A method based on the rate of depletion of a nutrient from solution was developed to characterize nutrient flux of plant roots. Nutrient concentration of the solution was measured at a series of time intervals to describe the complete depletion curve. An integrated rate equation, based on a Michaelis-Menten model, was developed and fit to the data of the depletion curve using a least-square procedure. The A procedure was also developed to measure uptake rate at constant concentration by adding nutrients to the pot at a constant rate that matched net influx into the root. This method also provides a means of measuring diurnal fluctuations in net influx rates.Knowledge of the relation between the rate of ion absorption by plant roots and the concentration of the ion external to the root is important for doing plant nutrition studies, for investigating ion absorption mechanisms, and for evaluating mechanisms supplying nutrients to roots growing in soil (6, 10). Hence, a convenient procedure that mathematically describes the kinetics of ion absorption would be useful.Ion absorption rate as related to ion concentration in the external solution has usually been measured using short term absorption by excised roots of isotopically labeled ions from solution (4) and long term absorption by intact plants from solutions maintained approximately at constant concentration (1). In the first, only ions with a convenient isotope for labeling can be studied and in both, separate measurements of uptake rate must be made for each of a graded series of ion con- THEORYWhen roots are in a solution containing salts, ions may move both into and out of the root. The terms we use to describe this ion movement are: influx, movement of ions from the external solution into the root; I, the rate of influx per unit length of root; efflux, the movement of ions out of the root into the external solution; E, the rate of efflux per unit length of root, and In, the net rate of influx per unit root length which is equal to I -E. The values of I, E, and ln in this paper are in terms of root length; however, root surface area, or root weight could be used if desired.When plants are grown in a solution of a constant volume, v, the decrease in ion content of the solution measures the net amount of the respective ion absorbed. The amount of ion in the nutrient solution, Q, is given by equation 1 where c is the ion concentration in solution. Q = cv (1) A plot of Q versus time, t, gives a curve showing the depletion of the ion from solution resulting from plant absorption and is called the depletion curve. The net influx per pot at any point on this curve is given by -dQ/dt. The slope of the curve can be estimated by expressing Q as a function of t and calculating the derivative. A relation Q = f(t) can be obtained by fitting the data using a parabolic spline function (3) or a cubic spline function (13). Both functions consist of a number of segments called splines fixed by the experimenter which describe the relation by a series of parabolic ...

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The rate of corn (Zea mays L.) root growth in the field and root distribution in the soil as related to stage of plant growth has not been studied in detail. To obtain more information we measured the length, fresh weight, and distribution of corn roots at time intervals between planting and harvest in 1970 and 1971. The study was made on corn growing on Chalmers silt loam soil at Lafayette, Indiana. Grain yields were 6,160 kg/ ha in 1970 and 11,700 kg/ha in 1971. Root length and fresh weight increased rapidly for 80 days following planting, remained relatively constant for 14 days, and then decreased rapidly when the plants were in the reproductive stage. A maximum root density of 4.1 cm/cm3 occurred in the 0 to 15 cm zone at 79 days. The lower soil zones reached maximum root density 1 to 2 weeks later than in the 0 to 15 cm zone. Root density in the 0 to 15 cm zone was greater in cores taken midway between plants in the row than at other locations. Maximum root length was 153 cm/cm2 of surface area at 86 days.

Soybeans (Glycine max L.) fertilized with either NH4‐N or NO3‐N were grown in a growth chamber using soil with four different initial pH levels. Liming the soil used in this research to increase pH decreased the P level in solution. Fertilization of soybeans with NH4‐N decreased the pH of the rhizocylinder (root plus strongly adhering soil); fertilization with NO3‐ increased rhizocylinder pH. The difference between the rhizocylinder pH of the NH4+ and NO3‐ treatments was as large as 1.9 pH units with an initial soil pH of 5.2 and as small as 0.2 units when soil pH prior to N application was 7.8.Ammonium‐fertilized soybeans absorbed more P and had a higher P concentration than NO3‐fertilized soybeans. The results for soybeans grown with NH4+ and NO3‐ treatments at four initial soil pH levels showed that the P content of the shoots and roots was closely correlated with the pH of the rhizocylinder, but not the pH of the bulk soil. This suggests that the increased availability of P from the soil where NH4+ was used was mainly due to the effect of the nitrogen source on the pH of the rhizosphere soil.Soybean root length decreased from 180 to 120 m/gram of dry roots as the pH of the rhizocylinder increased from 4.7 to 7.5.

Temperature affects each of the mechanisms involved in P uptake by corn. The extent to which each reduces P uptake, however, has not been resolved. The objective of this study was to determine the effect of temperature on each of the parameters used in a mechanistic mathematical model that predicts P uptake and then to use the model in a sensitivity analysis to evaluate the temperature dependency of parameters critical for predicting P uptake by corn. The effect of soil temperature, 18 and 25°C, and soil P status on root growth and P uptake by corn (Zea mays L.) was investigated in a series of pot experiments in a controlled climate chamber. Total yield of corn was 4‐ to 6.4‐fold, root growth 2.6‐ to 5.1‐fold, and P uptake 2‐ to 4‐fold greater at 25°C than at 18°C. Increasing air temperature to 25°C while holding soil temperature at 18°C resulted in a 2.7‐fold increase in root growth and a 2.2‐fold increase in P uptake. Predicted P uptake calculated with a mechanistic mathematical model by corn grown for 8, 15, and 21 d agreed with observed P uptake at both 18°C (y = 0.97x + 3.64, r = 0.99**) and 25°C (y = 1.07 x − 1.86, r = 0.99**). In the sensitivity analysis the parameters used to calculate P uptake at 18°C were changed individually to those measured at 25°C and then P uptake was again predicted. Rate of root growth, which increased 4.97‐fold when temperature increased from 18 to 25°C, had the most pronounced effect on calculated P uptake, indicating that this was the primary reason low temperature reduced P uptake by corn. The parameters describing the movement of P to the root surface and P uptake at the root surface appear to play only a minor role in causing reduced P uptake by corn at low temperature.