Spatial patterns of soil properties and nutrient concentrations need to be characterized to develop site‐specific farming practices that match agricultural inputs with regional crop needs. The spatial variation of soil organic C (SOC), soil water content (SWC), NO3‐N, PO4‐P, and K were evaluated in the 0‐ to 15‐cm layer of a 3.3‐ha field (Typic Haplaquoll and Argiaquic Argialboll) cropped with maize (Zea mays L.) and soybean [Glycine max (L.) Merr.]. The range of spatial correlation was determined from semivariance analyses of the data and was found to vary among and within fertility parameters. Nitrate had the shortest correlation range (<5 m) and SOC had the longest (>180 m), whereas SWC, PO4‐P, and K had intermediate spatial correlation ranges. In addition, SOC was found to have small‐scale spatial variation nested within large‐scale spatial variation. The spatial pattern of NO3‐N changed with time. Frequency distributions of SOC and SWC were close to normal, whereas the distributions of NO3‐N, K, and PO4‐P data were skewed. Median polishing detrending and trimming of outlying data were useful methods to remove the effects of nonstationarity and non‐normality from the semivariance analysis. The results suggest that reducing sampling intervals from 50 to 1 m would reduce the variance of SWC, SOC, NO3‐N, PO4‐P, and K estimates by 74, 95, 25, 64, and 58%, respectively. A useful sampling pattern for characterizing the spatial variation of several soil properties‐nutrients and scales should be random with sample spacing as close as 1 m and as far apart as the longest dimension of the field.
The objective of this work was to describe the relationship between elongation rate and diameter of maize roots and to estimate the length and growth duration of lateral roots of maize. Diameters and elongation rates of roots were measured in situ on plants grown 5 weeks in small rhizotrons under greenhouse conditions. At the end of the experimental period the roots were harvested and diameters of axile and lateral roots were measured. The frequency distribution of diameters of harvested roots was bimodal with a minimum at 0.6 mm; 97% ofaxile roots were larger than this value and 98% of the lateral roots were smaller. Root elongation per day increased as diameter increased but the slope of the relationship with lateral roots was about 2.5 times that with axile roots when separate linear regressions were fitted to the two populations. The length of lateral roots found on axillary roots between the base and about 30 cm from the apex was approximately 2.2 cm. All of the data was consistent with the hypothesis that the lateral roots grew for about 2.5 days and then ceased growing. The axillary roots continued to grow throughout the experimental period at a rate of about 3 cm day -~.
Sorption of NO 3 by different horizons of a highly weathered, acid tropical soil was measured in laboratory batch experiments. Sorption was found to increase with depth, ranging from small amounts in the 0-15 cm layer to amounts that would be roughly equivalent to 25 to 50% of the NO 3 in the 90-120 cm layer at water and NO 3 contents commonly found under field conditions. Calculations, based on sorption isotherms, demonstrated how sorption may be important for managing N in a tropical acid soil. Sorption of CI-was also found in the range of 0.1 and 2.0molm -3. In this range of concentrations sorption of NO 3 and chloride were found to be independent, suggesting that anion exchange sites were far from saturated.
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