Poultry litter and fresh mulch of Elephant grass improve the organic potato production

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INTRODUCTION
The potato (Solanum tuberosum L.) production will remain important for future food security, especially in developing countries, and crop management (e.g., N input) will probably contribute more to increase yields than the cultivar improvement itself (Kleinwechter et al. 2016).
Moderate N use is a recommended crop practice to delay the late blight development, because the excess of N fertilization can lead to increases in the disease severity, requiring more fungicides to prevent losses (Arora et al. 2014).With similar P and K tuber contents in organic fertilization, when compared to the conventional fertilization of potato, but with a 30 % lower N tuber concentration, there is an evidence that the N availability is the main reason to explain the lower tuber yields in organic cropping (Palmer et al. 2013).Regarding the N nutrition of potato plants, the ammonium (NH 4 + -N) and nitrate (NO 3 --N) combination is more important for the N uptake and tuber yield increase than the use of these forms alone (Cao & Tibbitts 1993, Serio et al. 2004).NO 3 --N inputs stimulate the stolon branching and growth, whereas NH 4 + -N stimulates the tuber swelling (Osaki et al. 1995).However, the NO 3 --N efficiency is greater for potato tuber by means of improving the shoot growth, what will subsequently provide more assimilates after the tuber initiation stage (Gao et al. 2014).
Poultry litter is an economical and technical alternative to the use of inorganic fertilizers, providing sufficient amounts of N, but adding P and K contents higher than needed by organic potato crops (Fioreze & Ceretta 2006).Poultry litter fertilization shows positive trends in improving marketable tuber yields, with reductions in small tubers yields and in specific gravities (Rees et al. 2014).Poultry litter is largely used by small growers in the southern region of Brazil, and its application provides NH 4 + -N amounts proportionally to the applied doses, with more rapid conversion into NO 3 --N when incorporated, rather than when applied on the soil surface (Rogeri et al. 2015).The soil incorporation of fresh manure in the spring is a recommended practice to supply N in organic farming, synchronizing the crop demands and N cycling in the soil (Berry et al. 2002).
Organic mulch on potato crop is a technique that can reduce the erosion and loss of soil organic matter, NO 3 --N and available P, K, Ca and Mg by runoff, increasing the total and marketable tubers yields (Rees et al. 2002).Mulching with organic residues decreases the soil thermal amplitude and soil water evaporation, thus increasing the N uptake and the efficiency of the N fertilization to enhance higher potato tuber yields (Singh et al. 2015).Besides this buffer effect on the soil hydrothermal conditions, organic mulching also adds plant nutrients to the soil after decomposition, but also can immobilize N by microorganisms (Kader et al. 2017) and, therefore, reduces the NO 3 --N leaching to the soil profile (Döring et al. 2005).
The hypothesis in the present study is that the fertilization with poultry litter incorporated into the soil at pre-planting may interact with fresh mulch and decrease the foliar late blight severity, with an increase of tuber yields.Thus, this study aimed to evaluate the effect of the interaction between fertilization with pre-planting doses of poultry litter and the use of fresh mulch after hilling on the severity of foliar late blight and yields and specific gravity of potato tubers cultivated in the organic system.

MATERIAL AND METHODS
The field study was conducted at the Instituto Agronômico do Paraná experimental station, in Lapa (25º46'11"S, 49º42'57"W and 908 m a.s.l.), Paraná state, southern Brazil.The local climate is classified as Cfb, according to the Köppen's climate classification, with mild summer and evenly distributed rainfall, no dry season and mean temperature of the warmest month inferior to 22 ºC.The precipitation volume varies from 1,100 mm to 2,000 mm, with the occurrence of severe and frequent frosts in the coldest months.The cumulative rainfall data and average maximum and minimum temperatures during the experimental period (from October 2017 to February 2018) were recorded by a weather station located 300 m away from the experimental plots (Figure 1).
After the distribution of all treatments, the soil was ploughed and harrowed at 15 days before planting.The poultry litter water content was determined by the difference between the natural and oven dried (65 ºC to constant mass) sample weight and transformed into percentage.Poultry litter samples were prepared by digestion with sulfuric acid, with a subsequent determination of ammonium-N by UV-visible spectrophotometry with blue indophenol, and by digestion with hydrochloric acid and determinations of P, K, Ca, Mg, Cu, Zn, B and Mn by inductively coupled plasma-atomic emission spectrometry.The chemical analysis (Tedesco et al. 1995) of the poultry litter (17.8 % of humidity) revealed 178.9 g kg -1 of C; 14.2 g kg -1 of N; 26.7 g kg -1 of P; 31.2 g kg -1 of K; 118.8 g kg -1 of Ca; 51.0 g kg -1 of Mg; 6.4 g kg -1 of S; 499 mg kg -1 of Cu; 5,059 mg kg -1 of Fe; 455 mg kg -1 of Zn; 27 mg kg -1 of B; and 774 mg kg -1 of Mn.
Each plot was composed of four rows spaced 0.80 m, with 5 m in length.In each planting row, 17 potato tuber seeds of the IPR Cris cultivar were planted (Oct.10, 2017), with about 0.25 m in depth and 0.30 m apart.After around a month (Nov.09, 2017), hilling was performed and then fresh mowed Elephant grass residue [Panicum maximum (Jacq.)],at a dose of 60 Mg ha -1 (84.8 % of humidity), was applied over the formed ridges, in the respective plots, with mulch.The grass distribution was performed on a mass basis onto the plot quadrants, similarly to the distribution of the poultry litter.The results for the chemical analysis (Tedesco et al. 1995) of the grass mulching residue were (dry base): 421.5 g kg -1 of C; 12.4 g kg -1 of N; 4.3 g kg -1 of P; 46.3 g kg -1 of K; 4.6 g kg -1 of Ca; 3.1 g kg -1 of Mg; 21.3 mg kg -1 of Cu; 522.0 mg kg -1 of Fe; 29.6 mg kg -1 of Zn; 3.7 mg kg -1 of B; and 123.4 mg kg -1 of Mn.
The experiment was conducted in an organic system, with mechanical weed control and inputs application authorized by the Brazilian regulation for organic products.A commercial mixture composed by 100 g of calcium oxide and 100 g of copper sulphate in 10 L of water (Bordeaux mixture) was applied twice a week, at the concentration of 1 %.A copper-based solution was applied in a similar way to that used by Brazilian organic family farmers.Similarly, a tobacco extract produced from rope tobacco (100 g) in 1 L of 70 % alcohol, applied at 1 % with solid soap (10 g) and 20 L of water, was used to control Diabrotica speciosa (Germar), when imminent pest symptoms began to appear.
The leaf blight symptoms were visually assessed every seven days, up to 101 days after planting, when the epidemic was covering almost the entire plot area.The disease diagram, key 3.1.1,proposed by James (1971) was used as a visual reference of the disease severity.Evaluations of leaf blight symptoms were obtained from all plants in each one of the plot quadrants.
The harvest was performed on Feb. 8, 2018, at 121 days after planting, considering the two central rows in each plot and leaving the outer rows as borders.The tubers were classified and weighed as e-ISSN 1983-4063 -www.agro.ufg.br/pat-Pesq.Agropec.Trop., Goiânia, v. 50, e57585, 2020 marketable (> 45 mm of diameter) and unmarketable (< 45 mm of diameter), with the aid of a coarse mesh sieve, on an inclined grading table.The tuber yields of both classes were summed to obtain the total tuber yields.From each plot, a random sample of ten marketable tubers was weighed in both the air (Wa) and water (Ww) to calculate the tuber specific gravity (SG) by: SG = Wa/(Wa -Ww).
The data were submitted to analysis of variance by the F test at 5 % of significance.The quantitative factors (poultry litter doses) were deployed in plots with and without mulching through polynomial regression analysis at 5 % and 1 % of probability.The difference between the treatments without and with mulching, in the case of significant interactions, was evaluated by the Tukey test (p < 0.05).Pearson's linear correlations were investigated among the dependent variables of interest.All statistical analyses were performed using the AgroEstat software (Barbosa & Maldonado Júnior 2015).

RESULTS AND DISCUSSION
Even with weekly Cu applications up to the end of the cycle, the plots without fresh Elephant grass mulch and poultry litter at the pre-planting stage presented an average late blight severity of up to 80 % (Figure 2).According to Finckh et al. (2006), the late blight control using Cu-based sprays is limited and inefficient, due to its action, in comparison to systemic fungicides.These same authors reported that the soil fertility, crop nutrient status and the ability of a given potato cultivar to make use of organic sources efficiently are very important factors to build strategies against late blight in the organic production.For the IPR Cris potato, a moderately resistant cultivar selected to be used in organic production systems in southern Brazil (Nazareno et al. 2015), both mulching (Figure 2a) and poultry litter doses (Figure 2b) decreased (p < 0.05) the leaf area infested by late blight without an interactive effect (p < 0.05) between these factors (Table 1).
The presence of fresh grass mulch with Elephant grass reduced the late blight severity by 19.9 %, from 79.6 % to 63.8 % for foliar severity (Figure 2a), confirming findings in Europe and in the USA (Nyankanga et al. 2008, Dvorák et al. 2010).A decrease in the late blight severity of 29.6 % (84.1 % to 59.2 %) was also observed with ns and *: non-significant and significant values at 5 % of probability, respectively.Poultry litter and fresh mulch of Elephant grass improve the organic potato production the application of the maximum poultry litter dose incorporated into the soil at the pre-planting stage (Figure 2b).By means of the linear regression, it was observed that 63.8 % of the late blight severity was obtained with fresh grass mulch with the equivalent to 24.5 Mg ha -1 of poultry litter incorporated into the soil at pre-planting.

Sources of variation
Straw mulch may lead to lower temperatures on the soil surface and on the air, with a cooler nocturnal microclimate, and may also prevent the rain splash dispersal of oospores, what probably led to a reduction (p < 0.05) of the late blight infestation in four experiments and a reduction trend in 13 of a total of 15 organic system experiments (Döring et al. 2006).Fresh and chopped Elephant grass residues with 84.8 % of humidity, as mulch, would have altered the microclimatic and epidemiologic conditions to the P. infestans propagation.But its high C:N ratio (34:1) also may have favored the multiplication and action of beneficial microorganisms from the organic compost, which may produce antibiotics and inhibit the development of phytopathogens (Islam et al. 2013), as a decreasing of late blight susceptibility with an excess of N (Ros et al. 2008).
There was a significant decrease (p < 0.05) of 43.2 % of leaves per plant infected by P. infestans with poultry litter application in the organic system of potato production.For this, Islam et al. (2013) compiled the following justifications: i) nutrient competition between the plant pathogens and beneficial microbes; ii) antibiotic compounds produced by these beneficial microorganisms; iii) parasitism and predation by beneficial microorganisms; iv) induction of systemic resistance in plants by microorganisms from manure or soil.Studying interactions of late blight susceptibility as a function of N availability to potato cultivars, Ros et al. (2008) observed that a high N availability significantly increased the susceptibility of three of the four potato cultivars, but, on the other hand, a better N nutrition induced more defense/ stress-related genes.
With freshly mowed Elephant grass mulch, the non-marketable tubers (d < 45 mm) increased as a quadratic response (R 2 = 0.81**) with the poultry litter dose of 17.7 Mg ha -1 , resulting in a maximum of 8.2 Mg ha -1 of tubers (306.9 % increase); whereas, without mulch, the poultry litter doses would not affect this result (Figure 3a).These results contradict those obtained by Rees et al. (2014), who observed that the incorporation of poultry litter at the pre-planting stage, during three years, decreased (p < 0.05) the average yield of smaller tubers.Associated to the poultry litter application, non-marketable tubers yields were similar (p > 0.05) between the treatments with and without mulching; but, in the control treatment without poultry litter fertilization, the presence of fresh grass mulch decreased by 5.8 Mg ha -1 (65.1 %) the yield of the non-marketable tubers, in comparison to plots without mulching.
In mulching made with C-rich materials, such as straw or stalks, microorganisms can use the soil N to decompose these materials, making them unavailable to plants (Kader et al. 2017).In fact, it was observed that, at the beginning of the vegetative phase, in plots with mulching, plants developed more slowly, if compared to plots without mulching, indicating that there could be less N availability for them (data not shown).Marketable tubers also increased linearly (R 2 = 0.76*) with the poultry litter doses in the fresh grass mulch treatment, but did not vary in plots without mulching (Figure 2b).A maximum increase of 19.2 Mg ha -1 (7.8 Mg ha -1 ; 68.9 %) was obtained with 30 Mg ha -1 of poultry litter with Elephant grass mulch at the pre-planting stage, being 23.4 % (3.4 Mg ha -1 ) higher (p < 0.05) than the same poultry litter dose without mulching.
The fresh Elephant grass mulching and maximum poultry litter dose of 30 Mg ha -1 provided 463 kg ha -1 of N, 304 kg ha -1 of P 2 O 5 and 988 kg ha -1 of K 2 O, respectively 32 %, 6 % and 55 % more N, P and K than the same dose of this manure alone.The total tuber yields increased as a quadratic response (R = 0.99**) only in the plots with fresh grass mulch, reaching 26.3 Mg ha -1 with 20.3 Mg ha -1 of poultry litter (Figure 3c).Moreover, with 20 Mg ha -1 of poultry litter, there was a 33.8 % (6.9 Mg ha -1 ) increase (p < 0.05) in the total tuber yield with fresh grass mulch, in comparison to plots without mulching.Elephant grass residues resulted in a higher soil surface persistence and lower nutrient release (Nascente et al. 2014).Possibly, the effect of the interaction between the use of mulch and poultry litter application could be more related to the physical protection of the soil, since there was no difference (p < 0.05) between the plots with and without fresh grass mulch on marketable tuber yields, in plots not fertilized with poultry litter (Figure 2b).
The soil incorporation of poultry litter favors the quick conversion of NH 4 + -N to NO 3 --N, due to the greater contact with soil nitrifying microorganisms, but not all the water soluble N in the poultry litter is mineralized and/or nitrified, due to possible denitrification losses and temporary retention by microbial immobilization (Rogeri et al. 2015).On a sandy soil under 719 mm of accumulated rainfall, at six months after organic fertilizing at the preplanting stage (Figure 1), the fresh Elephant grass mulch probably preserved the NH 4 + -N and NO 3 --N and other nutrients, providing an increase in the marketable tuber yields.On the other hand, the absence of 60 Mg ha -1 of fresh Elephant grass mulch, equivalent to about 9 Mg ha -1 of dry matter, applied over the potato planting rows under torrential rainfall (169 mm), at a tuber bulking stage 90 days after planting (Figure 1), probably favored the nutrient loss by leaching and reduced the marketable tubers yields.
In two experiments on sandy soils (557 g kg -1 and 590 g kg -1 of sand) in the organic system, Lynch et al. ( 2008) observed that 7.2 Mg ha -1 (300 kg ha -1 of N) of pelletized poultry manure incorporated into the soil at the pre-planting stage increased (p < 0.05) by 28.4 % (7.0 Mg ha -1 ) the marketable tuber yield.However, if compared to the control treatment, 14.4 Mg ha -1 (600 kg ha -1 ) of pelletized poultry manure did not increase (p > 0.05) the marketable tuber yield and also resulted in excessive NO 3 --N levels at harvest.Over 318 mm of accumulated rainfall, triticale straw mulch (2.5 Mg ha -1 ) resulted in less 17.6 kg ha -1 of NO 3 --N (p < 0.05) in the 0-0.6 m layer of a silty soil (730-760 g kg -1 of silt), in relation to plots without mulching after organic potato harvest, with no differences (p > 0.05) in the total tuber yield among these treatments (Döring et al. 2005).Rice straw mulch (6.0 Mg ha -1 ) under a 53 mm average rainfall regime, during two cropping seasons, improved the urea-N efficiency and, due to better hydrothermal conditions, tuber yields with 225 kg ha -1 of N in non-mulched plots were, on average, 11.1 % lower (p < 0.05) than with 135 kg ha -1 of N applied in mulched plots, in two experiments (Singh et al. 2015).
The reduction of the specific gravity, as the poultry litter doses increased only on mulched plots, reinforces the argument that nutrients were more available in this treatment (Figure 4).With 30 Mg ha -1 of poultry litter, the specific gravity of marketable tubers over mulching was 1.4 % higher (p < 0.05) than in the non-mulched treatment.Poultry litter and fresh Elephant grass mulch, as a physical barrier, may have a negative effect on the specific gravity of marketable tubers, due to the avoidance of N and K soil leaching and excessive K supply in both treatments, as this soil is very rich in this nutrient.The effect of poultry manures applied at the pre-planting e- ISSN 1983-4063 -www.agro.ufg.br/pat -Pesq. Agropec. Trop., Goiânia, v. 50, e57585, 2020 Poultry litter and fresh mulch of Elephant grass improve the organic potato production stage on the tuber specific gravity of potatoes was also observed under non-mulch conditions (Lynch et al. 2008, Rees et al. 2014).Without grass mulch, the specific gravity of tubers did not decrease with the poultry litter doses, due to the leaching of cations through the soil profile, which may also have been favored by the organic fertilizer.According to Gebrim et al. (2008), inorganic anions, such as chloride, nitrate and sulfate, and low molecular weight organic acids, such as malic and oxalic acids, present in poultry litter, help as an ion pair in the leaching of Ca, Mg, K and Na.CONCLUSIONS 1.The incorporation of poultry litter to the soil at the pre-planting stage of potato or the use of fresh Elephant grass mulch after potato hilling reduces the late blight infestation on leaves, in the organic system; 2. The application of fresh grass mulch after the hilling operation potentiates the effect of organic fertilization at the pre-planting of potato; 3. Pre-planting poultry litter doses increase the yields and decrease the specific gravities of marketable tubers, in association with fresh grass mulch; 4. The application of poultry litter at the preplanting stage and fresh grass mulch improve the sustainable potato cultivation in the organic production system.

Figure 2 .
Figure 2. Means of late blight (Phytophthora infestans) severity (Y) on leaves at 101 days after planting, as a function of the absence or presence of Elephant grass mulch (a) or poultry litter doses (X) incorporated into the soil in the pre-planting stage of organic potato crop (b).Vertical bars indicate the respective mean standard errors; means with distinct capital letters in (a) are statistically different (p < 0.05) by the Tukey test; and ** is a significant value at 1 % of probability.

Figure 3 .
Figure 3. Non-marketable (a), marketable (b) and total tubers (c) yields (Y), as a function of treatments without (○) or with (•) Elephant grass mulching combined with poultry litter doses (X) incorporated into the soil, at the pre-planting of organic potato crop.Vertical bars are the mean standard errors.ns , * and **: non-significant (p > 0.05) and significant values at 5 % or 1 % of probability, respectively.

Figure 4 .
Figure 4. Specific gravity of marketable tubers (Y), as a function of treatments without (○) or with (•) Elephant grass mulching combined with poultry litter doses (X) incorporated into the soil at the pre-planting stage of organic potato crop.Vertical bars are the mean standard errors.ns and **: non-significant (p > 0.05) and significant (p < 0.01) values, respectively.

Table 1 .
F-test values 1 in the analysis of variance for late blight severity (LBS), non-marketable tubers (NMT), marketable tubers (MT), total tubers (TT) yields and specific gravity of marketable tubers (SG), as a function of poultry litter doses and grass mulching with Elephant grass.