A field experiment was conducted in a farmer field in Nakhon Ratchasima province, northeast, Thailand objectively to investigate the response of cassava, Huay Bong 80 variety to phosphorus (P) fertilizer in Warin soil series (classified as Grossarenic Paleustults) amended with cassava tails and stalk (CTS) mixed with bentonite (BTN). The experiment was arranged in split plot with four replications. Main plot consisted of four rates of CTS +BTN: 0+0, 6.25+1.25, 12.5+2.5 and 25+5 t/ha while subplot comprising four rates of P fertilizer: 0, 25, 50 and 100 kg P2O5/ha. Cassava was harvested at 12-month of age and plant samples were collected at the harvesting time to study the effects of soil amendment on the uptake of primary plant nutrients in different plant parts of cassava. The results showed that the CTS+BN mixture at the rate of 25+5 t/ha highly significantly promoted the highest fresh tuber yield (FTY) and starch yield (SY) of 31.79 and 8.97 t/ha, respectively, which were 21.2 and 20.7 % greater than that of the control with no soil amendment addition. Increasing rate of P fertilization highly significantly induced greater FTY, SY and aboveground biomass (ABG) than did the zero-P fertilization. Amending the soil with 25+5 t/ha of CTS+BN along with the addition of 100 kg P2O5/ha interactively gave the significantly highest respective FTY, SY and ABG of 39.76, 11.48 and 17.35 t/ha whereas the plot amended with 12.5+2.5 kg CTS+BN/ha plus 50 kg P2O5/ha fertilization gave slightly lower STY and SY of 36.11 and 10.30 t/ha, respectively. The former combined rate of soil amendment and P fertilizer also had the interaction that highly significantly induced the highest uptake of all primary plant nutrients in the whole plant, accounting for 131.66, 14.23 and 124.13 kg/ha of respective nitrogen, phosphorus, and potassium. The result reaffirmed that amending the soil with proper rate of CTS+BTN mixture enhanced the efficiency of P fertilizer and subsequently improved yield of cassava. HIGHLIGHTS Coarse-textured soils in northeast Thailand are very poor physically and chemically, leading to low yield of cassava planted Improving the soils with proper soil amendments combined with phosphorus fertilization can be beneficial to an increase of cassava yield Amending Warin soil series, one of the coarse-textured soils in this region, with 25+5 t/ha of cassava tails and stalk + bentonite along with the addition of 100 kg P2O5/ha interactively gave the significantly highest fresh tuber yield, starch yield and aboveground biomass of cassava GRAPHICAL ABSTRACT

The pattern of soil carbon stock is atypical along the slope position in a seasonally dry tropical forest; the mean stock values increase from the lower, middle, to upper slopes, at 11.5, 13.2, and 15.5 kg m⁻², respectively. In sloping landscapes, soil organic carbon tends to accumulate in lower slopes, but our previous soil respiration study suggested that soil carbon stock distribution along the slope position in seasonally dry tropical forests is atypical. The aims of this study were: (i) to examine whether the atypical pattern occurs widely in the watershed; and (ii) to examine the pattern of root development in the soil profile as a source of soil carbon. The density and stock of soil carbon in three soil layers (0–10, 10–30, and 30–100 cm) of 13 soil profiles were compared in different positions on the slope (upper, middle, and lower). Root biomass at each slope position was also determined. Soil carbon density in each layer increased significantly with an increase in the relative position of the slopes, particularly in the 10–30 cm soil layer. The density of medium root (3–10 mm in diameter) in the upper slopes was significantly higher than that in the middle and lower slopes, especially for 15–60 cm soil layers. The atypical pattern of soil carbon accumulation along the slope position occurred widely in the studied watershed and appeared to be caused by the development of root systems in deeply weathered soil under xeric soil conditions in the upper slopes. Roots of bamboo undergrowth may also contribute to soil carbon stabilization by reducing soil erosion in the surface soil.

An Ustic Quartzipsamment was the soil that had light texture and very low fertility status with intense leaching. A field experiment was conducted in a farmer field, northeast Thailand. Experimental design was arranged in split plot. Main plot comprised four rates of chicken manure (CM) and subplot consisted of six rates of K2O. Soil leaching column was also studied in laboratory. Amending the soil with 3.125, 6.25 and 12.5 t/ha of CM highly significantly promoted greater respective fresh tuber yield of 14.9%, 24.2% and 31.6% over the control with no CM addition. Cassava responded best to 100 kg/ha of K2O. The manure evidently induced greater uptake of N, P and K in all plant parts while potassium (K) fertilizer increasing N, P and K uptakes in almost all plant parts. Soil properties at 0-30 cm depth were affected by the addition of 6.25 and 12.5 t/ha of CM as soil pH and cation exchange capacity, and quantities of total N, available P and K, and extractable Ca and Mg increased significantly and K fertilizer significantly affected a decrease of soil pH. In the leaching column study, greater K leached from soil column as well as the improvement of soil properties with increasing rates of CM addition was observed.

A study on the effect of composted pig manure and chemical fertilizers on the yield and plant nutrients uptake of the KDML105 and RD41 rice varieties grown on a Typic Natrustalf was carried out in a field of the Farmer Occupational Research and Development Centre, Ban Panead, Khok Samrong district, Lopburi province in 2013. The soil on the experimental area contained some soil constraints that adversely gave a low rice yield; therefore, the study investigated the response of two rice varieties to the application of composted pig manure and chemical fertilizers. A factorial, randomized, complete block design with three replications was employed. The first factor comprised the two rice varieties while the second factor consisted of six treatments consisting of no added fertilizer, sole and a combination of composted pig manure and chemical fertilizers. The results showed that both rice varieties produced similar amounts of grain yield—1.32 t/ha for the KDML105 variety and 1.28 t/ha for the RD41 variety. The KDML105 variety had greater vegetative growth than the RD41 variety. The application of 50% composted pig manure plus 100% chemical fertilizers highly significantly promoted the rice grain yield to the highest level achieved (1.88 t/ha), whereas the sole application of chemical fertilizer (T2) as commonly used by local farmers and 50% composted pig manure (T4) gave much lower yields, albeit greater than the control without manure added or fertilization. Almost all major and minor plant nutrients were statistically more concentrated in the grain of RD41 than in KDML105, while some micronutrients accumulated more in KDML105. In general, the application of 50% composted pig manure together with 100% chemical fertilizer (T5) effectively promoted the highest uptake of most plant nutrients in both the grain and straw, which coincided with the highest grain yield. While there were no statistically significant differences, RD41 with the addition of 50% composted pig manure together with 100% chemical fertilizer tended to give the highest grain yield of 2.29 t/ha when grown on this poor soil.

A tropical sandy Typic Paleustult soil has low fertility status with low nutrient retainability. A field experiment was carried out in a farmer field, northeast Thailand for two-consecutive years (2018–2019). Split plot design with four replications was employed with main plot consisting of five rates (0, 3.125, 6.25, 12.5 and 25 t/ha) of burnt rice husk (BRH) and subplot comparing six rates (0, 25, 50, 75, 100 and 125 kg K₂O/ha) of potassium (K) fertilizer. Amending the soil with 12.5 t/ha of BRH promoted the significantly highest fresh tuber yield (FTY) of 29.31 t/ha in the first crop and all amended plots significantly gave better FTY than did non-amended plot in the latter crop. Cassava significantly produced the highest FTY when received 125 kg/ha of K₂O in both growing seasons. In the second year, the addition of 25 t/ha of BRH along with 125 kg/ha of K₂O interactively gave the highest FTY which was 99.7% greater than that of the plot received no BRH nor K fertilizer. The waste, with increasing rates, evidently stimulated more N, P and K uptakes in all plant parts while N uptake in tuber and K in all plant parts were increased by increasing K fertilization. Two-consecutive year application of BRH significantly improved available P, K, Mg, Zn and Cu contents in the topsoil.