Glauconitic Siltstone as a Source of Potassium, Silicon and Manganese for Flooded Rice

The objective of this study was to evaluate the efficiency of glauconitic siltstone as a multi-nutrient source for flooded rice. Two experiments were carried out under greenhouse conditions, one using a Ferralsol and the another an Arenosol. Glauconitic siltstone was applied in different doses (0, 5, 20, 40, and 80 mg dm K2O) and potassium chloride, wollastonite, and manganese sulfate were respectively used as standard sources, at doses of 80 mg dm K2O, 270 mg dm Si, and 2 mg dm Mn. The sources were incubated for 90 days on the two soil types and, after the incubation period, rice plants were sown, and two consecutive rice growths were performed. The application of glauconitic siltstone in tropical soils promotes increases in the plant and grain dry matter of rice plants, as well as K, Si and Mn contents in soil samples and accumulated in plants. Greater effects following the application of glauconitic siltstone are obtained after the second rice growth due to its gradual release.


Introduction
Potassium chloride (KCl) is the most used source of potassium (K) fertilizer in the world (Ernani et al., 2007). However, crops fertilized with high KCl doses can present significantt chlorine (Cl) accumulation in leaves, which may inhibit some metabolic processes (Geilfus, 2018), as well promoting soil salinity and, as a consequence, water stress to plants (Watanabe et al., 2017).
Moreover, intensive application of KCl may also result in losses of K by leaching, since the K ion has a greater hydration radius than most other cations, resulting in a low retention cation-exchange capacity of soil (Dolcater et al., 1968). Potassium leaching is often a problem, especially in tropical areas with soil fertility constraints.
The total amount of K in Brazilian soils ranges from 0.5 to 25 g dm -3 (Ribeiro et al., 2010). Despite being the most absorbed element for the majority of crops, the content of soil available K to plants is low.
Recent investigations suggest that the addition of glauconite-rich rocks may increase the K content in soils (Zörb et al., 2014). In Russia, glauconitic sandstone is a K-rich rock used in K fertilizer production. In addition, there are reports of the use of glauconitic sandstone as K fertilizer in India and in some African countries, resulting in higher soil K contents and superior yields (Karimi et al., 2012).
In this context, GS could reduce the demand for KCl, which may deplete soils and their microbiota when inappropriately managed (Karimi et al., 2012). The objective of this study was to evaluate the efficiency of GS as a multi-nutrient source for flooded rice. This crop was chosen since yield responses in rice to soil K, Si and Mn amendments have frequently been recorded on weathered tropical or sub-tropical soils on which they are mainly grown. Owing to its capability to supply K, Si and Mn to plants, we expect to promote an innovative and sustainable use of GS powder in agricultural areas.
GS used in the experiment was characterized according to macro and micronutrient contents, determined on a flame photometer according to the method described by MAPA (2017), 10 g kg -1 total K 2 O, 270 g kg -1 total Si, and 0.5 g kg -1 total Mn. Wollastonite (230 g kg -1 Si, 303 g kg -1 Ca, and 110 g kg -1 Mg), KCl (600 g kg -1 K 2 O) and manganese sulfate (MnSO 4 ) (310 g kg -1 Mn) were used as standard sources for comparison purposes. The chemical characteristics of nutrient sources used in this study were determined according to methodology described by Teixeira et al. (2017) and Korndörfer et al. (2004) (Si analysis). The treatments were applied to soils as a < 2 mm powder.
Since wollastonite supplies Ca and Mg to plants, different rates of CaCO 3 and MgCO 3 were added to adjust the amounts of Ca and Mg in all treatments and increase soil base saturation to 70% and 90% in clayey and sandy soil, respectively.
The products were incubated for 90 days in 8 dm -3 of the two different soil types. In order to maintain humidity, around 80% of the field capacity value for each soil, deionized water was added to pots. Soil moisture was rigorously controlled by daily weighing of the plastic containers, replacing the volume lost through evapotranspiration with deionized water.
After the incubation period, 200 mg kg -1 N and 400 mg kg -1 P 2 O 5 were added to samples through ammonium sulfate and triple superphosphate, respectively. The micronutrients were supplied via solution at the rates of 1.5; 5.0; 0.5, and 0.05 mg dm -3 Cu, Zn, B, and Mo, through the sources CuSO 4 ·5H 2 O; ZnSO 4 ·7H 2 O; H 3 BO 3 and (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, respectively. Then, BRS Atalanta irrigated cycle rice cultivar was sown, which presents smooth grains and leaves, high tillering capacity, and strong stalks (Embrapa, 2007). Fifteen seeds were sown per pot, at a depth of 2 cm. After the emergence, thinning was carried out, maintaining 6 plants per pot. At this time, a 2 cm water irrigation level was also added, which was maintained until 15 days before harvesting.
Rice plants were harvested at 89 days after sowing (DAS), and at the same time, the soil samples were collected. Plants samples were dried in an oven and weighed to obtain the plant dry matter (DM) and grain DM values. Afterwards, the samples were ground and submitted to nitric-perchloric digestion. The K and Mn concentrations were analyzed by flame spectrophotometry and atomic absorption spectrophotometry (Teixeira et al., 2017), respectively, whereas Si concentrations were measured at 630 nm using an ultraviolet (UV) visible spectrophotometer (Korndörfer et al., 2004). The product of the plant DM and nutrient concentration in plant samples resulted in the values of K, Si, and Mn accumulated in rice plants.
Soil chemical analyses were performed according to Korndörfer et al. (2004) (soil available Si content) and Teixeira et al. (2017) (soil available K-Resin extraction and Mn contents).
Using the nutrient accumulated values, relative agronomic efficiency (RAE) of GS relative to KCl was calculated using the following equation as proposed by Fageria et al. (2010): where, GS is the plan treatment, After the applied. T without th manageme Normality distributio All analys

Results
The effect from 35 to ( Figure 1) Vol. 12, No. 9; After the second growth, these values ranged from 12 to 26 g pot -1 in clayey soil and from 5 to 12 g pot -1 in sandy soil (Figure 1). Linear increases of plant DM after the second growth were observed following the increase of GS dose to the soil; for every 10 mg dm -3 applied to the clayey and sandy soil, there was an increase in plant DM of 1.9 and 1.0 g pot -1 , respectively (Figures 1a and 1b). In general, higher response to GS application was observed in clayey soil, since the soil type affects the availability of K to plants. Potassium leaching is frequently related to the soil texture, being the most easily leached cation, especially in sandy soils, due to its displacement to the solution and to its percolation (Mendes et al., 2016), making it less available to plants.
After the first rice growth, the application of GS resulted in an increase in plant and grain DM, compared to the check, without K 2 O, yet the effects were not comparable with the KCl application ( Figure 1). The greater yields provided by the standard source were due to the greater solubility, which is mostly water-soluble and readily available to plants . However, in the second rice growth, GS promoted higher plant and grain DM values than the KCl application.
In general, relative agronomic efficiency (RAE) values following GS application were smaller than KCl values (values minor than 100%) after the first rice growth (Figure 2). In contrast, after the second rice growth, GS sources presented greater agronomic efficiency (values higher than 100%), indicating a good prospect for gradual release use, as previously discussed.   Vol. 12, No. 9; In addition to GS application, flooded conditions also directly influenced the Mn availability to rice plants.
Under flooded conditions, soils show low redox potential, thus Mn deficiency has decreased in lowland (Tanaka & Navasero, 1966). As Mn is an essential element for plants and its deficiency decreased growth and yield, as well as making plants more susceptible to pathogens (Socha & Guerinot, 2014), the use of GS associated to flooded rice cultivation may be a good alternative to increase Mn availability for plant uptake.
In general, GS efficiency to supply K, Si and Mn to rice plants shows an innovative and sustainable use of nutrient-rich rock to improve tropical soil fertility and rice yields, especially after consecutive crop cycles.

Conclusions
(1) Results described in this study provide an important understanding of the use of glauconitic siltstone not only as a K fertilizer, but as source of silicon (Si) and manganese (Mn) in tropical soils, being an efficient alternative to improve tropical soil fertility and increase rice yields.
(2) The application of glauconitic siltstone in tropical soils promoted increases in plant and grain dry matter of flooded rice, as well as K, Si, and Mn contents in soil samples and accumulated in plants.
(3) Greater effects following the application of glauconitic siltstone are obtained after the second rice growth due to its gradual release.