Little is known about the role of K fertilization, stover management, and tillage methods on soil K availability as they affect rice (Oryza sativa L.) and cowpea [Vigina unguiculata (L.) Walp.] productivity on Ultisols of the humid tropics. The effects of five K rates (0‐120 kg K ha−1), returning or removing stover, and three tillage methods (no‐till, strip, and conventional) were evaluated during 12 crops of rice and cowpea grown for a 4‐yr period. Fertilizer K was applied to the first seven crops. The site was a recently cleared, 18‐yr‐old secondary forest in the Peruvian Amazon Basin. The soil was a fine‐loamy, siliceous, isohyperthermic Typic Paleudult. Soils samples were collected at each crop harvest to 90 em in 15‐cm increments. Potassium fertilizer always increased grain yields when stover was removed. Conversely, responses to K additions were seldom obtained when the stover was returned. The extractable K (Modified Olsen) critical level for both upland rice and cowpeas was calculated to be 0.10 cmol L−1. Returning stover with no K fertilization maintained soil K concentrations above critical levels for both species up to the last crop of the rotation. Residual effects of fertilizer K were prolonged by returning the stover. When stover was returned, subsoil exchangeable K increased with increasing rate of K fertilization. Removal of stover resulted in greater increases in subsoil exchangeable K at the 40 kg K ha−1 rate than at 120 kg K ha−1, apparently because the higher rate resulted in K fixation. Tillage methods did not affect crop yields.
The interpretation of soil tests for K in highly weathered soils may be improved if the K buffering power is considered in addition to extractable K. This study was conducted to determine if cation exchange capacity (CEC) and humic matter (HM) should be included in the development of more meaningful K availability indices. Seventeen soils from the Coastal Plain, Piedmont, and Mountain regions of North Carolina and two soils from the Amazon Basin of Peru were studied. The buffering power tended to correlate positively with CEC, but that of soils high in HM was lower and less affected by CEC than that of the low-HM soils, indicating a lower capacity to replenish K removed from the soil solution upon depletion of K. The K concentration in the equilibrium solution increased linearly with percent K saturation of the CEC, and high HM soils had a higher concentration of K in the equilibrium solution at any given percent K saturation than low HM ones. A larger portion of extractable K was found in solution as HM increased and CEC decreased. It was noted that high HM soils were behaving similarly to low CEC and low HM ones so the soils were grouped into two categories: (i) soils of low K buffering power, which are those with HM 3* 1.9% and a CEC «s 4.6, and (ii) soils of higher buffering power, which are those with HM < 1.9% and CEC 3* 4.6. The use of this grouping should improve K fertilizer recommendations in highly weathered soils.
Sub‐optimum soil K levels often limit corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production in humid tropical Ultisols. The objectives of this study were to determine soil and plant critical levels of K and optimum rates of K fertilization for corn and soybean grown in the Amazon Basin of Peru. Two field experiments were conducted on Typic Paleudults, a loam and a sandy loam. Five K rates, ranging from 0 to 120 kg ha−1, were broadcast and incorporated prior to planting the first crop of a corn, corn, soybean rotation at both sites. At the loam site, K treatments were reapplied at the end of this rotation and three more corn crops grown. The corn stover was returned while the soybean stover was removed. Critical exchangeable K levels for corn were 110 kg ha−1 on the loam and 90 kg ha−1 on the sandy loam, while for soybean it was 75 kg ha−1 for both soils. The critical levels of K in plant tissue at flowering were 13 g kg−1 in corn and 12 g kg−1 in soybean. Soybean seemed to lower exchangeable K to where K was released from non‐exchangeable sources. An annual cropping system of corn and soybean with an intervening cover crop during the rainy season is proposed for the region. Potassium fertilization is recommended only prior to growing corn at 90 kg ha−1 on a loam and 60 kg ha−1 on a sandy loam.
Understanding the dynamics of soil K is important for the formulation of a sound fertilizer management strategy. Two experiments were conducted to determine the behavior of K in the soil after fertilization. Five rates of K (0 to 120 kg ha−1) were applied to two Typic Paleudults, a loam and a sandy loam, in the Peruvian Amazon Basin. A three‐crop rotation of corn (Zeu mays L.), corn, and soybean [Glycine max (L.) Merr.] was planted at each site. Three additional crops of corn were grown at the loam site. Potassium rates were applied only to the first crop of each three‐crop cycle. The corn stover was returned, while the soybean stover was removed. Soil samples were taken at germination, flowering, and harvest of each crop at 20‐cm depth intervals to 60 cm in the loam and 80 em in the sandy loam. Soil K was extracted with NaHCO3, NH4OAc, and concentrated H2SO4. Topsoil exchangeable K increased markedly with K addition, either as fertilizer or as stover, but rapidly decreased during cropping. Changes in the exchangeable pool were characterized with an exponential model. Increases in exchangeable K below 20 cm were of minor significance for the loam. For the sandy loam, however, subsoil exchangeable K was significantly increased to 60 cm. Evidences of K fixation and release were detected in the loam where the topsoil non‐exchangeable pool peaked after K additions and decreased during cropping. There was no evidence of fixation in the sandy loam. These differences suggest that higher K rates can be applied to the finer textured soils and there is less risk of loss of K from the rooting zone.
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