Methods of Incorporating Organic Matter With the Soil in Relation to Nitrogen Accumulations

Summary

The careful analyses of soils sampled annually where two and one-half tons of clover, corresponding to 106.2 pounds of nitrogen per acre, were turned under each year and where this corresponding amount was applied on the surface and turned under the following year, show that nitrogen accumulated only slightly more rapidly in the latter than in the former. This difference was relatively small, amounting to about 3 per cent of the nitrogen applied. The close agreement between the results of the two treatments, especially the greater amount of residual nitrogen in the soil where cloverwas put on the surface, suggests that this surface application doesn’t volatilize any nitrogen, nor does it have any loss; at least not more than where the clover is incorporated into the soil.

The study suggests that as a general average where clover is used for soil improvement in this manner, about one-third of the annual application may be considered as contributed to the residual or insoluble nitrogen supply in the soil while two-thirds are mobilized or transformed into soluble forms. With these heavy annual applications of clover, the soil supply of nitrogen was raised above its initial content at a rate amounting to more than 20 per cent of the annually applied nitrogen. Its effects in offsetting declines of nitrogen in the soil, added to this, make its effects in building up the soil nitrogen approach the rate of one-third of the nitrogen applied annually.

These results were obtained on a shallow surface soil underlain by a plastic clay subsoil. Slight increases in the upper subsoil content of nitrogen resulted where significant increases occurred in the surface soil. The low initial content of nitrogen in the surface and subsurface soils may have been a factor in the effects obtained during the 15 years. Since many soils of the state are of similar profile and of no greater nitrogen content, these results suggest that in much of the state heavy clover additions can build up the nitrogen reserve in the soil. Though such repeated heavy applications are not common practice, these results indicate the effects which might be expected from the application rates here used, when the intervals between applications may be three or four years as is common in practices aiming to maintain the soil.

Methods of Incorporating Organic Matter with the Soil in Relation to Nitrogen Accumulations

The incorporation of green manure by plowing it into the soil is the commonly recommended method in soil management. How effective legumes may be in soil improvement, when they are left to accumulate on the surface, has not been fully determined. Different biological conditions obtain in these two cases. The absence of sunlight within the soil as compared to its sterilizing effect on the surface is a distinct biological contrast. Moisture which is relatively abundant in the former case would favor decomposition, while the alternate wetting and drying in the latter case, with only short periods of significant moisture, would seem to militate against decomposition. Any volatile materials produced during intermittent and interrupted surface activities would quickly escape, but would readily be absorbed by the soil when produced within it.

By turning under green manures during the usual seasons of plowing, such as spring or fall, little consideration is commonly given to the fitness of the chemical composition of the green manure as a nitrogenous fertilizer, particularly its carbon-nitrogen ratio as determined by its growth stage. Possibly legumes intended for soil improvement can be left on the soil surface, or slightly disked in, with significant contribution to the nitrogen supply in the soil. If this can be done independently of the costly operation of turning the surface soil with the plow, green manure additions to the soil may be made more cheaply. The following study was undertaken in order to learn something about the differences in effects on the nitrogen content of the soil when red clover was turned into the soil promptly as compared to its application on the surface for much later incorporation.

Plan of Experiment

Soil and Plot Treatment–The soil used in this study was a phase of Shelby silt loam. It was in a wheat crop when the experiment was begun and had been in general crops preceding. It was decidedly silty with a low content of clay or colloidal material in its surface. At the beginning of the study the untreated plot had a pH of 5.75 and at the close the figure was 5.50. The plot which was limed in 1929 with no further treatment had a pH of 6.30 at the end of the samplings.

Two series (North and South) of three plots each with dimensions of 6 feet by 6 feet 8 inches or .0009182 acre, were laid out with board curbings around them to prevent erosion and to give specific limitations from which measurements could be taken for accurate location of samplings. They were covered by framed screens of a mesh no larger than the mesh of fly screen, fitted accurately on the curbing to prohibit wind effects as removals or additions. One series of plots was laid out in 1917 and the other in 1918. Both were kept fallow and any weeds that started were pulled and left on the plots.

In late June each year, or about the season when the regular cutting of red clover is made for hay, the plots were spaded to a guarded depth of seven inches. Dried and chopped red clover was spaded into the west plot of the three in each series or incorporated at the rate of 2,082.4 grams, which was the equivalent of two and a half tons per acre. This same amount of red clover was scattered on the surface of the east plot after it had been spaded. Any remnants of organic matter left on the surface from the application of the previous year were turned into the soil. These remnants were of small amounts as early, futile attempts to recover and measure them indicated. The central or middle plot was given no organic matter addition with the spading. No further tillage or soil disturbance occurred beyond that occasioned when a few weeds were pulled. In 1929 the plots of the South series were given limestone of 30 mesh fineness equivalent to three tons per acre.

Sampling and Soil Sample Preparation–Before the plots were spaded each year, nine samplings per plot were made, including the surface seven inches by means of a two-inch tube passing through a metal plate guard, and the subsurface five inches by means of an auger passing through the tube. The location of the samples each year was determined according to a scheme in which there was a shift of four inches in one or the other direction paralleling the sides of the plots. This means was used to obviate sampling into subsurface holes of previous samples which had filled with surface soil. The nine samples of the surface soil were combined (likewise those of the subsurface), dried, ground in a ball mill and then a portion of about 200 grams of each put through a Braun pulverizer to reduce it so that it would pass through a 100-mesh sieve. This portion was used for chemical analysis, while part of the larger remaining sample was put into storage for reference.

A portion of the clover hay, which was usually obtained from other nearby experimental plots and dried in the screened shed before it was chopped into one inch lengths, was also taken as a sample for chemical analysis. Due to differences in moisture and maturity in the clovers, a rather wide variation in the annual application of nitrogen occurred. This ranged from 80 to 124.8 pounds per acre, with an average of 106.2 for the many applications.

Chemical Analyses of Soils–After the samples had been collected for three years, analyses were made for their total nitrogen. Since analyses of this type involve errors that may be large in relation to the variable that is to be measured, all later samples were held for analysis at the same time, using the same reagents, the same standard acids, the same chemist,* and uniformity in all conditions as far as possible. Ten gram samples of the air dry, 100 mesh soil were weighed, oven dried for ten hours at 107° C., weighed again, transferred to 800 cc. Kjeldahl flasks and digested to a gray white condition by 25 cc. Of concentrated sulfuric acid given copper sulphate and sodium sulphate. They were then diluted, made alkaline and distilled into standard acid. This was.0714N, equivalent to 1 mgm. of nitrogen per cc, and was titrated with alkali of similar normality from supplies sufficiently large to serve for the entire lot of samples. Blank distillations of the reagents were made regularly and corrections applied accordingly. Duplicate titrations varied no more than .1 cc. in the majority of cases, in an attempt to hold the variations to .2 cc. Duplicate determinations would thus differ by 20 pounds per two million with the former figure and 40 pounds for the larger variation. Repeat determinations were made in a few cases when variations in duplicates were considered beyond toleration. All calculations were made on the basis of two million pounds of water-free soil.

Results

The data, as annual analyses, are assembled in complete form by plots, by soil strata, by treatments, and by years in Table 1. These are presented graphically as a scatter diagram in Figure 1. As a means of lessening the fluctuations, they have been calculated as advancing three year averages with the first data under the year 1920, which was the third year including 1918. These averages are assembled in Table 2. The data of 1917 are omitted for the North series in order that the dates may be more comparable. These advancing average data are given graphically in Figures 2 and 3 for the surface soils, and in Figures 4 and 5 for the subsurface soils of the North and South series, respectively. As a measure of the changes in nitrogen content by the treated plots, the arithmetic mean of the three year advancing average figures of the two untreated plots (surface soils only) for the various years was subtracted from the corresponding figure for those treated. These data are assembled in Table 3. The average nitrogen content of the surface soils of the two series of plots are given graphically in Figure 6. The increases in surface soil nitrogen where the clover was incorporated and where it was applied on the surface, as compared with the untreated soil, are given in graphs in Figure 7. Since there seemed to be no significant effect from the lime treatment on the surface soil of the South series as shown by the curves after the year 1929, this comparison was disregarded in handling the data.

 

 

As a convenient general survey of the nitrogen changes in the plots, the data have been assembled using the first three-year average figure for that at the outset, and the last three-year average figure for that at the close of the study. The difference between the two may be taken as the gain or loss in residual nitrogen. Included also are the averages of all the analyses made of the samples taken annually during the period. These data are assembled in Table 4.

Nitrogen Supply in the Untreated Surface Soils–As for the untreated surface soils, the fallowing treatment during the period gradually depleted their supply of total nitrogen. There were some periods of variation but during the time as a whole it is evident that the nitrogen supply in the soil was becoming less as shown in Figures 1, 2 and 3. If one takes the final three-year average figure as contrasted with a similar period for the outset, as given in Table 4, the loss during the 15 years under study was 115 pounds per acre.

The situation as to nitrogen loss would have been little different had the soil been cropped and all the crops removed, since the nitrogen taken by crops is that in the soluble form, and would be leached were it not taken up by the plant roots. Crop removal leaves roots that represent a part of the soluble or leachable nitrogen returned in the soil in the organic and less soluble form. Other than for this item, the conditions under fallow and cropping would be similar with respect to nitrogen removal.

Nitrogen in the Surface Soils Receiving Clover–The most pronounced feature of the tables and figures is the increase in total nitrogen of the surface soil whether clover was incorporated or left on the surface and turned under the next year. The rising curves with time for the treated surface soils are most noticeable for the separate plots in Figures 2 and 3, and also for these as averages in Figure 6. This effect of the clover is marked, first in raising the nitrogen content of the soil above that of the soil at the outset by 346 pounds as an average of the gains as given in Table 4, and second, in bringing it much above the untreated soil in which the nitrogen content was lowered with time. The effect of the clover addition on the nitrogen of the soil must be measured by this difference, which, therefore, makes the increases in nitrogen the more significant. The changes in the total soil nitrogen by the organic matter additions are summarized graphically in Figure 8.

Incorporation Versus Application of Clover on Soil Surface–Where the clover was incorporated regularly when applied, the nitrogen content of the treated soil was raised less over the untreated plot than where this legume was applied on the surface. This indicates a more complete decay of this material and a less amount of its nitrogen remaining in the soil in the former than in the latter condition. The decay processes liberating nitrogen and changing it into a soluble form to be lost by leaching are more active in the former conditions, so that the accumulated nitrogen residue is smaller where the green manure is promptly mixed with the soil. This difference as based on the figures from the three-year advancing averages is indicated by the distance between the two upper graphs in Figure 6 and shown as a separate graph in Figure 7. Calculated from the data in Table 3, this average annual difference is 69.5 pounds, or calculated from the averages of annual analyses, the figure is 60 pounds.

Residual and Mobilized Nitrogen–If the increases by each year over the preceding year as given in Table 3 and Figure 7 are averaged on a single year basis, then 36.5 pounds of nitrogen are added to the nitrogen in the soil each successive year where the organic matter is immediately incorporated, and 40.1 pounds where it is applied on the surface to be incorporated the next year. This represents the residue left in the soil from an application of 106.2 pounds of nitrogen. Hence the difference between 106.2 and 36.5 or 69.7 pounds is the amount that became soluble annually in the former while 66.1 pounds were mobilized in the latter. Thus, 3,6 pounds more, or slightly more than 3 per cent, of the applied nitrogen was made soluble and available for plant use where clover was plowed under immediately than where it was left on the surface and incorporated into the soil a year later.

If one uses the final figure of the three-year advancing averages for the four treated plots, or the average of the two treatments in Table 4 which was 2,582 pounds, and that for the untreated plots which was 1,970 at the close of the treatment, then the clover additions increased the total nitrogen in the soil by 612 pounds during the 15 years, or 40.8 pounds per acre per year. Since 106.2 pounds were applied per acre annually, about 38 per cent of the annual application contributed to the increase of the residual nitrogen in the soil as based on the 15 year period. Consequently, the balance of the application, 62 per cent, or 65.4 pounds per acre, was mineralized by conversion into soluble nitrogen through the decay processes of the soil. We may say then that slightly less than-two-thirds of the green manure equivalent was transformed annually into available or leachable nitrogen, while slightly more than one-third was converted into the more stable soil organic matter as an addition to the reserve or stored nitrogen in the soil.

Clover Needed to Maintain the Soil Nitrogen in a Fallow Soil–While the clover treatments of 21, tons per acre annually raised the residual nitrogen in the soil above the untreated soil by 612 pounds during the 15 years, the untreated soil was declining for a total of 115 pounds. Thus, almost one-fifth (18.8 per cent) of this addition would have been needed to maintain the soil at the level present at the outset. In terms of the clover application, this would have required a half ton per acre annually. In the light of this it would seem that a two-ton application once in a four-year rotation would be required to maintain the nitrogen reserve in a fallow soil at the level found in the soil at the beginning of the study.

Nitrogen Levels in the Subsurface Soils–An average of the fluctuations in the nitrogen contents of the subsurface soils as given in Table 2 and graphically in Figures 4 and 5, reveals that under the plots with no treatment there is a slight decline in the nitrogen content, while the differences between the nitrogen contents at the outset and at the close in Table 4 suggests insignificant change. For the treated plots, there are slight subsurface gains which are larger where the organic matter is incorporated into the surface soil than where left on the surface for one year.

Since the subsurface soil is more impervious than the surface, and since the still deeper layers are even more impervious, this suggestion of loss of nitrogen in the untreated and gain in the treated soils indicates its movement downward on this more level and limited area even into and through this layer of heavier texture. These changes, however, are very small and can be taken only as indications. They are of much less dimension than those in the surface soils and seem suggestive mainly because they are in agreement with the direction of the changes in the surface layers.