Optimizing Cassava Growth with Localized Struvite Application: Root Proliferation and Fertilization Efficiency

Abstract

Cassava is a root storage crop that is important to the starch industry and food security. In this study, the sustainable fertilization of cassava using local placement of struvite, a fertilizer recovered from wastewater, rich in nitrogen, phosphorus, and magnesium, was investigated. It was asked if struvite is a suitable fertilizer for cassava, if it is likely to spread through the substrate (leach), and if roots can proliferate and utilize a concentrated placement of struvite. Cassava was grown in rhizoboxes under different fertilizer placement strategies: unfertilized control, homogeneous fertilizer distribution in the top 20 cm (‘homogenized’), a strip placement (‘layer’) at 20 cm depth, and a localized ‘depot’ at the same depth. Shoot and root growth responses were monitored over 8 weeks. Cassava growth was significantly improved with struvite fertilization. The fertilizer remained localized, with minimcnal spread during the 8 weeks of experimentation. Both the ‘layer’ and ‘homogenized’ struvite placements resulted in comparable biomass production, significantly greater than the unfertilized treatment. Plants in the ‘depot’ placement initially grew similar to the unfertilized treatment as roots took time to locate and proliferate into the fertilizer depot. Afterward, plants in the ‘depot’ treatment grew quickly, resulting in an intermediate biomass at harvest. Notably, cassava exhibited strong root proliferation in response to concentrated struvite, which did not compromise deep rooting but instead appeared to enhance it, increasing specific root length. These findings suggest that strip fertilization with struvite may offer a sustainable fertilization strategy for cassava, warranting further investigation in field trials.

1. Introduction

The chemical composition of a fertilizer and its placement in time and space can strongly influence crop production and fertilizer use efficiency. Fertilizer uptake can be increased by placing the fertilizer closer to the roots and timing the fertilizer application to the crop’s demand [1]. At the same time, crops can optimize their root placement through nutrient tropism, i.e., growing roots toward fertilizer, and root proliferation, i.e., growing more roots in areas with greater nutrient availability. Root proliferation is well known from the images of Drew [2], which show the proliferation response of barley to localized placements of phosphate, nitrate, ammonium, and potassium. This seems a logical response to take up nutrients, but using a ¹⁵N labeling study, Vuuren et al. [3] concluded that the proliferation response of roots into a patch did not increase total N uptake very much. The following question arose: why do plants bother? The answer given was that the proliferation response mostly provides a competitive advantage by depleting a patch faster [4,5]. This questions how valuable this response is to agriculture and how it might be harnessed through the breeding and management of fertilizers.

Schneider and Lynch [6] suggested that root proliferation is mostly useful in low-nutrient soils but not in fertilized agricultural fields since the metabolic costs would be ‘too high’. They claim that it would be better for the crop to invest in growing deeper roots. Drew’s images seem to promote this idea, as they show poor rooting of the deeper layer compared to the control treatment [2]. As Drew kept the concentrations constant, the control treatment had much more fertilizer and grew a larger plant [2]. In a large meta-analysis of fertilizer placements, Nkebiwe et al. [1] concluded that localized placement tends to increase yield compared to broadcasting and that localized placement can promote deep rooting. Although this does not prove that the proliferation response is useful, root architectural modeling studies have suggested that without it, plants cannot take up concentrated fertilizer as the concentrations are well above the saturation point of the nutrient transporters (e.g., [7,8,9]). Thus, to increase uptake locally, they need to increase the root surface area through increasing the local root length density. In search of the optimal proliferation response, Croft et al. [10] concluded that it depends on several factors related to resources, competition, and mycorrhizal colonization. One important factor is that the nutrient patch provides a stable supply of nutrients and does not move to other locations, e.g., via leaching or diffusion.

To achieve sustainable and circular agriculture, it is essential to enhance nutrient uptake efficiency and to utilize fertilizers derived from waste, especially wastewater. An important source of three plant-relevant (macro-)nutrients is struvite, a precipitate consisting of MgNH₄PO₄·6H₂O or MgKPO₄·6H₂O. This mineral can be recovered from domestic wastewater and other nutrient-rich waste streams [11], ensuring the nutrient recycling of N and P [12]. The use of struvite as a fertilizer has been demonstrated successfully for agricultural crop production in various soils and regions worldwide [13,14,15]. The positive fertilizer properties of struvite have often been associated with its slow nutrient release and physical–chemical properties, which can improve soil quality and plant growth. In particular, losses of N and P via leaching and surface runoff could be significantly reduced by using struvite as a fertilizer while crop yields were maintained [16,17]. Struvite is a slow-release fertilizer with a strongly pH-dependent dissolution rate [18,19]. Roots acidify the rhizosphere by exuding organic acids and protons, a process enhanced by ammonium uptake as the plant needs to maintain electrical neutrality and balance the uptake of anions versus cations by pumping protons. The cation–anion balance is determined by the type of nitrogen taken up. Studies have shown that combining struvite with an additional ammonium fertilizer can improve phosphorus uptake [20]. It was hypothesized that the pH-dependent slow-release properties of struvite help to localize the fertilizer and minimize environmental losses. This controlled release also supports a root proliferation response, allowing roots sufficient time to absorb nutrients before fertilizer diffuses from the area of increased root length density.

Cassava (Manihot esculenta) is an economically relevant crop grown for its large high-starch-content storage roots in predominantly tropical regions, including Sub-Saharan Africa, Southeast Asia, and parts of Latin America. [21]. Edible varieties contribute substantially to food security in African and South American countries, especially in Nigeria, Congo, and Brazil (UN Food and Agriculture Organization (FAO)), whereas higher-yielding bitter varieties are grown in Southeast Asia for the well-developed starch industry. Cassava is typically grown on nutrient-poor and drought-prone sandy loam soils, as it can yield comparatively well in low-input agriculture but responds well to fertilization. Many studies have tested yield responses to fertilizers in different locations of the world and different soils with reported yield increases of over 80%, although typically, the yield increases are more moderate [22]. A negative response has also been reported, as over-fertilization can lead to excessive growth of shoot biomass and a reduced root yield [23]. Planted from stem cuttings in the rainy season, its 9–12-month cropping cycle typically ends at the end of the dry season. Deep rooting and a low and slowly developing leaf area index make the crop highly nutrient and water-efficient. It also means that cassava needs a long time to establish. Its canopy often does not close until the end of the third month, and the roots progress toward deeper layers at a slow pace of about 1 cm a day [24,25,26,27]. Fertilization can thus promote faster crop establishment and yield [28,29,30]. Furthermore, continuous cultivation can deplete the soil’s nutrients. Depletion might be avoided if nutrients can be recycled back from the starch factory. Recently, it was shown that a large portion of phosphorus could be recovered as struvite from wastewater from manioc processing plants [31]. However, so far, struvite has not been tested as a fertilizer for cassava production.

Although cassava grows deep roots, its root system is coarse and sparse compared to other crops [32]. It is often suggested that cassava’s nutrient uptake efficiency is primarily due to its symbiotic association with mycorrhizal fungi, and several studies have demonstrated a positive response to mycorrhizal inoculation (e.g., [33]). However, symbiosis is not the sole factor behind cassava’s nutrient efficiency. It was wondered if cassava’s root architectural plasticity may also play a role by placing more roots in soil domains with greater nutrient availability. Such a plasticity response offers the potential to be harnessed in cropping systems. Cassava is typically cultivated on soil ridges, where fertilizer can be applied in strips within the ridge. For efficient nutrient uptake, cassava roots must grow toward the fertilizer strip and proliferate into the nutrient-rich zone. Understanding this dynamic could help optimize fertilization strategies for the crop.

To explore these dynamics, the following key questions were addressed: (i) can cassava roots proliferate into nutrient hotspots, and can this root response be harnessed to improve nutrient uptake when struvite is locally placed? (ii) to what extent does struvite fertilizer spread in sandy, low-sorbing soils when concentrated in a hotspot, and how does this affect nutrient availability? Finally, it was examined whether localized fertilization with struvite compromises deep rooting or whether cassava can maintain deep root development while responding to concentrated nutrient zones.

By addressing these questions, the aim was to explore the potential of struvite as an effective and sustainable fertilizer for cassava while evaluating how root plasticity and targeted fertilization can be used to enhance nutrient use efficiency in cassava cropping systems, particularly in marginal sandy soils.

2. Materials and Methods

A greenhouse experiment was conducted using cassava plants, cultivar Rayong 9, grown in rhizoboxes to assess the impact of varying struvite placements and used as a slow-release fertilizer, as illustrated in Figure 1. The study focused on evaluating the shoot and root growth parameters, root distribution, and architectural responses to different fertilizer concentration patterns. Two controls were included: an unfertilized control and a homogeneously fertilized control, referred to as ‘control’ and ‘homogenized’, respectively.

To investigate strip fertilization effects, the rhizoboxes were positioned either parallel or perpendicular to the strip. Due to the flat design of the rhizobox, the parallel orientation ensured that roots would inevitably encounter the fertilizer strip, simulating uniform nutrient availability. In contrast, the perpendicular orientation created a concentrated nutrient hotspot, requiring roots to actively grow toward and proliferate within this localized zone. These configurations were designated as ‘layer’ (parallel) and ‘depot’ (perpendicular) treatments.

In total, three fertilization treatments were applied, all with the same total amount of nutrients but varying placement and local nutrient concentration: ‘depot,’ representing the most concentrated fertilizer placement, ‘layer’, with an intermediate concentration, and ‘homogenized’, the least concentrated. This experimental approach enabled a comprehensive analysis of root system plasticity and nutrient uptake efficiency in response to different spatial distributions of struvite.

2.1. Rhizobox Preparation and Experimental Set-Up

Previous studies with other crops proved flat rhizoboxes, also called rhizotrons or rhizoboxes, useful for studying root responses to localized fertilizer [34,35]. The used rhizoboxes (600 × 300 × 30 mm, 5.4 L) were filled with previously dried and homogenized sand and a dried and sieved nutrient-depleted peat-based substrate (“Nullerde”, Einheitserde/Patzer Erden, Germany; 1:1 vol. %), and struvite fertilizer according to one of four treatments (Figure 1). The mixture without fertilizer was very low in nutrients with N, P, K, and Mg values of 70, 30, 130, and 130 mg kg⁻¹, respectively, but with sufficient micronutrients. In the ‘unfertilized control’ treatment, rhizoboxes were filled up to 2 cm below the upper edge with the sand–peat substrate. For the homogenized treatment, rhizoboxes were filled up to 20 cm below the top. The residual space was then filled with the sand–peat substrate previously mixed with 3 g struvite (CrystalGreen^®, 100 SGN grade (1.0 mm), OSTARA, Vancouver BC, Canada; containing 150 mg of nitrogen (N); 366 mg of phosphorus (P); 300 mg of magnesium (Mg), according to the provider’s analysis (w/w): total nitrogen (ammoniacal-N): 5.0%, phosphorus pentoxide (P₂O₅): 28.0% (12.2% P), total magnesium oxide (MgO): 16.7% (10.0% Mg)). For the ‘depot’ treatment, rhizoboxes were filled up to 20 cm below the top. Then, a depot of 3 g of struvite was carefully placed on the substrate at a 5 cm distance from the right border, holding it temporarily in locating with a rubber tube, and the residual space was filled with the unfertilized substrate. For the ‘layer’ treatments, rhizoboxes were filled as aforementioned up to 20 cm below the top. Subsequently, 3 g of struvite was homogeneously spread across the entire surface, and on top of this thin layer, 20 cm of unfertilized substrate was placed. Thus, all fertilized treatments received the same total amount of struvite, but the fertilizer was most concentrated in the depot and least in the ‘homogenized’ treatment. All treatments were replicated 3 times in a fully randomized design.

Earlier studies with struvite [20,36,37] or with struvite composite fertilizers [35,38] were able to show a clear effect on plant growth in lupine, maize, and soy within an experimental period of 27 days to 8 weeks. Even though struvite is considered a slow-release fertilizer, it has been shown that significant (fertilizer) effects of struvite can be observed within a period of 4 to 8 weeks. Based on all these earlier studies on struvite and the results obtained for the various plants, a study period of as long as 8 weeks was set for the cassava trial presented here. We assumed a pronounced fertilizing effect of the applied struvite due to the continuous struvite dissolution over time.

2.2. Plant Material and Planting

One stem cutting of approx. 20 cm of cassava (Manihot esculenta, cult. Rayong 9) was placed centrally, vertically, and approximately 10 cm into the substrate in each rhizobox. Stem cuttings were obtained directly from donor plants previously grown in the same greenhouse.

2.3. Growth Conditions

The substrate was watered close to saturation, and all rhizoboxes were placed at an angle of 45° in a semi-controlled greenhouse with day/night temperatures of 29/24 degrees Celsius (°C), a relative air humidity of 75%, and a daily light integral (DLI) of approximately 15 mol·m⁻²·day⁻¹, supported on darker days by supplemental lights. Plants grew over a total period of eight weeks. Watering was conducted whenever necessary, keeping the substrate at approximately 50% water holding capacity (corresponding to a volumetric soil water content of approximately 25%).

2.4. Shoot Measurements

Before final harvest, SPAD measurements were acquired from three fully developed leaves with a Chlorophyll Meter SPAD-502Plus (Konica Minolta, Langenhagen, Germany). Eight weeks after planting, the freshly developed shoots were cut off and divided into leaves and stems, excluding the original cutting. Leaves were scanned with a Li-Cor 3100c leaf area meter (LI-COR Environmental, Lincoln, NE, USA). Subsequently, the shoot biomass was dried at 60 °C until constant weight, after which dry mass was determined with a scale.

2.5. Substrate Nutrient Concentrations

After harvesting the shoots, the rhizoboxes were opened, and from each box, 13 substrate samples from the in and around the fertilized regions were collected, according to Appendix A Figure A1 and as the basis for the heat map in Figure 2. The scheme was set up to evaluate the potential spreading of nutrients in the substrate via diffusion and advection. The extracted substrate samples were homogenized after drying at 70 °C in a drying oven. A subsample of approximately 50 mg substrate was extracted using microwave-assisted acid extraction (H-Cl, H-NO₃). The P, K, and Mg concentrations were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES; Thermo Scientific iCAP6500, Dreieich, Germany). Another substrate subsample was analyzed for its N concentration using an elemental analyzer (Leco TCH 600, Mönchengladbach, Germany).

2.6. Root-Washing and Measurements

A custom-made root-washing device carrying a nail board was inserted into rhizoboxes before root-washing, and the back of the rhizobox was carefully removed. The root-washing device was designed to hold the roots in place during washing (Appendix B Figure A2; compared to [39,40]) and was made of a plastic grid with nails mounted in an aluminum frame. Nails were placed every 13 mm in a rectangular plastic grid. Construction details are provided in Appendix B. This root-washing device, which is simple and inexpensive to reproduce, allowed the used substrate to be washed away quickly and, at the same time, guaranteed that the roots remained in place (Appendix B Figure A3 and Figure A4).

After washing, three random root sections from three layers, i.e., 20 cm below the top (layer a), 20–25 cm below the top (layer b), and 25–60 cm below the top (layer c) were cut out. Each root section contained a section of an adventitious root with its lateral branches and was stored in 50% (v/v) ethanol solution at 4 °C and later scanned with WinRHIZO using the link analysis to obtain lateral root traits.

Next, the root-washing frame was turned over onto an even surface, and, with a light tap, the roots slid off onto a smooth surface, which was scanned for total root length analysis with the current version of GROWSCREEN-Root [41].

Finally, the root biomass, except for the subsamples placed in alcohol, was collected, oven-dried, and weighed just as the shoot biomass.

2.7. Plant Nutrient Analysis

After determining the plants’ biomass, shoot and root dry matter were ground with a ball mill (Retsch MM400, Haan, Germany). A subsample was digested using microwave-assisted acid digestion with nitric acid and water peroxide and analyzed for its P, K, and Mg content using inductively coupled plasma optical emission spectrometry (ICP-OES; Thermo Scientific iCAP6500, Dreieich, Germany). Another ball-milled subsample was analyzed for its C, H, and N content using an elemental analyzer (Leco TCH 600, Mönchengladbach, Germany).

2.8. Roots Length Determination

After root-washing, the software GROWSCREEN-Root [41] (Version 2024) was used to analyze the final root images from the rhizoboxes. Briefly, the primary, secondary, and tertiary roots were marked using the colors green, red, and blue, respectively. Thus, the root length per root class and depth layer were obtained.

2.9. Lateral Root Traits Using WinRHIZO

The collected root sections stored in alcohol were analyzed with WinRHIZO Pro V 2020a software (Regent Instruments Inc., Quebec, QC, Canada) using the ‘link analysis’. Roots were removed from the ethanol solution, carefully washed in water, spread out in a tray with water, and scanned. The link analysis identifies ‘II’ links, which corresponds to the distances between forks, i.e., branches, and it was assumed that the median length of all II links is a good estimate of the interbranching distances (IBD) as defined in the root handbook by Freschet et al. [42]. Note that very few root crossings were found, which would otherwise also result in ‘II’ links. The authors also recommend determining the unbranched root apex, which is the root apex with the cell stretching zone behind it. It was assumed that the median length of the ‘EI’ (external–internal, between root tips and forks) links in the link analysis is a reasonable estimate of this trait for the lateral roots (including the higher-order lateral roots). Care was taken to keep the root sections intact and not crossing, which was evident from a low crossing count and a close agreement between the number of tips and the number of forks. Thus, it was assumed that the average lateral root length could be estimated by dividing the total length by the number of root tips.

2.10. Calculations

The ‘root mass fraction’ (RMF, g g⁻¹) as the ratio between the ‘root dry mass’ and the ‘total plant dry mass’ was calculated; the ‘root length density’ (RLD cm cm⁻³) as the ratio between ‘root length’ and ‘soil volume’ for a given sample; and ‘specific root length’ (SRL, cm g⁻¹) as the ratio of root length to root dry mass. The net light use efficiency (nLUE, g Mj⁻¹) as the ratio of the ‘plant dry mass’ (DM, g) to the ‘integral of intercepted photosynthetic active radiation over time’ was estimated. For different light use efficiency models and their assumptions, see a recent review by Pei et al. [43]. To compute this, an exponential increase in leaf area (LA) over time with a starting leaf area of 1.8 cm² and a negligible effect of self-shading (exponential fit, with exponential growth constant k, and initial leaf area, LA_init, supported by unpublished data) was assumed. Thus, the light captured by the canopy is 1.0 times the DLI (Section 2.3) times the time integral of the exponential function for leaf area development. Combined, we get the following:

$$\mathrm{n}\mathrm{L}\mathrm{U}\mathrm{E}\cong {\displaystyle \frac{\mathrm{D}\mathrm{M}\ast 4.6\frac{\mathrm{m}\mathrm{o}\mathrm{l}}{\mathrm{M}\mathrm{j}}\ast k}{\mathrm{D}\mathrm{L}\mathrm{I}\ast \mathrm{L}{\mathrm{A}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}}\ast {\mathrm{e}}^{k\mathrm{t}-1}}} \mathrm{a}\mathrm{n}\mathrm{d} k={\displaystyle \frac{\log \left(\frac{\mathrm{L}\mathrm{A}}{{\mathrm{L}\mathrm{A}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}}}\right)}{\mathrm{t}}}$$

with t being the full growth period of 56 days.

2.11. Statistical Analysis

All data were analyzed with R (R version 4.4.2, 2024 [44]). Linear models were fitted when comparing treatments and mixed linear models with the REML (restricted maximum likelihood) method when analyzing the subsampling of the boxes at specific locations, indicating the nested structure of the error terms. Means were computed with the ‘emmeans package’ [45] using REML and analyzed using ANOVA and Tukey’s honest sign. distance. Log transformations were applied where appropriate, and plots were plotted with a log-transformed y-axis accordingly.

3. Results and Discussion

3.1. Root Length Density Distribution Is Associated with Nutrient Placement

Root growth was strongly affected by fertilization, with significantly (3.3 times, α = 0.05) more root length in the fertilized compared to the unfertilized conditions (Figure 2 and Figure 3). The placement of roots in the box was associated with the placement of the fertilizer. This was especially strong in the depot treatment and answers our first question: cassava exhibits a strong root proliferation response to localized placement of struvite. Root proliferation has been reported for many plant species [46], but to our knowledge, this is the first report of a root proliferation response in cassava. Root proliferation in response to struvite was reported in soybean [35]; however, root proliferation in response to struvite still remains an understudied area of research [18].

Fertilized boxes also had more roots in the unfertilized lower layers, where the bottom of the rhizotron obstructed root growth, causing roots to accumulate. Fertilization thus clearly led to deep rooting, especially in the ‘depot’ and ‘layer’ treatments. These treatments represent strip fertilization, as explained in the Materials and Methods section, and suggest that localized fertilizer placement might enhance deeper rooting, which may be advantageous for water uptake. It also indicates that the metabolic ‘trade-off’ between root proliferation and deep rooting, suggested by Schneider et al. [6], did not occur. Rather, fertilization increases the size of the root system and, thereby, the number of deep roots. Compared to the homogenized fertilizer treatment, root proliferation reduced root length density in the shallow layers where less fertilizer was placed (especially, comparing in Figure 2, in the rhizotron with the depot treatment, the RLD on the left (RLD = 0.17 cm cm⁻³, i.e., no fertilizer placement), and right side (RLD = 0.78 cm cm⁻³, i.e., ‘depot’ placement of struvite)).

3.2. Nutrients Did Not Spread Much

Although the roots proliferated where the fertilizer was placed, we verified that the fertilizer did not spread through the box. The heat maps for the P fertilizer (Figure 2) show slightly higher concentrations in locations neighboring the fertilizer placement, but overall, samples from unfertilized locations did not differ significantly from unfertilized soil in N, P, or Mg content (Figure 4, α = 0.05). As a control, K concentrations were also measured and did not differ in any location or box, except for the sample taken in the depot, where the potassium concentration was lower. At that location, pure struvite was placed, replacing the soil substrate. The used struvite contained no potassium. It can be concluded that most of the residual fertilizer remained in location at the end of the experiment, confirming the findings of Rech et al. [47]. This suggests that struvite could continue to feed the plants much longer as this fertilizer did not diffuse far into the soil, as suggested by Talboys et al. [19] and Rech et al. [47].

The presented experiment was terminated 8 weeks after planting when roots had reached the bottom of the rhizotron, and the plant dry mass was estimated at approximately 2 g per liter substrate, which was confirmed by the measurements (compare biomass results in Section 3.4 to rhizotron volume of 5.4 L). A meta-analysis has shown that pot size effects become strong when plants grow larger than 2 g biomass liter⁻¹ media [48]. At this stage, residual struvite was still present (Figure 2 and Figure 4). Therefore, it could have supposedly supported plant growth for much longer, which deserves further testing in field experiments lasting a full growth season of 9–12 months. Still, the experimental period of 8 weeks in this study was relatively long compared to numerous other greenhouse experiments with struvite [20,35,36,37,38]. Further, some of these earlier studies with struvite were also carried out in a similar, low-sorbing, low-buffering sandy substrate. Due to the similar methodology in this study, it could therefore be assumed that an experimental duration of 8 weeks was sufficient to observe clear struvite fertilization effects on cassava.

Although crop response to struvite might depend on soil pH and crop, our cassava experiment showed that struvite is suited for cassava production. As mentioned in the introduction, struvite appears to have a positive fertilizer effect on various crops and in various soil types. This was also shown in an earlier study on maize cultivation in tropical soils (sandy loam and clayey soils; ref [13]). Therefore, it can be speculated that the positive fertilizer effect of struvite is also valid in soils of cassava cultivation regions, in particular with a possible interaction with soil-borne arbuscular mycorrhizal fungi (AMF), which could promote the release and plant availability of nutrients from struvite. However, the recovery of struvite requires specific conditions and resources, such as magnesium, which may not be readily available or cost-effective in all casava-producing regions. This can limit the scalability and economic feasibility of struvite production and use [49].

3.3. Fertilization Increased Nutrient Concentrations in the Biomass

Fertilization increased the N, P, and Mg concentrations in the plant biomass significantly (Figure 5). Unfertilized plants had concentrations of N, P, and Mg at the lower end of the reported spectrum, whereas fertilized plants had average to high concentrations when compared with earlier published data [50,51,52]. The highest concentrations were achieved with the homogeneous spreading of fertilizer, which made the nutrients available to the plant from day one. Concentrations in the homogeneous treatment were 2.5, 4, and 2.6 times greater than in the control treatment for N, P, and Mg, respectively (α = 0.05). In contrast, in the ‘layer’ and ‘depot’ treatments, the plants had to grow toward the fertilizer and proliferate roots into it. This led to somewhat lower nutrient concentrations (and less total uptake, multiplying the concentrations of Figure 5 with the total biomass in Figure 6B) compared to the ‘homogenized’ treatment, but to significantly (2, 1.9, 1.7 times for N, P, and Mg, respectively, α = 0.05) higher concentrations in the ‘unfertilized control’. The differences in plant nutrient concentrations between ‘homog.’ and the ’depot’ treatments were greatest for P and smallest for N. This may be consistent with the soil-nutrient-mobility concept, which suggests that P is the least and N is the most mobile nutrient [53], and thereby, root placement and root length density are more critical to P uptake. We conclude that localized placement of fertilizer may alter the elemental stoichiometry in the plant. Cassava has a relatively high demand for potassium [54]. The struvite used in this study did not contain potassium as the substrate used was potassium-rich (Figure 4). However, in agricultural settings, potassium struvite may be a good alternative, perhaps blended with monoammonium phosphate. The positive fertilizing effect of blended struvite and monoammonium phosphate at various ratios was demonstrated successfully in an earlier study with corn and soy using a comparable struvite source obtained from the same supplier [55]. These and other aspects must be considered when designing locally adept fertilization with struvite and require further research.

Cassava is known to grow in close symbiosis with arbuscular mycorrhizal fungi (AMF). A previous study on the use of the mass-produced in vitro mycorrhizal fungus Rhizophagus irregularis demonstrated its positive effects on cassava yield increase independent of P fertilization [56]. Another study successfully demonstrated the positive influence of AMF on the struvite nutrient supply of tomato plants [57]. The study concluded that N and P uptake from struvite is improved by AMF-associated tomato plants in that AMF positively influences the solubility limitations of struvite as a fertilizer [57]. Based on these findings and due to the high natural AMF symbiosis in cassava in agricultural cultivation, it is assumed that the nutrient supply and fertilizer effect of struvite can also be positively influenced by AMF. Therefore, it can be postulated that AMF in cassava cultivation might promote the struvite dissolution rate, resulting in improved plant nutrient availability. This assumption needs to be verified in future field trials under agricultural conditions using targeted struvite fertilization, evaluating AMF species-specific effects on struvite dissolution either by characterizing the native species colonizing the host plants and/or by targeted inoculation.

As struvite can be successfully extracted from wastewater from the anaerobic fermentation of cassava starch [31], using struvite as a locally produced fertilizer for cassava cultivation could contribute to close local nutrient cycles while at the same time reducing waste streams and dependencies on fossil P fertilizers.

3.4. Unfertilized Plants Were Small and Had Signs of N Deficiency

The total leaf area and biomass production were low in the ‘unfertilized control’ and highest in the ‘homogenized’ and ‘layer’ treatments (Figure 6A,B). The ‘depot’ treatment had more biomass than the ‘unfertilized control’ but less than the other treatments. The nutrient concentrations of the ‘unfertilized control’ (Figure 5) were critically low for N, suggesting that these plants suffered mostly from nitrogen deficiency. By visual evaluation, small leaves, slow leaf appearance, thin stems, and light green color supported this conclusion. SPAD values, a measure for leaf greenness and chlorophyll content, were indeed low in the control plants (Figure 6C). Low chlorophyll leads to low photosynthesis rates, and the estimated net light use efficiency was reduced in the ‘unfertilized control’ plants (Figure 6D). Mahakosee et al. [58] report solar radiation use efficiency values of 1–1.5, which, assuming PAR/solar radiation = 0.45, correspond to 2.2–3.3 on the y-scale of Figure 6D. This suggests that the control treatment had truly low-efficiency values. Although the N content of struvite is relatively low, it was sufficient to increase plant growth. While N can be regarded as the ‘most limiting’ nutrient in the unfertilized control plants, they also had low P and Mg values, suggesting that these nutrients would quickly have become the growth limiting factor when N had been supplied.

3.5. Functional Ratios Show Less Stress in the ‘Homogenized’ and ‘Layer’ Treatments

Low nutrient availability typically alters the allocation between root and shoot, resulting in a larger root mass fraction [59]. The root mass fraction (and its counterpart root-to-shoot ratios), however, did not differ among treatments (Figure 7A). Although commonly used to indicate a functional equilibrium between root and shoot, we postulate that the RMF is a bad measure in a crop that forms storage roots as one would have to distinguish the heavier storage roots from the fine roots, which is difficult in younger plants [32]. Furthermore, RMF does not take changes in specific leaf area or specific root length into account. The functional balance between root and shoot is proposedly better described by the ratio of their respective surface areas [60]. Determining root surface area reliably is challenging, and we took root length as a proxy and computed the root-length-to-leaf-surface-area ratio (Figure 7B). Indeed, ‘unfertilized control’ plants had a much greater root-length-to-leaf-surface-area ratio than ‘homog.’ fertilized plants, whereas ‘depot’ and ‘layer’ were in between, indicating that concentrating nutrients tended to make nutrient uptake harder, presumably because it renders much of the root length outside the nutrient hotspots ineffective. The specific root length is the root length per root dry mass and is strongly related to the ratio of fine roots (accounting for most length) and thick roots (accounting for most mass) and a measure of the metabolic efficiency of the root system. Figure 7C shows that plants responded to low nutrient availability by increasing their SRL.

3.6. Struvite Increases Foraging Intensity

Changes in specific root length hint at changes in root architecture. From subsamples, the length of the unbranched root apex was estimated. This length seemed longer in ‘unfertilized’ and ‘depot’ treatments, although it was hard to establish significance for this trait (α = 0.05, Figure 8A). Still, longer unbranched root apices suggest faster root elongation of individual lateral root tips [42]. The individual lengths of lateral roots followed a similar pattern, consistent with this theory, and were significantly different between homogeneous- and depot-fertilized plants with 2.6 and 4.0 cm, respectively (α = 0.05, Figure 8B). Finally, the interbranching distances, i.e., the median distance between branches, also showed the same pattern, however, with a greater relative difference between control and depot-fertilized plants, with 0.48 and 0.33 cm, respectively (α = 0.05, Figure 8C). Altogether, this suggests that unfertilized plants and ‘depot’-fertilized plants, which had most of their roots in unfertilized soil, tended to have less branched but more elongated lateral roots, resulting in sparser root systems aimed at soil exploration rather than local foraging intensity.

There are currently no studies describing how fertilization placement influences cassava root architecture. Thus, the mechanisms controlling this root architectural plasticity are unknown and have scope for further research. Although progress has been made in discovering mechanisms controlling root primordia formation, which is the precursor for branching, this research is dominated by studies on first-order lateral roots in Arabidopsis thaliana and not readily translated to other crops, which often show opposite responses [61,62,63].

4. Conclusions

This study shows that cassava can proliferate into a struvite hotspot and can thus improve nutrient uptake. The struvite did not move much in the used sand–peat mixture. The low mobility is attributed to the slow-release characteristics of struvite and makes a proliferation response extra useful. It has been asked if such a proliferation response has the negative side effect of shallow rooting, but it turned out that it actually did not have this trade-off, but rather nutrients promoted deep rooting. Thus, struvite can be used to fertilize cassava efficiently to increase plant growth and nutrient concentrations. Being a wastewater-recovered mineral with slow-release properties that disperses very slowly in soil, it may be useful in the sustainable production of this long-season crop with less risk of leaching during heavy rain seasons. Furthermore, continuous cassava cultivation depletes the soil of important nutrients. Therefore, a belowground strip application in the ridges may be a particularly interesting application of struvite for cassava cultivation, especially in continuous cassava production. The positive fertilizing effects of struvite in cassava production need testing in field trials, considering appropriate positive fertilizer controls and biotic and abiotic soil conditions potentially influencing nutrient release and availability.