The phosphorus (P) sorption index and efficiency of P extracting methods were investigated to quantify the available P content for accurate recommendations regarding cassava P fertilizer application in the sandy soils of Thailand. Phosphorus sorption isotherms were examined to predict the response of soils to the P fertilizers applied. Six rates of P fertilizers (0, 12.5, 25, 37.5, 50 and 62.5 kg P₂O₅ ha⁻¹) were applied in a randomized complete block design at five field sites in major cassava-growing areas. The efficiency among the Bray II, CaCl₂ and Mehlich III extracts was also tested by comparing their capacity to predict crop response. The P adsorption capacities of all soils were in the range 39.4–106 mg kg⁻¹; therefore, there were classified as low P-sorbed soils. Phosphorus fertilizer produced significantly higher aboveground biomass than did zero-P fertilization. The critical P level adequate for 95% of the maximum yield was in the range 0.14–0.26 mg P L⁻¹. The equation for calculating P fertilizer using soil P extracted based on the Bray II, CaCl₂ and Mehlich III methods had accuracy levels of 73, 65 and 72%, respectively. Bray II was slightly superior to Mehlich III for use in soil-P availability indices, both of which were recommended for P fertilizer assessment.

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.

Effect of magnesium chloride (MgCl₂) and dolomite on magnesium (Mg) availability cassava was observed by a pot experiment under greenhouse conditionin three Paleustults. Their water solubilities varied greatly, with increased Mg availability in the soils. This resulted in cassava taking up more Mg and producing 23.3–45.2% and 18.8–26.2% greater aboveground biomass over the control when respective MgCl₂ and dolomite were added. Without Mg fertilization, Mg contents, mainly in a component of minerals, were low and constant throughout 120 days of the study, indicating that initial Mg content in these soils was insufficient for cassava growth. Magnesium became available at 1-month after dolomite was applied. Magnesium in MgCl₂-added soil was predominated by available Mg; moreover, MgCl₂ evidently boosted the soluble Mg portion within 15 days after applying. After that, the proportion decreased similarly to that of the other treatments. It indicated that dolomite was a better Mg source in supplying Mg for a longer period than MgCl₂. In addition, the Mg contents in the soils gradually increased early and then sharply decreased at 30 days, and then the contents were constant until the end of the study. The result clearly signified that cassava absorbed Mg rapidly within 1.5 months after planting. Among soil extractants tested in the study, 0.05 M HCl was the best available Mg extraction suitable for cassava crop, whereas 1 M NH₄OAc, the routine soil test in Thailand, was the least suited extractant.

Boron (B) release mechanism is vital in supplying available B, particularly in acidic soils, because a temporary B deficiency can be triggered when liming the soil. This research was conducted with the aim at elucidating the role of Ca on B release kinetics in three acidic soils of Thailand to ascertain the kinetic release of B as affected by added calcium (Ca) from two sources: ground limestone (GL) and calcium chloride (CaCl ₂ ). Topsoil samples were incubated with either GL or CaCl ₂ at 0, 0.5, 1.0, or 1.5 times of the lime requirement for 7 days; thereafter, the soil pH and the kinetics of B release were analyzed based on successive extractions with 0.01 M CaCl ₂ solution for 72 h, which was comparatively described using four kinetic models: Elovich, parabolic diffusion, power function, and first‐order of which each model would differently provide unique insights into the kinetic release of plant nutrient from soil. The added Ca significantly affected B release into the soil with the release kinetics of B in the two‐time segments only conforming to the power function model in which approximately 70% of B was released rapidly during the initial phase and the rest was in a slow‐release phase later on. Compared at the same rates of Ca applied, CaCl ₂ accelerated more B release than GL with no statistical difference in some soils. This was directly impacted by a decrease in soil pH as significantly caused by the addition of CaCl ₂ . In addition, the B release rate significantly correlated with soil organic matter ( r = 0.699 ^∗∗ , p < 0.01). Our findings suggested that increasing soil organic matter should be recommended to provide more B availability but liming or using GL as a Ca source should be cautious as it can lead to B deficiency for plants grown in these acidic soils.