Growth and Nutrient Uptake of Rice at Different Growth Stages and Nitrogen Levels 1

Dry matter synthesis and uptake of N, P, and K by field sown rice ( Oryza sativa L.) growing in Crowley silt loam soil was measured at weekly intervals throughout the growing season. K as KCl and N as anhydrous ammonia were applied preplant at rates of 56 of K and O, 123 and 157 kg/ha of N. ‘Vegold,’ ‘Nato,’ and ‘Blue‐bonnet 50’ varieties (cultivars), classed as very‐short‐season, short‐season, and midseason in maturity, were used. Approximately half of the total uptake of N and K and two‐thirds of the P uptake occurred after dates previously established as optimum for midseason topdress N applications, e.g., 50, 67, and 79 days after seedling emergence for Vegold, Nato, and Bluebonnet 50, respectively. About two‐thirds of the dry matter production also occurred after these dates. Consequently, the requirements for nutrient elements by the rice plant are great during the reproductive stages of growth. These data partially explain why topdress N applications at midseason (jointing stage) are more efficient than topdress applications near seeding for increasing grain yields. Uptake of N, P, and K at dates of maximum nutrient accumulation (near soft dough stage) reveal a greater uptake by Nato than Vegold or Bluebonnet 50. As an average of all varieties at N levels producing maximum grain yields, 232, 48, and 171 kg/ha, respectively, of N, P, and K were taken up. At N levels producing maximare yields, about 25 kg of additional grain were produced for each kg of N applied. Losses of N and K from the plant occurred during a growth stage between the tillering and jointing stages for each variety. Additionally, N and K content at maturity was much lower than 1 to 4 weeks prior to maturity (soft dough stage). These facts should be considered when selecting sampling dates for measuring nutrient uptake or when uptake data are used as bases for fertilizer rate recommendations.