Green manures
Sunn hemp was cut at 92 days after sowing and the lablab at 107 days, a time indicated by the flowering of at least 50% of the plants. At the full flowering stage plants have high levels of N and are more humid, with residue readily decomposable for soil microbiota (GOSS et al., 2013; MESSIGA et al., 2015). This allows a faster recycling of nutrients present in its biomass and the greater incorporation of these in microbial biomass, the main source of nutrients for plants in highly weathered soils, such as tropical ones (LIMA FILHO et al., 2014).
In this period spontaneous plants and legumes covered the soil completely (100%). This good plants growth can be explained by the fact that they were grown in summer, because when these legumes were grown in the same area, but in autumn-winter (March to October), despite being fertilized with P and K, the soil cover after 100 days it was 73% for sunn hemp and 65% for lablab in 65% (PEREIRA et al., 2012).
Species used as cover crops in no-tillage systems or as green manures have different characteristics in terms of the rate of decomposition and nutrient cycling. Thus, probably due to differences in plant material such as C/N and lignin content, it was observed by visual evaluation that sunn hemp (C/N = 23) remained on the soil surface for a longer time compared to lablab ( C/N = 18) (FERREIRA et al., 2019). This expresses the plant residues potential to protect the soil against erosion and for greater maintenance of soil water and nutrients, however with nutrient cycling slower compared to species with softer tissues / tender (VENDRUSCOLO et al., 2018). This behavior may require a mineral nutrients supply in first years of using this green manure to allow for higher levels of cultivated agricultural crops productivity.
Another factor that interferes with the decomposition process and the consequent nutrients mineralization by green manures is the way they are handled. When green manure is incorporated into the soil the material decomposition process is faster, unlike the methodology used in this study, where green manure was spread over the soil surface without incorporation. However, when deposited on the soil, there is greater control of spontaneous plants and better soil protection (JAVAID et al., 2015), which can be a strategy of producer interest.
The dry weight (DW) differed between cover crops (p < 0.05), with the mean DW production of crotalaria and lablab (6.3 t ha-1) being 2.2 times greater than that of Spontaneous plants DW (2.9 t ha-1) (Table 1). These results are related to legumes ability to fix atmospheric nitrogen, favoring plants development including dry weight accumulation (VENDRUSCOLO et al., 2018). The DW determination is important as the minerals are contained in this portion and this is intrinsically related to availability of nutrients present in it for subsequent crops (GONÇALVES et al., 2016).
Table 1
Dry weight (DW) and nutrient content of spontaneous plants and legumes Crotalaria juncea (sunn hemp) and Dolichos lablab (lablab) used for green manure and legumes fertilization effect with organic compost. Spontaneous plants and lablab with 107 days and sunn hemp after 92 days of sowing.
| Green manures | DW | N | P | K | Ca | Mg | Mn | Zn | Cu |
| | t ha-1 | -------------------- g kg-1 ------------------- | --------- mg kg-1 --------- |
| Spontaneous plants | 2.86 b 1/ | 88 b | 12 c | 49 b | 6 b | 20 d | 286 c | 86 c | 131 b |
| Sunn hemp | 6.23 a | 194 a | 32 b | 73 b | 21 a | 69 b | 610 ab | 232 b | 389 a |
| Lablab | 6.43 a | 188 a | 38 ab | 134 a | 24 a | 44 c | 494 bc | 153 bc | 520 a |
| Sunn hemp + Compost | 8.28 a | 268 a | 48 a | 109 ab | 30 a | 99 a | 703 a | 183 bc | 554 a |
| Lablab + Compost | 8.30 a | 270 a | 40 ab | 142 a | 27 a | 50 bc | 600 ab | 393 a | 318 ab |
| Mean | 6.42 | 201 | 34 | 101 | 22 | 56 | 549 | 209 | 382 |
1/ Means followed by the same letter in the column do not differ by Duncan's test at 5% significance.
Legumes fertilization with organic compost, although not significantly different by the Duncan test, increased the lablab and crotalaria DW in a similar way, with a mean increase of 32% in relation to plants grown without organic compost, a mean increase of 2 t ha-1 (Table 1). This result confirms the compost's usefulness as an organic fertilizer.
In this study, in which the legumes were grown in the summer for 100 days (mean), the DW production of the legumes fertilized with compost was 8.3 t ha-1 (Table 1) while in this same area, however with cultivation in the autumn-winter period (March to October) and chemical fertilization (50 kg ha-1 of P2O5 and 25 kg ha-1 of K2O) the lablab DW production lablab was 4.5 t ha-1 and crotalaria was 10.4 t ha-1 after 167 days of planting (PEREIRA et al., 2012).
Comparing these results, the opposite is observed between the two species. Lablab produced 84% more DW in the summer than in the winter while crotalaria produced 25% less DW. The higher lablab DW production in the summer in a shorter time may have been due to a lesser adaptation of this legume in the winter and, or to the type of fertilization used. The lowest DW production observed in sunn hemp grown in summer in relation to winter may have been the 72 more days of growth in winter, allowed for later flowering, when the green manures must be cut. Another possibility is that C. juncea grows best in times of short days (FONTANÉTTI et al., 2006), so the cultivation in winter may have favored its growth.
Although the DW production of spontaneous plants was lower than that of legumes, studies indicate that time regardless they remain in the area, spontaneous plants also have potential for biomass production for ground cover (HIRATA et al., 2014; GUZMÁN et al., 2019).
The nutrient content varied between cover crops, reflecting also in the difference in DM production (Table 1). The variation in the nutrients exported amount by different plant species may be a result of both the biomass (DW) produced and these plants capacity for absorption, distribution and use of nutrients. Different plants with different rooting systems explore different soil depths within the soil, so they may have different capacities to absorb different nutrients amounts, in addition to producing different root exudates (organic acids) resulting in benefits for soil, microorganisms and plants (FAO, 2018).
N, Ca and Cu content did not differ significantly between legumes (Table 1). Between sunn hemp and lablab, Mg content was 80% higher in sunn hemp and K content was 53% higher in lablab (Table 1).
The organic compost increased N, P, K, Ca, Mg, Mn and Zn content in sunn hemp and lablab, on mean 41, 24, 21, 29, 33, 18 and 50% respectively compared to plants without fertilization with organic compost (Table 1). This justifies the mean DW increase presented by legumes fertilized with organic compost (FIG. 1). The lowest evaluated nutrients content was in spontaneous plants (Table 1) which is mainly associated with the lower DW production of these plants types.
It is important to note that although a species of green manure immobilizes a large nutrients amount in its biomass, this does not mean that these nutrients will be readily available for subsequent cultivation. Depending on the decomposition degree, the organic matter added to the soil can have an immediate effect or a residual effect, so it is economically important and conserves the soil physical, chemical and biological properties (Bento et al, 2014).
The results obtained with regard to the soil cover rate together with the high production of biomass and nutrient content demonstrate that both lablab and sunn hemp, especially fertilized with organic compost, have relevant characteristics and are of great importance in protecting the soil and production of crops in succession. The green manure effectiveness is related to the vegetable residues produced quality and quantity, the coverage percentage and the persistence of these residues on the soil surface, especially in the rainy season beginning when summer crops have not yet covered all the soil and this is exposed to rain (ANGELETTI et al., 2018).
Lettuce agronomic performance
The different fertilizations significantly influenced the shoots fresh weight (SFW), the shoots dry weight (SDW), head diameter and lettuce leaves number (p < 0.05) (FIGURE 1).
In general, SFW, SDW, head diameter and lettuce leaves number grown in succession to sunn hemp and lablab, with or without organic compost addition, were higher than unfertilized lettuce production (control) and equal to conventionally fertilized lettuce (Fig. 1). These results indicate the efficiency of using these legumes as a fertilization alternative form.
The lettuce fertilized only with the organic compost or grown in spontaneous plants residues did not differ from the control regarding the production of SFW, SDW, head diameter and lettuce leaves number (FIGURE 1).
The SFW of lettuce grown in succession to spontaneous plants was 226% lower than those grown conventionally (FIG. 1). This may be due to some inhibitory effect that some spontaneous plants may have as a competition form (STEVENS & TANG, 1987; WAKJIRA et al., 2005) and/or to the plant biomass lower production and lower spontaneous plants nutrient content in relation to legumes (Table 1) and consequently insufficient nutrients supply to lettuce demands.
The smallest SFW, head diameter and leaves number of lettuce grown in soil fertilized only with the compost is possibly related to this fertilizer having lower nutrient contents than commercial mineral fertilizers and, even though it is a stabilized product its application in the soil it can stimulate microbial activity and growth which, even though small, can immobilize soluble nutrients such as N, present in the compost and in the soil and consequently temporarily decrease the availability of nutrients for plants (ERHART & HARTL, 2010).
The current observation of lesser effect on crop growth with the organic composts use is not isolated. Montemurro et al. (2015) reported that, depending on the maturity stage, olive pomace-based compost could lead to lower lettuce yield compared to unfertilized control.
The organic compost beneficial effects, in general are expected over time by continuous use, as it improves the soil physical and chemical properties and increases the soil's organic matter and microbial C (FONTANÉTTI et al., 2006; PINTO et al., 2017). The compost beneficial effect on lettuce growth was observed when it was applied 80 to 110 days before cultivation (SANTOS et al., 2001). Sunn hemp fertilization with compost increased the lettuce SFW compared to that grown in succession with the same legume, but without using the compost as a fertilizer (FIG. 1). In this case, the compost may have stimulated the nutrients decomposition and cycling immobilized by sunn hemp and causing the nutrients mineralization to coincide with the phase of greatest demand for lettuce (SORATTO et al., 2012).
Analyzing the organic compost effect associated with legumes, when added to the sunn hemp, this effect was positive for lettuce production, improving all the characteristics evaluated when compared with fertilization with sunn hemp only (FIG. 1).
SFW production of lettuce grown in succession to sunn hemp fertilized with compost was 46% higher than that of lettuce grown in succession to sunn hemp without fertilization with compost and equaled the highest yields observed in lettuce in succession to lablab, lablab + compost and fertilized conventionally. The use of sunnhemp (C. juncea) and white lupine (Lupinus albus) as green manure fertilized with compost increased the cabbage production by 19% and green corn by 5% in relation to cultivation without compost use (SOUZA et al., 2015). However, this positive compost effect was not observed when the legume used was the lablab (Fig. 1). This can be due to higher levels of nutrients cycled by the lablab and, or by the faster decomposition of its biomass. On the other hand, the compost application in leguminous cover crops can also buffer excess nitrogen to reduce the risk of N leaching (LYNCH et al. 2004).
The biomass loss in leafy vegetables is due to the water loss and is directly related to its useful life, which is the period of time between its production or handling and that in which the product retains its quality characteristics suitable for consumption, being both directly related to the loss of turgor of the product. The results of accumulated percentage of fresh fresh weight loss (APSFWL) did not differ between treatments (p < 0.05) and are shown in Fig. 2.
The highest APSFWL was in the first 12 hours after harvest. The lettuces with greater dehydration were those grown with lablab and lablab + compost (mean of 18%) which was, on mean, 1.5 times higher than those of conventional and control cultivation (average of 13%) and 2.2 times greater than that fertilized with sunnhemp + compost, with a lower APSFWL (9%) (FIGURE 2). After 24 hours of the harvest the lowest APSFWL were in plants conventionally fertilized and with sunnhemp + compost, showing higher product quality, providing less waste and greater use of the and income to the producer and trader. After 48 hours this loss was greater in control plants without fertilization and intermediate in the others up to 72 hours (FIGURE 2).
The lower APSFWL observed in conventionally fertilized and sunn hemp + compost crops may be associated with greater plant rusticity at harvest time and this is directly associated with the nutritional status of the plants and with some nutrients contents, such as lower N levels compared to conventional cultivation. This characteristic can be very important for maintaining the leaves quality during the harvesting, transportation and commercialization processes.
The apparent quality of a post-harvest product is the first criterion used by the consumer when choosing the product in vegetables purchase and consumption (GUERRA et al., 2017) and the minimum APSFWL is desirable for commercialization and vegetables crocrancy or appearance maintenance. The biomass loss during storage is due to the post-harvest plant metabolic activity, as well as the water loss resulting in physiological effects of wilt and color change (TAIZ & ZEIGER, 2013). Environmental and nutritional factors as well as fertilization have an influence on the vegetable’s physical characteristics in the post-harvest (CHITARRA & CHITARRA, 2005).
The lettuce shoots nutrients levels were influenced by the type of fertilization, except for Fe and Cu (p < 0.05) (Table 2). The highest leaf contents of N, P and K were in lettuce with fertilizations with lablab, lablab + compost, sunn hemp + compost and conventional (Table 2), which may have favored the greater SFW yields and greater head diameter and leaves number, as well as better quality regarding the senescence of lettuce grown in succession to these sources of fertilization (FIGURE 1).
Table 2
Lettuce nutrient content grown with different fertilization management and cover crops.
| Fertilização | N | P | K | Ca | Mg | Fe | Mn | Zn | Cu |
| | ----------------------- g kg-1 ----------------------- | ------------- mg kg-1 ------------ |
| Spontaneous plants | 24 de 1/ | 6.1 bc | 25 bc | 15 abc | 10 bc | 1.0 | 95 d | 45 b | 17.3 |
| Sunn hemp | 28 cd | 6.0 bc | 21 cd | 17 a | 14 a | 1.4 | 83 d | 44 b | 17.2 |
| Lablab | 31 bc | 7.6 a | 32 ab | 12 bc | 8 c | 1,5 | 124 d | 49 b | 17.3 |
| Sunn hemop + Compost | 31 bc | 8.2 a | 39 a | 16 a | 9 bc | 1.3 | 461 a | 71 a | 17.3 |
| Lablab + Compost | 33 b | 8.1 a | 39 a | 11 c | 6 d | 1.2 | 433 ab | 68 a | 17.3 |
| Organic compost | 24 de | 5.5 c | 27 bc | 14 abc | 6 d | 1.2 | 363 bc | 65 a | 17.4 |
| Conventional | 42 a | 7.2 ab | 35 ab | 14 abc | 12 ab | 1.3 | 305 c | 67 a | 17.3 |
| Control | 23 e | 5.3 c | 14 d | 15 ab | 13 a | 1.4 | 106 d | 42 b | 17.2 |
| Means | 29 | 6.8 | 29 | 14 | 10 | 1.3 | 246 | 56 | 17.3 |
1/ Means followed by the same letter in the column do not differ by Duncan's test at 5% significance.
The N content was higher in lettuce with conventional fertilization, intermediate between lettuces fertilized with green manure with and without compost, and lower in those fertilized only with compost and spontaneous plants and without fertilization (Table 2). Despite the concern with high N levels plants due to imbalance in plants making them more susceptible to attack by pests and diseases and for being harmful to human health, the levels of N observed in plants with the above fertilizations, are within the considered suitable for lettuce (30 to 50 g kg-1) (TRANI & RAIJ, 1997). On the contrary, N levels considered below the adequate levels were observed in the lettuces produced in the control and in those fertilized with compost, spontaneous plants and sunn hemp, which may be correlated to the lower lettuce FW production in these treatments (Fig. 1), because this is one of the nutrients most quantitatively demanded by most plants and its lack in the soil reduces leaf growth (FAQUIN, 2005).
The highest P levels were observed in lettuce in succession to sunn hemp + compost, lablab + compost and lablab, not differing from lettuces with conventional fertilization (Table 2), which indicates that phosphorus mineralization and release by both lablab and composting guarantee this nutrient supply for lettuce culture. Cover crops can improve the P nutrition of subsequent crops, converting native and residual P fertilizers that are relatively unavailable to chemical forms, more available for subsequent crops and can provide a larger mineralizable organic P pool (CAVIGELLI & THIEN, 2003; CIACCIA et al., 2015).
The K content was higher in lettuce in succession to green manures fertilized with organic compost followed by those with conventional fertilization and with lablab (TAB > 2). The K applied as organic fertilizer behaves as mineral since the application, since it is not part of any stable organic compost, therefore, it does not need to suffer the action of the microorganisms (SCHERER et al., 2010). On the other hand, the Mg content of lettuce plants (Table 2), as well as the content in lablab and sunn hemp (Table 1), were lower in the conditions of compound addition. This may be due to the large amount of K in the compost and in the soil, which can cause an antagonistic effect on the absorption of divalent cations Ca and Mg (RODRIGUES & CASALI, 1999).
The micronutrients Mn and Zn levels were higher in treatments that received organic compost (Table 2) being within the range considered adequate (WEIR & CRESSWELL, 1993). Studies comparing the accumulation micronutrient levels in the edible parts of three crops commonly grown in Tunisia, clearly suggest a significant micronutrient content reduction in conventional agricultural crops when compared to organic cultivation (HATTAB et al., 2018).
Between the lettuce fertilization with the two legumes, the P and K levels were higher in lettuces fertilized with lablab, while the Ca and Mg levels were higher in those fertilized with sunn hemp. The leaf contents of N, Fe, Mn, Zn and Cu did not differ between the plants grown in lablab and sunn hemp residues (Table 2).
The compost added to the sunn hemp provided an increase in the lettuce plants P, K, Mn and Zn levels in relation to fertilization with the sunn hemp without organic compost (Table 2). This increase was also observed in most nutrients content of the sunn hemp fertilized with compost as well as greater production of DW (Table 1). Therefore, it can be inferred that the compost added to sunn hemp increased the lettuce SFW production equaling the production with conventional fertilization (FIG. 1).
The N and P levels increase in lettuce fertilized with lablab + compost is an legume effect and not of the compost, since the contents that increased in plants fertilized exclusively with compost were Mn and Zn (Table 2). This is probably due to legumes ability to fix atmospheric nitrogen (CALIL et al., 2016; VENDRUSCOLO, et al., 2018) and possible root colonizations by naturally occurring arbuscular mycorrhizal fungi that improve nutrient absorption by extension roots, especially P (COSTA et al., 2019).
In relation to the control, fertilizations with lablab, lablab + compost and sunn hemp + compost increased the N, P and K levels in lettuce plants, equaling plants with conventional fertilization (Table 2) which may have guaranteed the highest lettuce productivity in these crops.
It is important to note that these indications are very general, as soil conditions, climate and genetic material can influence nutrient levels (MALAVOLTA, 1997).
Carrots agronomic performance
All parameters studied in the carrot cultivation were influenced by the fertilization (p < 0.05) (Table 3). The marketable roots productivity in succession to lettuce cultivation fertilized with lablab and lablab + compost did not differ from the productivity with conventional fertilization and was, on mean, 269% higher than the productivity in the control without fertilization (Table 3). In these crops the highest marketable carrot roots frequency was also observed (93% in lablab and 85% in lablab + compost) which was, on mean, 17 and 55% higher than in cultivation with conventional fertilization and control, respectively.
It is important to consider that only conventional fertilization was carried out again before the carrot was grown. Fertilizations with an organic source were only carried out before lettuce was grown. This result together with the result observed in lettuce productivity (FIG. 1) show the potential of lablab used as green manure in crops succession.
The soil balanced exploration using the crops succession is fundamental in vegetables production, since it allows to explore nutrients rationally avoiding the soil depletion through the alternation of species with diverse nutrient requirements and root systems (SOUZA et al., 2015). This management is becoming well known and practiced by producers who aim at greater productivity and crops profitability in a more sustainable and environmentally friendly way. Several green manures together with organic composts application are recommended to organic producers to increase crop productivity during short-term organic vegetable rotations (MURAMOTO et al., 2014; SOUZA et al., 2015; PINTO et al., 2017; BRENNAN & ACOSTA-MARTINEZ, 2017).
Table 3
Carrot roots marketable and unmarketable productivity, length and diameter, shoots dry weight production and height of carrot plants grown under the residual effect of previous lettuce cultivation with different fertilization managements and cover plants.
| Fertilização | Root | | Shoots |
| Marketable | Unmarketable | Length | Diameter | | Dry Weight | Height |
| | t ha-1 | t ha-1 | cm | mm | | t ha-1 | cm |
| Spontaneous plants | 5.8 c 1/ | 1.9 b | 17 cd | 19 cd | | 1.0 d | 17 bc |
| Sunn hemp | 7.1 c | 1.2 b | 20 bc | 22 bc | | 1.5 c | 19 bc |
| Lablab | 19.7 ab | 1.9 b | 20 bc | 25 bc | | 2.5 b | 20 bc |
| Sunn hemop + Compost | 12.6 bc | 1.6 b | 23 b | 25 ab | | 1.7 c | 23 b |
| Lablab + Compost | 19.8 ab | 0.8 b | 22 bc | 25 ab | | 1.6 c | 22 bc |
| Organic compost | 12.7 bc | 1.3 b | 23 b | 26 ab | | 1.7 c | 22 bc |
| Conventional | 27.7 a | 7.5 a | 30 a | 29 a | | 3.0 a | 32 a |
| Control | 5.4 c | 1.2 b | 14 d | 15 d | | 0.8 d | 15 c |
| Means | 13.8 | 2.2 | 21 | 23 | | 1.8 | 21 |
1/ Means followed by the same letter in the column do not differ by Duncan's test at 5% significance.
The yield of carrots classified as unmarketable (small and/or deformed) was higher with conventional fertilization (7.5 t ha-1), on mean, 5 times more than when grown with green manure (average of 1.5 t ha-1) (Table 3).
Organic fertilization in the carrot culture plays a fundamental role in increasing the commercial roots productivity and in reducing deformed unmarketable roots, especially in soils with low organic matter content (OLIVEIRA et al., 2001).
The lablab residual effect after lettuce cultivation was greater than that of sunn hemp for the carrots production grown in succession to both crops. The marketable roots productivity fertilizad with lablab did not differ from conventional cultivation and was 177% higher than productivity with sunn hemp and 278% higher than control (Table 3).
However, the dose of 16 m3 ha-1 of organic compost added to the sunn hemp plantation and before the lettuce plantation increased the marketable roots productivity, the carrot grown in succession by 77% in relation to fertilization only with the sunn hemp and equaled the productivity with compost fertilization. This positive effect of the organic compound added to the sunn hemp is related to the nutrient’s availability by the compost, which was also observed in the previous lettuce cultivation (Table 3). The carrots productivity fertilized with organic compost and sunn hemp + compost increased by 135% in relation to the control and did not differ from crops with lablab and lablab + compost, but it was 120% lower than in conventional cultivation (Table 3).
The increase in lettuce, turnip and carrot productivity was associated with an increase in the N availability in the soil by adding 20 and 40 t ha-1 of organic compost to green rye and vetch fertilizers, compared to fertilization with only green fertilizers and control without fertilization (PINTO et al., 2017). Similarly, a higher mean cabbage weight was observed when more N was supplied to the crop, through the association of the crotalaria legume with organic compost (20 t ha-1) (FONTANÉTTI et al., 2006).
When added to the lablab, the organic compost did not influence the carrot yield compared to those fertilized only with the lablab (p < 0.05) (Table 3), the same observed in lettuce cultivation (FIG. 1).
In general, the organic compost positively influenced the productivity, length and diameter of the root and the dry weight and carrots shoots height (Table 3). The roots length and diameter were greater in the carrots grown with conventional fertilization, smaller in the control and intermediate in the others (Table 3).
Sunn hemp and spontaneous plants did not increase the productivity in relation to the control, however, the sunn hemp increased the roots length and diameter (mean of 46%) and shoots dry weight (83%) (Table 3).
The carrot shoots dry weight was higher with conventional fertilization followed by lablab, it was lower in the control and spontaneous plants and intermediate in the others (Table 3). The length of the carrot shoots was also greater with conventional fertilization, followed by sunn hemp + compost. The latter did not differ from the plants height in the other treatments, except the control plants without any fertilization, which produced the lowest height and one of the lowest DW of carrot plants (Table 3).
All of these results show the good sunn hemp and lablab fertilizers efficiency associated or not with the organic compost in pre-cultivation of lettuce and carrots, considering that the succession practice brings benefits to the small producer. The gradual plants decomposition will favor the nutrients cycling and may increase the productivity and profitability of crops, reducing costs with chemical fertilizers and pesticides. This practice of crop succession benefits the subsequent crop for its exploration within the same agricultural year, in addition to preserving the soil productive capacity in the long term.