Effect of Integrated Nutrient Management Through Targeted Yield Precision Model on Soil Microbes, Root Morphology, Productivity of Hybrid Castor on a Non-Calcareous Alfisol

Abstract

Precision application of fertiliser nutrients based on soil-available nutrients is a vital means of increasing castor (Ricinus communis L.) productivity. Fertiliser application based on the targeted yield model under inorganic fertilisers alone and Integrated Plant Nutrition System (IPNS) differ from the blanket recommendation practices. Field experiments were conducted in two locations to validate the Soil Test Crop Response (STCR) targeted yield model developed for hybrid castor on non-calcareous Alfisol. The main objective was to determine the effect of inorganic fertilisers and organic manures on microbial populations, enzyme dynamics in soil, and productivity of castor. Experimental field data revealed that combined application of inorganic fertilisers along with 12.5 t ha⁻¹ farmyard manure increased the soil microbial population and enzyme activity in the rhizosphere soils of castor. Castor responded positively with an increase in highest targeted yield level. The highest yield of 2726 and 2695 kg ha⁻¹ were attained in the treatment T₈ (STCR-IPNS −2.75 t ha⁻¹) in both locations, and Treatment T₅ (STCR-NPK alone −2.75 t ha⁻¹) was on par with T₈. The IPNS treatments showed higher percent achievement than the NPK treatments alone. Root length and dry matter production increased significantly with the application of a higher dose of fertiliser along with farmyard manure. Root dry matter production significantly contributed towards the castor seed yield. More soil-beneficial microorganisms and enzyme dynamics were observed in the IPNS treatment.

1. Introduction

Castor (Ricinus communis L.; 2n = 20), belong to the Euphorbiaceae family, is the most important non-edible oilseed crop and has significant economic importance, and it is grown all over the world [1]. Though the centre of origin for castor is Ethiopia, its cultivation is mostly concentrated in Asia, accounting for about 92.2 percent of the world’s production [2]. Castor is cultivated on a commercial scale in an area of 1.22 million ha, with a production of 2.051 million tonnes of seed [3]. India is the world’s leading producer, accounting for about 79 per cent of total worldwide castor seed production and meeting 80–90 percent of global demand for castor oil. The majority of the global castor seed production was mainly contributed by small farmers [4].

Castors are produced on marginally arable areas in India, which causes them to be thirsty and starving and lowers their seed production and oil quality. Castor is a widely spread, long-duration oilseed crop that ensures maximum growth and productivity when the best fertiliser management strategies are followed [5].

Castor oil is solely an industrially important product and castor production cannot compete with edible crops for cultivation on fertile soils. As a result, castor cultivation in these locations is extremely uneven, yielding just 1417 kg ha⁻¹. Compared with other nutrients, nitrogen, phosphorus, and potassium are vital to the seed yield of castor. The plant roots take up nutrients applied to the soil. Thus, the nutrient uptake mainly depends on the root length, surface area, and root distribution, which is the spatial and physiological efficiency of plant [6]. Root system distribution is uneven and adapts to the heterogeneous environment of soil nutrients to absorb them. Therefore the root length, biomass, and surface area of contact with soils are essential for nutrient absorption. Revealing the root length and root biomass in the soil, microbial population around the rhizosphere soil, and their interaction improve the availability of nutrients in the rhizosphere soil and increase plant nutrient uptake, morphological growth, and seed yield of castor.

Castor must be cultivated with the best fertilisation practices to attain maximum productivity. The fertiliser recommendation for crops requires knowledge on soil inheritance fertility status, nutrient supply capacity of the soil, fertiliser efficiency and manure efficiency, the fate of applied nutrients, and crop response to applied nutrient [7]. Troug [8] developed the “Prescription method” of fertiliser use to obtain high yields; however, the theoretical foundation, field experiment demonstration, and confirmation of Liebig’s law of minimum plant nutrition were established by Ramamoorthy [9]. The specific yield equation and level of yield target are based on the soil-available nutrients and financial status of the farmers to use the costlier fertiliser input, ensuring sustainable yield [10].

The present study was conducted at the North Western Agro-climatic Zone of Tamil Nadu, India to examine the fertiliser prescription equations developed for the desired yield target of hybrid castor on Alfisol. In addition, the study also explored the effect of fertiliser, farmyard manure, and initial soil nutrient on the growth, uptake, and seed yield of hybrid castor and the dynamic changes in the soil microorganism population and enzyme activity in the rhizosphere soil.

2. Materials and Methods

2.1. Experimental Design

Field experiments were conducted in the two locations to validate the fertiliser prescription equations developed for hybrid castor for red non-calcareous Alfisol of Tamil Nadu, and the study site was located at Tamil Nadu, India—Figure 1 (

Table 1. Characteristics of initial surface soil sample of experimental field.

Parameter	Location I	Location II
Latitude	11°28′ N	11°49′ N
Longitude	77°54′ E	77°99′ E
pH	7.23	7.66
EC (dS m−1)	0.45	0.32
Organic Carbon (%)	0.53	0.62
CEC c mol (p+) kg−1	24.8	28.5
Soil texture	Sandy clay loam	Sandy clay loam
Major Nutrients (kg ha−1)
KMnO4-N	261	265
Olsen-P	16	20
NH4OAc-K	207	261
Micronutrients (mg kg−1)
DTPA-Zn	3.42	2.01
DTPA-Fe	3.87	3.58
DTPA-Mn	12.41	8.54
DTPA–Cu	3.21	2.06
Field operation
Date of Sowing	22.12.21	29.12.21
Date of Harvest	24.05.21	29.05.21

). The soil at the experimental site belongs to Alfisol soil order, with a sandy clay loam texture and they are non-calcareous. Ten treatments were imposed consisting of inorganic fertilisation alone, inorganic cum organic fertilisation, and absolute control, and used a randomised block design with three replications. In this study, the data from 30 plots was studied. Each plot was a size of about 25 m², and all were sown with the same hybrid YRCH⁻¹ of castor. The growing period for castor in our study was from December 2021 to May 2022. The initial soil-available nitrogen, phosphorus, potassium, soil organic carbon, and the micronutrients (i.e., zinc (Zn), iron (Fe), manganese (Mn), and copper (Cu)) can be seen in Table 1. The Experiments include ten treatments viz: T₁-Blanket (100% Recommended Dose of Fertiliser (RDF)), T₂-Blanket (100% RDF + FYM (Farm Yard Manure)@12.5 t ha⁻¹), T₃-STCR-NPK alone-Target 2.25 t ha⁻¹, T₄-STCR-NPK alone-Target II 2.5 t ha⁻¹, T₅-STCR-NPK alone-Target 2.75 t ha⁻¹, T₆-STCR-IPNS-Target 2.25 t ha⁻¹, T₇-STCR-IPNS-Target 2.5 t ha⁻¹, T₈-STCR-IPNS-Target 2.75 t ha⁻¹, T₉-Farmer’s practice, and T₁₀-Absolute control.

The fertiliser prescription equations developed for hybrid castor [11] for red non-calcareous Alfisol of Tamil Nadu are shown below:

Inorganic fertiliser alone

$$FN=10.38 T-0.70 SN$$

$$\mathrm{F}{P}_2{O}_5=4.62 T-3.60 SP$$

$$F{K}_{2}O=6.30 T-0.44 SK$$

Inorganic fertiliser with FYM

$$FN=10.38 T-0.70 SN-0.69 ON$$

$$\mathrm{F}{P}_2{O}_5=4.62 T-3.60 SP - 0.89 \mathrm{O}\mathrm{P}$$

$$F{K}_{2}O=6.30 T-0.44 SK - 0.60 \mathrm{O}\mathrm{K}$$

where FN, FP₂O₅, and FK₂O are fertiliser of nitrogen, phosphorus, and potassium in kg ha⁻¹, respectively; T represent desired yield target in q ha⁻¹; SN, SP, and SK are soil-available nutrients in kg ha⁻¹; and ON, OP, and OK are the quantities of nitrogen, phosphorus, and potassium supplied through farmyard manure in kg ha⁻¹.

The fertiliser quantity to be applied for each treatment was calculated using the above-mentioned prescription equations. Farm Yard Manure (FYM) and phosphorus fertilisers were applied basally at the time of sowing, while nitrogenous and potassium fertilisers were applied in three batches (i.e., 50% during basic fertilisation and 25% on the 30th and 60th day after sowing). The sources of NPK fertilisers were Urea, Single Super Phosphate (SSP), and Muriate of Potash (MOP), respectively. The crop was grown along with all the management practices.

Initial soil was taken from each plot and analysed for microorganism diversity. The microbial diversity (i.e., bacteria, fungi, actinomycetes), and soil enzyme activity (i.e., dehydrogenase, urease, and phosphatase) were studied in the rhizosphere soils of castor in each plot after harvesting the crop. The best-diversified plot was made into the sequencing sample for identifying the beneficial microorganisms.

2.2. Sampling and Processing of Experimental Field Soil

The rhizosphere is the narrow zone of soil directly influenced by root secretions and associated microbial activity. Its collection requires careful separation from bulk soil to ensure that the microbial and physicochemical characteristics specific to the root zone are preserved. Castor rhizosphere soil samples were taken from experimental plots with various yield targeted levels under STCR-NPK alone, STCR- IPNS, inorganic fertilisation alone, inorganic fertilisation combined with FYM, and control plots.

Rhizosphere soil was collected from healthy, representative plants at the experimental plot. Plants were carefully uprooted using a sterilised spade to ensure minimal damage to the root system. Immediately after uprooting, loosely adhering bulk soil was gently removed by shaking the roots. The soil particles tightly adhering to the root surface, considered as the rhizosphere fraction (approximately 1–3 mm from the root zone), were collected.

For microbiological analysis, roots with adhering soil were placed in sterile polyethylene bags, kept in insulated boxes with ice packs, and transported to the laboratory. In the laboratory, the adhering soil was carefully detached from the roots by gentle brushing and used for further analysis. Samples were processed within 24 h of collection to minimise microbial alterations.

For physicochemical analysis, a portion of the rhizosphere soil was air-dried under shade, ground gently, and passed through a 2 mm sieve prior to analysis. All tools used during sampling were sterilised to avoid cross-contamination, and samples were properly labelled with details of crop, growth stage, location, and date of collection.

2.3. Isolation and Counting of Microorganisms

Soil bacteria, actinomycetes, and fungi were isolated using serial plate technique [12]. Nutrient agar medium was employed to isolate bacteria and the Petri plates were incubated at 30 ± 1 °C for 24 h. For fungi, Martin’s rose Bengal agar medium was used and Petri plates were incubated for five days at 25 ± 1 °C. Colony-Forming Units (CFU) were observed and accounted after the incubation period, and expressed in moisture-free basis per gram of soil.

2.4. Soil Enzyme Properties

Soil enzyme activity was analysed in the rhizosphere soil of castor. Dehydrogenase (DHA) activity was determined using the tri-phenyl tetrazolium chloride method as given by Casida [13] by monitoring the triphenyl formazan (TPF) production. Phosphatase activity was measured by colorimetric method at 410 nm [14] using p-nitro phenyl phosphate as substrate; urease activity was determined by Tabatabai and Bremner [15].

2.5. Plant Harvest

Castor capsules were harvested at 90, 120, and 150 DAS and the capsules were sun-dried and dehulled. The samples from 120 DAS were processed and analysed for plant N [16], P, and K [17]. In each plot, three whole plants were harvested with the root and divided into root and shoot. Plant and root samples were dried at 70 °C until a constant weight was attained. The total uptake was computed by multiplying the mineral content with the dry-matter yield.

2.6. Statistical Analysis

In this study, the data were analysed using the IBM SPSS Statistics (Statistical Product and Service Solutions 22.0, IBM, Armonk, NY, United States). The correlation and regression analysis were successively studied and the result were revealed by Pearson correlation coefficient (r) as the result of correlation analysis and adjusted R square (R²), root mean square error (RMSE), and p-value for regression analysis. The comprehensive method of calculation above can be found in the help section of an SPSS document.

3. Results and Discussion

3.1. Castor Seed Yield and Response Ratio (RR) Under Inorganic Fertilisation Alone and Inorganic Fertilisation Cum Organic Manuring

In both field experiments, castor seed yield showed a significant yield increase with an increase in the dosage of the fertiliser. In both locations, the highest target yield treatments (T8 and T5) recorded the highest yields of 2726 and 2682 kg ha⁻¹ in location I (

Table 2. Results of validation experiment on hybrid castor in location I.

Sl. No.	Treatments	FYM (t ha−1)	Fertiliser Doses (kg ha−1)			Seed Yield (kg ha−1)	Per Cent Achieve-ment	RR (kg kg−1)	B.C Ratio
			N	P2O5	K2O
T1	Blanket (100% RDF)	-	90	45	45	2298 d	-	5.78	2.43
T2	Blanket (100% RDF + FYM @12.5 t ha−1)	12.5	90	45	45	2467 bc	-	5.12	2.52
T3	STCR-NPK alone −2.25 t ha−1	-	51	46	51	2325 d	103.3	7.21	2.47
T4	STCR-NPK alone −2.5 t ha−1	-	77	58	66	2512 b	100.5	6.24	2.60
T5	STCR-NPK alone −2.75 t ha−1	-	103	67.5 **	67.5 **	2682 a	97.5	5.98	2.77
T6	STCR-IPNS −2.25 t ha−1	12.5	45 *	26	22.5 *	2371 cd	105.4	7.52	2.51
T7	STCR-IPNS −2.5 t ha−1	12.5	45 *	37	37	2539 b	101.6	6.37	2.64
T8	STCR-IPNS −2.75 t ha−1	12.5	67	49	53	2726 a	99.1	6.17	2.79
T9	Farmer’s practice	-	40	20	20	1615 e	-	4.46	1.77
T10	Absolute control	-	0	0	0	1258 f	-	-	1.43

* maintenance dose ** maximum dose (maintenance dose—50% of blanket dose, maximum dose −150% of blanket dose).

) and 2695 and 2628 kg ha⁻¹ in location II (

Table 3. Results of validation experiment on hybrid castor in location II.

Sl. No.	Treatments	FYM (t ha−1)	Fertiliser Doses (kg ha−1)			Seed Yield (kg ha−1)	Per Cent Achieve-ment	RR (kg kg−1)	B.C Ratio
			N	P2O5	K2O
T1	Blanket (100% RDF)	-	90	45	45	2247 d	-	5.19	2.38
T2	Blanket (100% RDF + FYM @12.5 t ha−1)	12.5	90	45	45	2380 bc	-	4.49	2.43
T3	STCR- NPK alone −2.25 t ha−1	-	48	32	27	2282 cd	101.4	9.07	2.47
T4	STCR-NPK alone −2.5 t ha−1	-	74	44	43	2439 b	97.6	7.00	2.59
T5	STCR-NPK alone −2.75 t ha−1	-	100	55	58	2628 a	95.6	6.18	2.74
T6	STCR-IPNS −2.25 t ha−1	12.5	45 *	22.5 *	22.5 *	2385 bc	106.0	10.03	2.52
T7	STCR-IPNS −2.5 t ha−1	12.5	45 *	24	22.5 *	2461 b	98.4	7.14	2.60
T8	STCR-IPNS −2.75 t ha−1	12.5	63	36	28	2695 a	98.0	6.49	2.80
T9	Farmer’s practice	-	40	20	20	1612 e	-	3.75	1.77
T10	Absolute control	-	0	0	0	1312 f	-	-	1.49

* maintenance dose (maintenance dose—50% of blanket dose, maximum dose—150% of blanket dose).

), respectively, and were almost double the yield of the absolute control treatment. Comparing the whole field experiment, location I recorded a higher yield compared to location II. Regarding percentage achievements, the lowest targeted yield treatment (T3, T4, T6, and T7) showed the highest percentage compared to T5 and T8. Comparing the treatments that used inorganic fertilisation alone (T3, T4, and T5), inorganic cum organic treatment (T6, T7, and T8) recorded a higher percentage. Regarding Benefit Cost ratio (B.C ratio), T8 and T5 recorded higher in the both locations. Response ratio (RR) has been recorded in the lower target yield treatment (T3, T6) and lower RRs were recorded in the blanket recommendation plot (T1,T2).

The experimental results revealed that NPK fertiliser application is an essential means to increase the castor seed yield, and the castor crop respond well to higher fertilisation practices. Xue [6] reported that castor seed yield increased with the increased fertilisation levels. Comparing the treatments that used inorganic fertilisation alone, the inorganic fertilisation along with 12.5 t ha⁻¹ FYM (equal level of nutrient supplement) showed a higher yield and higher percentage in both locations.

The increase in castor seed yield in STCR-IPNS treatment may be due to the early vegetative growth nutrient requirement met by inorganic fertilisers, and the slow and continuous release of nutrients from FYM during all growth stages of the crop. Padmavathi [18] reported a higher yield in the STCR fertilisation-treated plot compared to inorganic fertilisation alone and organic manure-treated plots. However, in different locations, the maximum yield and the percentage achieved may differ due to the growing conditions and the soil’s inherent productivity [5]. Seed yield showed a higher positive correlation between phosphorus and potassium fertiliser application than nitrogen. Application of the optimum dose of phosphorus and potassium fertiliser showed significant increase in castor seed yield.

In blanket application of the fertiliser (T1), a higher dose of fertiliser was applied and all other treatments showed lesser seed yield, on par with the lowest targeted yield treatments (T3 and T6). Result revealed that imbalanced addition of N, P, and K fertilisers or absence of organic manure can affect the soil nutrient availability, plant uptake, and use of nutrients (including micronutrients). Imbalanced nutrient application negatively affects the soil organic matter, which is vital for the growth of beneficial soil microorganisms, and plant rhizobacteria, which in turn reduces the crop yield and quality [19].

3.2. Impact of Different Fertilisation Approaches on Soil Microbial Population

Soil microbial populations are aroused by external supplies of nutrients [20]; generally, application of balanced fertiliser and organic manures leads to an increase in microbial population [21], enhancing microbial enzyme activity [22], and influencing a change in microbial diversity. A diversified soil microbial population is vital for a stable and productive agroecosystem [23].

In the present study, soil samples were taken before sowing and after harvest of the castor, the bacteria, actinomycete, and fungi populations were seen in each plot. Both location data reveal that the presowing microbial population significantly increased under different fertilisation plots compared to the control plot (Figure 2). Comparing the inorganic fertilisation treatments (T₁, T₃, T₄, and T₅), the treatment along with farmyard manure (T₂, T₆, T₇, and T₈) significantly increased the microbial population. An increase in the population may be due to a higher carbon source for the microbial population by farmyard manure application, and, in the case of inorganic fertilisers, plot microbial population solely relies on the exudates and root residues of castor crop as a carbon source [24]. Several investigation results revealed that inorganic fertilisation alone drastically reduced microbial population and its diversity, including plant growth-promoting microbial species [23].

Laboratory data found that the abundance of the soil bacterial and actinomycete population was more than the fungi population, and result revealed that bacteria and actinomycetes were more sensitive to fertilisation practices than fungi [25]. Nakhro and Dkhar [26] reported the highest microbial population and microbial biomass carbon in organic manure-treated soil than the inorganic fertilised one. In contrast, inorganic fertilisers showed negligible effect on soil microbial population and diversity than other organic soil treatments. In fact, inorganic fertiliser-treated soil relies solely on the root exudates and crop residue to increase carbon input [24].

Simple correlation between soil applied with mineral fertiliser and FYM with soil microbial population showed that soil microbial populations significantly increased with the application of FYM (Figure 3). An increase in soil microbial population by mineral fertilisers has been found only when they were applied in combination with organic manures [24].The microbial population showed positive correlation with seed yield (Figure 3). Application of balanced fertiliser along with 12.5 t ha⁻¹ FYM increased the microbial population, in turn allowing for better plant growth, which results in higher rhizodeposition. Soil organic matter has a significant impact on soil microbial diversity structure and enhances soil quality and crop productivity [27].

3.3. Impact of Different Fertilisation Approaches on Soil Biological Properties

Soil enzyme activity is an indicator of microbial community and functions [28]. It also reflects changes in soil biochemical processes and human-induced abiotic and biotic factors in soil [29]. During the examination of soil quality, the number of microorganisms belonging to key groups is not the only significant factor, but the enzyme produced by these microorganisms’ activity and its effect on the soil nutrient mobilisation is equally essential.

In the present study, among different fertilisation practices, the treatments T₈, T₂, and T₇ recorded higher DHA activity (Figure 4). The increased activity of DHA was due to improved fertilisation, incorporation of organic manure, and higher root exudation due to increased root growth. The organic manures and root exudates act as carbon sources for the soil microbial populations, and in turn, it increases the microbial population and enzymatic activity in the soil. Balanced NPK fertilisation along with organic manures showed higher increases in DHA activity than inorganic fertilisation alone [30]. On the other hand, among the experimental plots, absolute control T₁₀ recorded lower DHA activity followed by farmer’s practice. Lower enzyme activity in the control plot may be due to the lesser availability of carbon sources for microbial population [31]. Simple correlation between DHA and microorganism population showed that an increase in microbial population increased the DHA activity in soil (Figure 3). As a result, dehydrogenase activity can be used to predict the soil microbiological activity [32].

Phosphatase enzyme production by microbes and the root hairs are important factors in improving the availability of orthophosphate needed for the plant and microbial growth [33]. In the mineralization process of phosphorus (P), phosphatase enzymes act on mineral P to convert the unavailable form to a plant-available form of P [34]. Among different treatments, organic manure applied treatments (T₂, T₆, T₇, and T₈) showed higher phosphatase activity compared to inorganic fertilisation treatment alone (T₁, T₃, T₄, and T₅). Phosphatase activity in location I was recorded higher than location II, which may be due to deficiency of soil phosphorus in the plant system response to alleviate the phosphate deficiency. Study results revealed that the application of organic manures increases phosphatase activity and the inorganic phosphorus suppressed the phosphatase activity [35]. Simple correlation (Figure 3) revealed that an increase in the soil bacterial population (phosphate solubilizing bacteria) increases the phosphatase enzyme activity in the soil.

Urease is an enzyme produced by microorganisms that hydrolyse urea into ammonia and carbon dioxide [36]. Soil urease enzyme is an intra- and extracellular enzyme mostly derived from root exudates, plants debris, and microorganisms. [37]. The highest urease activity was recorded in T₂ and T₈ treatments (36.0 and 36.3 NH₄ μg g⁻¹ hr⁻¹, respectively) in location I. In location II, T₈ recorded higher urease activity of 37.3 NH₄ μg g⁻¹ hr⁻¹. In both the locations the farmyard manure-applied treatment showed higher urease activity than the inorganic fertilisers-alone applied field. Among inorganic fertilisation, the increasing application of nitrogen in the form of urea increases urease activity. The absolute control plot recorded the lowest urease activity due to lesser organic inputs and lower root biomass production [38].

3.4. Effect of Different Fertilisation Approaches on Growth Parameters of Castor

Experimental results revealed that castor crop responds to balanced fertilisation in both inorganic-alone fertilisation and inorganic fertilisation cum organic manuring in locations I (Figure 5) and II (Figure 6). Growth parameters viz., plant height, Leaf Area Index (LAI), number of branches, number of spikes plant⁻¹, length of primary spike, and dry matter production showed positive response to an increase in the fertilisation level. Among the treatments, the treatment along with the FYM application showed a significantly higher response than treatment with inorganic fertilisation alone. The reason might be due to the application of FYM, which showed an increase in soil beneficial microorganism population and in turn produced soil enzyme activity that mobilises the unavailable soil nutrient into plant-available form. On the other hand, growth parameters viz, shelling percentage, seed test weight, and oil content percent were not significantly improved because of various treatments, and the result might be due to the inherent characteristics of the hybrid castor YRCH-1 variety not affected by fertilisation practices.

3.5. Effect of Different Fertilisation Approaches on Root Length and Root Dry Matter

The root is the main component of the plant system through which plants absorb water and nutrients from the soil, and the root growth and root surface area which has contact with soil have a marked impact on the soil nutrient uptake. The nutrient uptake by the plant root system depends on the fertility status of the soil, nutrients applied externally, biological status, and soil physical properties. Fertilisation of plants had a marked impact on the root length and the root biomass growth. In general, nutrient deficient conditioned plants stimulate root growth to explore greater soil volume to increase the plant nutrient uptake [27,39]. Prolonged nutrient deficiency stunts plant root growth and affects the soil nutrient availability to plant [39]. In addition, overuse of fertilisers may lead to high concentrations of nutrients in the soil, resulting in inhibition of root growth [40,41]. Only at the balanced level of nutrient supply along with good soil condition did it result in the optimal root development and higher nutrient uptake. In the present field experiment, in both location root length and root dry matter production increased with an increased balanced level of nutrients (Figure 7). Treatments along with the organic manure showed higher root activity and root biomass. Higher root length was absorbed in the treatment T₈ STCR –IPNS- 2.75 t ha⁻¹ (48.1 and 43.4 cm) in locations I and II, respectively. Under blanket application, T₁ (higher level of nutrient) showed lower root length compared with blanket application along with FYM (T₂)_. The results revealed that incorporating FYM improves the soil physical properties and microbial population, which in turn induces root penetration and elongation and increases the plant nutrient uptake. However, root length decreased significantly in the control plot, and may be due to lesser nutrient availability to promote root elongation.

Nutrient management practices affect the root dry matter production and result were in trend with root length (Figure 7), and root dry matter production was maximum in T₈ (412 and 398 g plant⁻¹) in location I and location II, respectively. The simple correlation results between castor seed yield and root length and root dry matter production (Figure 3) revealed that root dry matter production showed a higher correlation towards the castor yield than root length.

The reason might be that higher root surface area contact with soil increased the plant nutrient uptake and in turn increased the seed yield of castor. Compared with root length, root dry matter clearly depicted the status of root surface area, fine roots, and its root hairs, which comprise a vast surface area. Study results revealed that an increase in the root length and biomass released higher root exudate, which improved the microbial population and soil enzyme activity. This result supported the conclusion that root biomass production promoted higher nutrient uptake and yield. In addition, castor seed yield and root size and dry matter showed a positive relationship [6].

4. Conclusions

The balanced application of fertiliser based on soil test value is an effective approach to increase the castor seed yield production. It was evident from the two validation experiments that the percentage achievement of yield targets of castor was within ±10 per cent variation, proving the validity of the Fertilizer Prescription Equations developed. Fertiliser Prescription Equations developed in the present investigation can be used to prescribe fertiliser N, P₂O_5, and K₂O doses under IPNS for desired yield target of castor on Yethapur series (Kanhaplic Rhodustalf). Castor yield increased significantly with an increase in fertilisation level to achieve the target yield. The highest yield of 2726 and 2695 kg ha⁻¹ was recorded in the treatment T8 (STCR-IPNS target 2.75 q ha⁻¹) in locations I and II, respectively. Yield targeting with IPNS recorded relatively higher percentage achievement than that aimed under their respective NPK alone treatments. STCR-IPNS-based fertilisation showed higher soil microbial population, enzyme activity, and higher target achievement comparing the STCR-NPK alone and absolute control treatment. Inorganic fertilisation treatment showed lower microbial population and enzyme activity in the rhizosphere soil comparing treatments along with organic manure application. Organic manuring sole practices are not feasible because organic manures do not provide high amounts of NPK to support plant productivity. A combined application of inorganic fertiliser with organic manures could be the best approach to attain a sustainable yield particularly for marginal fertile soils with low organic carbon. Adoption of soil test crop response-based integrated plant nutrition system (STCR-IPNS) ensured efficient and economic use of inorganic fertilisers.

The Fertiliser Prescription Equations for castor are to be adopted under the following situations.

✓

The equations should be used for similar and allied soils occurring in a particular agro-eco region.

✓

Targets chosen should not be unduly high or low and should be within the range of experimental yields obtained and also based on the yield potential of castor in an agro-eco region.

✓

Prescription equations must be used within the experimental range of soil test values and cannot be extrapolated.

✓

Based on soil test values, secondary and micronutrient deficiencies should be corrected.

✓

Recommended agronomic practices must be followed for raising castor.