Setting Up of a New Local and Ecological Substrate for Tomato Soil-Less Cultivation to Cope with Saline Soils ^†

Abstract

Soilless culture is one of the alternative techniques to cope with saline soil. The objective of this study is to identify a new local substrate based on cactus fiber. Three mixtures of cactus fiber and coconut fiber substrates (M0, M1, M2, M3, and C) at a ratio of (1:0, 3:1, 1:1, 1:3, and 0:1), respectively, were evaluated on round tomato cultivation under greenhouse. The mixtures had an organic matter content that exceeded 82%, a C/N ratio between 41.3% and 46%. The mixtures did not cause any phytotoxicity in lettuce and bean. The economic study showed that the use of M2 substrate was the most profitable.

1. Introduction

Global agriculture faces the challenge of ensuring optimal productivity while respecting the environment and preserving natural resources [1]. Soilless farming offers an alternative to problems associated with agricultural intensification such as soil fatigue, salinization, and soil diseases [2]. However, this cropping system requires a high level of technical expertise and must be associated with high value-added crops in order to make a return on an initial heavy investment. In Morocco, this production system is growing rapidly, particularly in the Souss-Massa region, which faces water scarcity problems [3]. Imported organic substrates such as coconut fibers are expensive in terms of foreign exchange outflow, especially since the quantities imported in potting soil and the areas of soilless crops are constantly increasing over the years [4]. On the other hand, mineral substrates such as sand, despite its good stability of properties that offers the possibility of use over several years [5], remain subject to problems such as low water retention capacity, risks of contamination by pathogens or the presence of limestone that disrupts the pH, and substrate compaction that leads to root asphyxia [6]. It is then necessary to look for local substrates that can replace imported substrate and that have valid physicochemical, mechanical, sanitary, and economic properties. In the present study, we will study the evolution of physico-chemical parameters of substrate mixtures based on cactus fiber and coconut fiber as well as their impact on the productivity and profitability of a round tomato variety in the Souss-Massa region (Morocco).

2. Materials and Methods

2.1. Preparation of the Cactus Fiber Substrate

The dried cactus paddles were sorted, crushed, and then sieved in two times; the first in a 10 mm × 10 mm sieve and then with another 2 mm × 2 mm sieve to have a granulometry between 2 and 10 mm.

2.2. Physico-Chemical Analysis of the Three Mixtures

The study consisted in determining the physical properties (water retention capacity, apparent density (Da), real density, and porosity), chemical properties (organic matter content (OM), pH, electrical conductivity (EC) at 20 °C, cation exchange capacity (C.E.C in meq/100 g of dry matter), the ratio carbon/nitrogen: C/N, the mineral nitrogen NH₄⁺ and NO₃⁻, and the assimilable phosphorus (H₂PO₄⁻); the potassium K, calcium Ca, and the trace elements in mg/liter (Fe, Zn, B and Mo)) and sanitary of the substrates.

2.3. Growth Attributes Assessment and Fruit Quality and Folaire Analysis

Plants growth was monitored by measuring: Root length, Stem height length, Leaf area. Moreover, the mineral elements were analyzed from leaves. Total nitrogen was analyzed by the Kjeldhal method, total phosphorus by colorimetry by complexing with molybdate and ammonium vanadate, and organic carbon by the loss on ignition method at 600 °C and K+, Na+, Mg++, and Ca++ ions by spectrophotometry. An analysis of fruit quality was also made, i.e., firmness, acidity rate, ripeness index, and juice rate according to the studied mixtures.

2.4. Phytotoxicity Study of the Different Mixtures Used

The test was carried out in vitro to analyze the effect of each aqueous extract of the mixture of cactus fiber and coconut fiber (1:0; 3:1; 1:1; 1:3; 0:1) on the germination of the seeds of the lettuce (Lactuca sativa) variety SUCRINE; distilled water was used as the control. The experiment layout used is a completely randomized device with five replicates. Each treatment consists of 20 seeds.

2.5. Economic Study of New Substrates Based on Cactus Fibers and Coconut Fibers for Greenhouse Tomatoes

In order to demonstrate that the use of cactus fibers as a growing substrate for greenhouse tomato can be advantageous on a commercial scale. The trial was conducted in a Canary Islands-type greenhouse on an area of 80 m². The containers filled with the different substrate mixtures were 1 m long, 20 cm wide and 20 cm deep, offering a volume of 13.33 L/plant. The planting density was 2 plants/m² (i.e., 20,000 plants/ha). For the determination of production costs, the fixed charges related to the structure were determined: greenhouse and irrigation system. On the other hand, the variable charges related to the consumption of water, fertilizers, and phytosanitary products were taken in account. On the other side, the marketable yield after eliminating the sorting differences was calculated.

3. Results and Discussion

3.1. Physico-Chemical Characteristics of the Mixtures Based on Cactus and Coconut Fibers

The physico-chemical characteristics of the mixtures of substrate of cactus fiber and coconut fiber for which its ratio are, respectively, (M0, M1, M2, M3, and C) at a rate of (1:0, 3:1, 1:1, 1:3, and 0:1) are represented in

Table 1. Physico-chemical characteristics of the substrates mixtures (M) and the Control (C).

	M0	M1	M2	M3	C
% OM	82	85	90	92	96
Cation exchange capacity CEC (meq/100 g of dry weight)	13.33	14.50	16.66	20.80	26.90
Water retention (mL/L)	225	240	260	330	350
Bulk density Da (g/cm3)	0.30	0.25	0.23	0.20	0.18
Real density Dr (g/cm3)	1.67	1.60	1.57	1.55	1.55
Total porosity in % of volume	82	84	85	87	90
pH at 23 °C	8.4	7.9	7.4	7.1	6.6
EC (mS/cm)	3.32	3.15	2.92	2.08	0.40
C/N	43.5	41.3	43.48	46.00	52.00

below.

According to [7], the C.E.C of an ideal substrate is between 10 to 30 meq/100 g dry weight, which coincides with the results of the analysis of the substrates studied and which are between 13.33 and 26.9 meq/100 g. In terms of water retention, the results obtained show that the addition of coconut fiber has generated an increase in water retention capacity. Concerning total porosity, the substrates meet the considered standard of total porosity of the culture substrates (Pt ≥ 50%). It should be remembered that in soilless culture, the physical characteristics (granulometry and porosity) of the culture substrate are considered among the decisive factors of the morphological quality of the plants. They act directly on all the root functions of the plants, in particular on the absorption of water, mineral elements, and respiration [8].

It is considered that ECs between 0.6 mS/cm and 1 mS/cm are suitable for most of the plants. Between 1 and 4 mS/cm, most plants experience a significant decrease in growth and development. Above 4 mS/cm, only halophytes can tolerate these conditions [9]. The EC measurements of the different substrates, except for the control substrate, revealed that the electrical conductivity of the substrates belong to a “saline” class. This salinity higher than 2 mS/cm of the substrates is related to their high minerals content (

Table 2. The content of mineral elements in the different substrates.

Substrat	NH4+	NO3−	Phosphorus	Potassium	Ca	Mg	Fe	Mn	Cu	Zn
M0	12.60	42.00	710	180	0.19	0.007	0.071	0.004	0.0017	0.0013
M1	28.00	38.50	700	176	0.19	0.004	0.063	0.004	0.0017	0.0014
M2	57.75	38.75	340	80	0.15	0.004	0.042	0.003	0.0018	0.0013
M3	56.00	38.60	90	40	0.11	0.004	0.014	0.001	0.0018	0.0090
C	58.00	37.00	80	30	0.10	0.003	0.010	0.001	0.0015	0.0080

). A washing of these substrates before planting eliminates the excess of salts. A good fertilizing irrigation management reduces the risks of this salinity.

3.2. Evaluation of the Phyto-Toxic Power of the Different Mixtures Used

Statistical analysis showed that there was no significant difference between the germination rates by substrate. In 1987, Mustin reports that the germination rate for a non-phyto-toxic substrate is higher than 50% [10]. The values obtained in this trial show that our substrates are not toxic for the germination of lettuce seeds.

3.3. Economic Return of the Substrate Mixtures Based on Cactus Fiber and Coconut Fiber on the Cultivation of Round Tomato under Greenhouse

After calculating the profitability in an open system, the use of the substrate (M2) appeared to be more economical beneficial with a profit margin of 224,782.5 Dh/ha against 179,187.5 Dh/ha for substrate (M1), 223,180 Dh/ha for the control substrate (C), and 197,015 Dh/ha for substrate (M3). The conclusions will be different, however, if we place the trial in the case of a closed-circuit soilless production system because the substrate (M1) is less efficient in terms of water consumption.

4. Conclusions

Substrates (T) and (M2) offered the best commercial yields of 9.66 kg/m² and 9.52 kg/m² respectively. In terms of water and fertilizer consumption, substrates (T) and (M2) were more efficient than (M3) and (M1). Concerning mineral nutrition and water consumption/plant, the analysis showed that all substrates were within the standard norms except for a slight water deficiency in the case of substrate (M1) due to its low water retention capacity. Physiologically, there were not many differences between the treatments. Economically, in the case of an open system that does not allow for recovery or recycling of drainage solutions, and under the conditions of the trial, the use of the substrate (M2) is profitable since it provides profit margins of about 224,782.5 Dh/ha.