bags only for the last crop (S2) and the rest of the bags for the three crops (S1, W, S2). A sample was taken from each bag plus one blank, but some of the results obtained were not conclusive and a second batch of samples was collected to observe the variability among the bags. In this second batch, two samples were taken from two perlite bags used in S1 and W (four samples in total), three samples were taken from two perlite bags used for crop S2 (six in total) and 12 samples of the blank were taken from six unused perlite bags. The concentrations of nutrients found in these analyses can be found in Appendix VI.

The process for the collection of samples and the analysis of perlite is illustrated in Figure 7.3 Each bag (number 1 in Figure 7.3) was opened (number 2) and spread on the floor, mixing well the perlite with the hands and making a layer of perlite with uniform thickness. To take a representative sample, this layer of perlite was divided into 20

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sections (number 3), and approximately the same amount was taken with a spoon from each section. The sample was then carried to the lab in a plastic jar (number 4), and a similar process was conducted using a plastic tray to obtain a smaller sample for its analysis (number 5). For each sample (except the blank) the roots of the plant were carefully removed to measure only the nutrients retained in perlite, not the organic matter (number 6). The sample was then dried at 383 K and ground using an analytical mill (number 7).

Figure 7.3 Pictures of the methodology for the collection of samples from perlite bags and their preparation.

The samples of perlite were digested in duplicate with concentrated HNO3 in a microwave oven together with digestion blanks. Then, they were lixiviated, and the solids were removed, remaining the liquid solution with the extracted nutrients from perlite, which were analysed using ICP-OES for nitrogen, phosphorus, potassium, sulfur, magnesium, calcium, sodium, iron, zinc and manganese.

The results from this assessment provided the variation of the nutrient concentration (mg of nutrient per g of perlite) in the perlite along the crops. The correlation between these values and certain key variables of the crops was assessed to look for patterns in the accumulation of nutrients. The variables considered were: concentration of the nutrient in the nutrient solution and the leachates at the end of the crop, nutrient load of the nutrient solution and the leachates (quantity of nutrient supplied and leached) and duration of the crop (number of days that perlite was used).

The accumulation of nutrient in perlite was obtained from the difference between the concentrations measured and the total amount of perlite in the crop. For the nutrients that presented a clear pattern of accumulation, the quantity of nutrient accumulated in each crop was estimated.

Finally, an elementary analysis was done for all the solid samples discussed (fruits, biomass, perlite) to get the content of carbon, hydrogen and nitrogen in the sample. The results from these analyses were used similarly as the concentrations of the nutrients in the samples.

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7.3 Results