Humic Substances With Mineral Fertilization on Nutrient Availability in Irrigated Soil

This study aimed to evaluate the alternative use of humic substances (humic and fulvic acid) combined or not with mineral fertilization containing Nitrogen (N), Phosphorus (P) and Potassium (K) in the process of nutrient retention in the soil. For this, an experiment was conducted in a greenhouse with PVC columns of 40 cm high and 5 cm in diameter, where they were filled with Oxisol sample, being half of the treatments fertilized with NPK and half without at. Humic substances were added at doses equivalent to 0, 60, 120 and, 240 L ha. After this, 10 irrigations were made with 32 mm rain simulation. After 30 days, soil samples were collected in the upper (0-20 cm) and lower (20-40 cm) layers of the columns. Soil samples were evaluated for P, K, Ca, Mg content, pH, and soil conductivity. The experiment was in a completely randomized design, with a factorial statistical scheme of 2 × 2 × 4 with three replicates. The results showed that mineral fertilization complemented with the use of humic acid promoted a higher residual effect of Ca and K in the soil after 30 days. The fulvic acid, when used in a complementary way to mineral fertilization, promoted a higher amount of P in the soil. In the absence of mineral fertilization, the effect of humic substance use is low on the availability of nutrients in the soil.


Introduction
Brazilian Cerrado soils are naturally low in available nutrients. To maintain high agricultural productivity, the addition of fertilizers is required alongside careful management to preserve soil longevity and water quality.
The main limitations in most Cerrado soils, especially Oxisols, are insufficient development of negative electric charges, high P adsorption, and low nutrient availability (Souza & Lobato, 2004;M. A. Baldotto & L. E. B. Baldotto, 2014;Silva & Lana, 2015) mainly due to the intense weathering caused by high rainfall and temperatures in the region. These limitations can be addressed by increasing the organic matter content of the soil. The functional groups present in organic matter help to improve cation exchange capacity (CEC), increase nutrient availability, decrease the specific adsorption of P, and reduce the toxicity of aluminum and other heavy metals (Rose et al., 2014;Guo, Liu, & Wu, 2019).
The functional groups of soil organic matter consist of simple and complex organic molecules. Simple molecules, such as proteins or carbohydrates, have a high lability and short residence times in soil. Complex molecules, such as humic substances (HSs), have longer residence times and are capable of improving the chemical, physical and biological properties of the soil (M. A. Baldotto & L. E. B. Baldotto, 2014). According to Qian et al. (2015), HSs can be an important reservoir of N, P, and K, while also improving the physical structure, aeration, and drainage of soil. The use of HSs to improve soil properties has long been an interest for the fertilizer industry which has developed products based on fulvic acid (FA) and humic acid (HA). Rose et al., 2014), increased soil microbiota , and increased efficiency of mineral fertilization (Karčauskienė et al., 2019).
There is skepticism regarding the effectiveness of HSs, despite their proven positive effects. Part of the reason for this is their variation in physical and chemical properties (Rose et al., 2014). They can be formed in a diversity of environmental conditions which makes them heterogeneous and their molecular structure difficult to define (M. A. Baldotto & L. E. B. Baldotto, 2014). Furthermore, the recommended application rates of commercial products based on HSs are generally small in relation to the natural amount present in the soil, and consequently, the effects of HSs are substantially less predictable than those of inorganic fertilizers (Rose et al., 2014).
This study tested the effect of HSs (HAs and FAs) in various combinations with NPK fertilization on nutrient retention in soil. The aim was to identify the limitations and potential applications of commercial products based on HSs in Brazilian Cerrado soils. It was hypothesized that the addition of HSs in combination with NPK fertilization would promote greater nutrient retention in the soil.

Method
The experiment was conducted in an acclimatized greenhouse for 30 days (November to December) under natural light, 70% relative humidity, in a constant temperature range between 25 and 30 °C. It was conducted in the city of Rio Verde, Goiás, Brazil (Koppen-Geiger classification = Aw) (Lopes Sobrinho et al., 2020).
The soil bulk density was mesured using a volumetric ring of 100 cm -3 . Based on a density of 1.2 g cm 3 , it was calculated that each column contained approximately 0.9 kg of soil. The HSs (fulvic or humic acid) were added in doses equivalent to 0, 60, 120, and 240 L ha -1 , which equated to 0, 20, 40, and 80 µL column -1 , respectively. These were administered in solutions of 5.0 mL column -1 due to the small quantity of HSs per dose.
The HS treatments used two commercial products: one with a greater amount of HA and the other with a greater amount of FA (Table 1). These treatments were applied in combination with the presence or absence of NPK fertilization. The experimental plots were arranged in a completely randomized design (DIC) in a 2 × 2 × 4 factorial arrangement. Factors included the presence or absence of mineral fertilization, two types of HSs and four application doses. There were three repetitions which totaled 48 sampling units.
A Oxisoil soil of medium texture with 34% clay was used. It was collected in an area of native vegetation at a depth of 0-20 cm, and its characteristics are represented in Table 2. The soil was air dried and sieved at 2.0 mm to maintain uniformity of porosity between treatments.  The soil samples (columns containing approximately 0.94 kg soil each) had their pH corrected with lime (Calcium Carbonate) using an amount equivalent to 4.0 t ha -1 (0.9 g column -1 ). To calculate the liming, the base saturation method (Raij, 1997) was used to raise the value to 60%, which is ideal for the development of crops such as corn. The soil was kept under incubation for 30 days at 70% water retention capacity (WRC), following the methodology of Ruiz, Ferreira, and Pereira (2003), where the WRC specific to Oxisoil tropical soil was estimated by determining the soil moisture equivalent (EU) using the equation WRC = 0.081 + 0.888 EU (R 2 = 0.910), where WRC and EU are expressed in kg kg -1 .
Half of the samples were fertilized with NPK using concentrations equivalent to those needed for a local corn (Zea mays L.) productivity of 6-8 t ha -1 . were applied the equivalent of 100 kg ha -1 of P 2 O 5 in the form of simple superphosphate; 70 kg ha -1 of K 2 O in the form of KCl; and 120 kg ha -1 of N in the form of urea. The columns were 40 cm high with a diameter of 5 cm; made of PVC; and were open at both ends. At the base of each column, an cloth filters were attached to retain the soil.
Each column received 10 irrigations over 30 days, from 10/10/2018 to 11/12/2018. Irrigations were performed every 3-4 days. The initial amount of water represented a rainfall of 32 mm, which is ideal for the start-up of grain crops. Subsequent additions of water were weighed and administered with the objective of maintaining the soil at field capacity. After 30 days, soil samples were collected from the columns in the upper (0-20 cm) and lower (20-40 cm) depths. These were air dried and sieved through a 2 mm mesh sieve (TFSA) prior to analysis of the P, K, Ca, and Mg concentrations; and the pH and electrical conductivity in the soil as per the methodology of Teixeira, Donagemma, Fontana, and Teixeira (2017).
An analysis of variance (ANOVA) was performed to test the effect of different treatment combinations; and where significant effects were found, Tukey's test was applied at 5% probability to compare the means. A regression analysis was used to test the effect of HS doses. The data were analyzed using SAS software (Schlotzhaver & Littell, 1997).

Results and Discussion
The ANOVA results showed that mineral fertilization had a significant positive effect (p ≤ 0.05) on P, K, and Ca concentrations; and electrical conductivity (EC) in both the upper and lower layers of the columns (Table 3). The combined treatment of fertilization with HSs had a significant effect on Ca content in the upper soil layer (p ≤ 0.05). The use of HSs had a significant effect (p ≤ 0.05) on Mg in both soil depths as did the dose of HSs (p ≤ 0.05). The three-way interaction between fertilization, HSs, and the dose of HSs, had a significant effect (p ≤ 0.05) on P and K in the upper soil layer.  The Tukey's test confirmed that, regardless of treatment with HSs, mineral fertilization promoted higher concentrations of P, K, and Ca, as well as a greater EC, than treatments without mineral fertilization. EC increased significantly at both sample depths. P, K, and Ca only increased significantly in the lower soil layer (Table 4). Interestingly, even though 50 mg kg -1 of P was added to the soil via mineral fertilizer, the amount of P remaining in the soil after 30 days was 8.1 mg kg -1 (16.2 kg ha -1 ) in the lower layer of the column, which is too little to supply the needs of plants. According to Souza and Lobato (2004), the average P requirement for plants is 30 kg or 60-100 kg of P 2 O 5 . Table 4. Mean values of P, K, Ca, and electrical conductivity (EC) in the upper and lower soil layers after 10 irrigation events, with the presence or absence of NPK fertilization Means followed by identical letters in each depth do not differ by the Tukey test at 5 % probability.
It is possible that the mixing of P fertilizer with the soil enabled the high adsorption and consequent reduction in the availability of this element for plants (Malavolta, 1981). Moreover, a study by Machado, Souza, Andrade, Lana, and Korndorfer (2011) observed that in medium-textured soils, a higher dose of P fertilizer resulted in higher P adsorption. Novais, Smyth, and Nunes (2007) reported that the retention of exogenous P occurs via its precipitation in solution with Fe 3+ and Al 3+ in an acidic environment, and with Ca 2+ in an alkaline environment. This adsorption effect is significant in weathered tropical soils where Fe 3+ and Al 3+ hydroxides are present in larger quantities. To reduce adsorption, localized application of P fertilizer is used wherein a large part of the plant's root system meets the added P.

Conclus
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Acknowle
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