We report three N rate experiments conducted on a gravelly loam soil to assess the N status of potato (Solanum tuberosum L.) using a Minolta SPAD-502 chlorophyll meter. Highly significant linear and quadratic trends were obtained for the regression of N rate on marketable tuber yields and SPAD readings. SPAD readings were taken at four times during the growing season and decreased as plants aged. Based on regression analysis, the early season SPAD readings, associated with N rates giving maximum marketable tuber yields, ranged from 49 to 56 units depending on year, variety, and location. Potato variety significantly affected SPAD values in eight of the 12 situations where readings were obtained. Precision in interpretation was improved when the highest N rates were considered “reference strips” to standardize the SPAD readings across varieties and growing seasons. Our results suggest that field SPAD readings can readily identify severe N deficiency in potatoes, have the potential to identify situations where supplementary sidedressed N would not be necessary, but would be of limited value for identifying situations of marginal N deficiency unless reference strips are used.
Nitrate uptake patterns by nitrogen‐depleted wheat seedlings (Triticum vulgare var. Atlas 66) exhibited an initial lag phase which was lessened, but not completely overcome, by presence of solution Ca. Maintenance of a more rapid subsequent rate of nitrate uptake, which developed after the initial lag phase, depended upon presence of solution K. The rapid phase was enhanced when both Ca and K were present and was curtailed by shoot excision and by Ca deficiency. Sizeably larger amounts of nitrate were recovered in the shoots after 24 hours in KNO3 as compared to Ca(NO3)2, and with the former salt, smaller proportions of the absorbed nitrate were reduced. The data suggest a beneficial influence of Ca both on initial nitrate uptake by root tissue and on transport to shoots while a continual K supply was apparently beneficial in nitrate transport.
Tomato (Lycopersicon esculentum Mill.) plants grown on polyethylene (PE) mulch in New York State frequently have more branches and increased mineral nutrient uptake and yield than plants not mulched. In four field experiments conducted on a silt loam soil, clear PE mulch stimulated root extension shortly after transplanting. One week after transplanting, roots were significantly longer for mulched than for unmulched plants in all four experiments, whereas aboveground dry matter differences did not become significant until 14 days after transplanting in two of four trials. Mulching increased branching, hastened flowering on basal branches, and increased concentration of major nutrients in the aboveground parts. In the field, stimulation of aboveground growth due to mulch might be brought about by warming of the stem by air escaping from the planting hole in the mulch. However, an experiment with black, white, or clear mulch, in which the planting hole was either left uncovered or covered with soil, showed no effect of hole closure on branching even though air temperature near the stem was increased when holes were left uncovered. The results taken together imply that the increased aboveground growth observed with mulching is a consequence of enhanced root growth and nutrient uptake.
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