Ann Draycott

Quantitative relationships for the dependence of growth rate of arable crops on their nitrogen content, dry weight and aerial environment

Quantitative relationships for growth rate and its dependence on plant nitrogen concentration are developed from the results of experiments on potatoes, cereals and vegetables. The relationships appear to be of general applicability and most coefficients in them are similar for widely different crops.

Response of potatoes to N fertilizer: Dynamic model

SummaryA simulation model is described to interpret N fertilizer experiments on potatoes. It calculates the total growth of dry matter, the N uptake, the partition of dry matter and of N between tuber and foliage and the distribution of inorganic N down the profile for each day during the growing season.The validity of the model was tested against measurements of these parameters made at approximately fortnightly intervals on plots that received N fertilizer and those that received none in 4 experiments on a sand, 4 on a sandy loam and 3 on a clay soil.Simulated values were in reasonably good agreement with the measured values in all experiments. Overall the sums of squares of the differences between the simulated and measured values of the %N in the total plant, and the total amount of inorganic N in the top metre of soil and the logarithms of the total weight of dry matter, tuber dry weight, N uptake in the entire plant and N uptake in the tubers were each less than 25% of the sum of squares of the measured values about the mean.Only 9 inputs were required for the model. It was found essential to take account of differences in spring and summer leaching but not of inter-site differences in mineralization rate.

Response of potatoes to N fertilizer: Quantitative relations for components of growth

SummaryQuantitative relationships for key processes influencing N response were derived from measurements of inorganic N in soil, the weights and N contents of foliage and tubers made at intervals during growth of maincrop potatoes in 11 N fertilizer experiments.Apparent mineralization rates (calculated from measurements of N uptake and inorganic N in the top metre and averaged over the growth period) were remarkably similar from site to site despite wide differences in the textures, water contents and organic matter contents of the soils. They were mostly about 0.78 kg N ha−1 m−1 d−1.Inorganic N in the top 50 cm of soil was rapidly removed by the crop until it fell on all sites to a low value (about 4 μg N cm−3) which was maintained for the remainder of the growth period. When N fertilizer was applied, growth rate until at least the end of July was always well defined by a single coefficient in a previously derived equation. Average values of this coefficient for each of the soil types and for each of the years in which the experiments were carried out were within 20% of each other.The minimum %N in the dry matter needed to permit maximum growth rate declined with increase in plant weight in a similar manner to that previously found for other crops.Equations were found for the partition of assimilate and of nitrogen between the foliage and tubers. The coefficients in them were little affected by whether or not N fertilizer was applied.According to these relationships the maximum potential dry weight yield of tubers is 20 t ha−1 and requires the crop to contain at least 290 kg N ha−1.

Apparent recovery of fertilizer n by vegetable crops

Apparent recoveries of fertilizer N by crops were studied as their treatment in simulation models is a serious problem. Multi-level N fertilizer experiments in which fertilizer was broadcast and incorporated in soil immediately before drilling were carried out on a range of vegetable crops on adjacent sites of the same field. At final harvest, apparent recovery always declined approximately linearly with the increase in fertilizer N even when less was applied than was needed for maximum uptake; this contrasts with the well-known constancy of apparent recovery of Gramineae (winter wheat and grass) over widely different rates. At the seedling stage (of vegetables), when N-uptakes were very small, N-uptake and invariably dry weight, unexpectedly increased with the increase in fertilizer N until very high levels were reached. A single simulation model, with species differences accounted for by variations in the value of only one coefficient, defined, at least qualitatively, all the various phenomena. It is ar...

Experimental validation of an N-response model for widely different crops

A simulation model, developed from a previously published one generally gave a reasonably good description of the effects of N-fertilizer on plant dry weight and N-content of 12 different vegetable crops grown in 26 experiments on separate sites within the same field.Apart from standard weather and soil data, the model required as inputs, the maximum yield of dry matter, the fractional recovery of N by the crop (with minimum fertilizer), the plant mass and N content (at the time of planting or drilling), the dates of planting or drilling and of harvest. Simulations always started from the previous autumn. A constant value of soil mineral-N at that time and the same temperature dependent mineralization was assumed for all experiments.The validity of the model was tested by a range of statistical procedures. In 19 out of the 26 experiments there were no detectable trends in the deviations of the simulated from the measured dry matter yields with increase in fertilizer-N. The model over-estimated the %N in the dry matter of some crops at the highest level of fertilizer-N and was unsatisfactory for one of the crops. With these exceptions, the sum of squares of the differences between measured and simulated %N was 22% of the sum of squares of the measured values above the mean. When N-fertilizer was withheld, the average N-uptake over all experiments was 69 kg N ha−1, whereas that simulated was 59 kg N ha−1; the average difference between simulated and measured uptake for each experiment was 20 kg N ha−1. Simplification of the model by incorporating the same relationship between critical %N and plant weight for all crops did not lead to appreciable loss of accuracy. A user-friendly version of the model has been compiled so that it will run on IBM-compatible microcomputers with outputs that can be coupled with high level graphics packages.

Quantitative relationships for growth and N content of different vegetable crops grown with and without ample fertilizer-N on the same soil

Unexpected relationships were found to cover various features of N-response of 12 different crops grown in 26 experiments on adjacent sites in the same field. When N-fertilizer was withheld the % N in the dry matter of the different crops declined in a regular way with increasing plant weight and was always about 0.6 × the critical % N; crop weight was approximately proportional to the weight with ample fertilizer and the constant of proportionality was also about 0.6. For drilled crops, the maximum plant dry weight obtained with any level of fertilizer-N was almost linearly related to the duration of growth. These results are in approximate agreement with those predicted with a previously described simulation model. On the basis of this work, a simple field method is suggested for estimating the rate of mineralization of soil organic matter in some soils.The apparent recovery of fertilizer-N by the different vegetable crops always declined linearly with increase in fertilizer-N over the entire range of applications which is in marked contrast to the constancy of apparent recovery by crops like cereals and grass that have more extensive root systems. In some instances, the apparent recovery of the vegetable crops also declined linearly with increase in the % N in the plant dry matter. The apparent recovery of an infinitely small amount of fertilizer-N by the different crops, when plotted against plant weight fell approximately about the same ‘diminishing returns’ type curve. The implications of these findings to the further improvement of the model are discussed.

Response of winter wheat to N-fertiliser: Quantitative relations for components of growth

Measurements were made of yield of dry matter, plant-N content, and the distribution of mineral-N down the soil profile in 10 fertiliser-N experiments. In one of them detailed measurements were made throughout growth. Rate of N-uptake by the crop was unaffected by the amount of mineral-N in the upper 90 cm of soil when it was above about 30 kg N ha−1. The %N in plants that received ample N-fertiliser declined with increase in plant mass according to a previously derived equation. During senescence there was an apparent loss of N from the crop.N-nutrition in the different experiments had little effect on the partition of assimilate between grain and straw. At harvest grain and straw weights were well related by a linear model which had the same gradient but different intercepts for each experiment. Grain %N was about four times greater than straw %N. Regression analysis supported the view that high evaporative conditions or temperatures during the growing period induced earlier harvest dates, less grain relative to straw, and a higher %N in the plant when ample N-fertiliser was applied but not when N-fertiliser was withheld.Other analyses indicated that cereal roots were generally unable to extract mineral nitrogen from the soil when the concentration was less than about 0.18 kg N ha−1 cm−1, that at low levels of N-nutrition the recovery of available inorganic-N from soil by the grain and straw was about 80%, and that the average mineralisation rates from early spring to shortly after harvest date varied between 0.22 and 0.88 kg N ha−1 d−1 from site to site.

Weather, nitrogen-supply and growth rate of field vegetables

A new procedure was developed for interpreting data from multi-harvest N-fertilizer experiments on 5 different vegetable crops. Per cent N in the plant dry matter of each N-deficient crop was, throughout growth, almost proportional to relative growth rate (standardized to the average weather). After correcting for the effects of plant mass, growth rate of each N-sufficient crop varied considerably during the growth period and approximately in proportion to the growth rate when N-fertilizer was withheld. Some of the variation was associated with small changes in soil water which also greatly influenced C-partition between foliage and storage roots of at least one crop. Some of the variation was related to temperature and radiation. The significance of these findings to modeling the effects of N-nutrition and environment on growth is discussed.

Measurement and simulation of the effects of N-fertilizer on growth, plant composition and distribution of soil mineral-N in nationwide onion experiments

To aid the development of simulation models for N-response, N-fertilizer experiments with onions (Allium cepa L.) were carried out on 5 different sites. In each experiment, there was little loss of fertilizer-N in soil during the period between application and rapid crop growth and little loss of mineral N by leaching at any time. Even so, a substantial proportion of the N applied as fertilizer could not be accounted for in the crop and soil at harvest; the sum of soil mineral-N plus crop N (excluding fibrous roots) was always linearly related to N rate applied over the entire range (0–300 kg N ha−1) and the gradient was always approximately the same, 0.64, irrespective of soil type or the amount of nitrate remaining in soil at harvest. Evidence was obtained that the phenomena resulted from roots retaining N and inducing immobilization at a rate proportional to soil nitrate concentration and that the proportionality constant was similar on all sites.Throughout plant growth there was little luxury consumption of N and the critical %N was related to plant mass by an equation previously deduced for other C3 crops (Plant and Soil 85, 163); plant nitrate concentration in the early stages increased with soil mineral-N (0–30 cm) to a maximum which varied from site to site but the nitrate concentration in the mature crop was always negligible. Plant yield in the early stages of growth generally declined with increase in fertilizer-N, despite the crops having been planted as sets and no more than 150 kg N ha−1 broadcast at one time; but at maturity, yield always increased asymptotically with increase in fertilizer-N. Mineralization rates were approximately the same in the first as in the second half of each experiment. At harvest, residual soil mineral-N in the upper 30, 60 and 90 cm of soil increased with increase in fertilizer-N even when crop demand for N exceeded supply. At harvest in every experiment, the ratio of crop dry weight in the absence of added N to the maximum obtained was approximately equal to the ratio of plant %N (with no fertilizer) to critical %N.The various phenomena concerning yields, plant-N contents, and values of soil mineral-N at harvest were quite well simulated by a slightly modified version of a previously published model (Fert. Res. 18, 153) with few site-dependent inputs.

Response of winter wheat to N-fertiliser: Dynamic model

A concise computer simulation model is described for calculating the growth and N-content of winter-wheat. The validity of the model was tested by means of a new application of statistical theory against the results of nationwide fertiliser experiments having different designs. There was agreement within the limits of experimental error between the measured and simulated values of both total plant and of grain dry weight over the entire range of N-fertiliser treatments in the different experiments. The model also gave good estimates of the %N in the grain provided N-fertilizer levels were not high. Response curves, calculated from the model for grain yield, grain %N, are given for different combinations of potential yield, mineralisation rate and the distribution of inorganic-N down the soil profile.