Saturation Degree of Soil and Nutrient Delivery to the Crop

With the better understanding of the physico-chemical aspects of the clay fraction of the soil, the mysteries of the migration by the nutrient ions from the soil into the plant are rapidly submitting to solution. In fact, we now know that the reverse movement is possible, so that in fertility-depleted soils the nutrients may be going back to the soil to reduce the cation stock in the plant originally contributed by the seed. The colloidal phenomenon of exchange of cations is helping us to understand soil fertility more clearly. With more research attention to the anions, even these may let their behavior come within the pale of understanding. If the colloidal system in the plant root is opposed to the colloidal clay system with the not wholly invulnerable root membrane interposed, we may look to equilibrium of forces within and without as a helpful explanation. Should we reason on this simple physico-chemical basis, then the questions naturally arise whether soil fertility applications should be an attempt to provide but that needed for most economic service to the plant, or that for modification of the soil. It becomes a question whether applied nutrients should be used to saturate highly limited soil areas in the immediate root zone or to give low degree of saturation of the soil throughout the plowed layer of the common 2 million pounds.

This viewpoint prompted an experimental study of the degree of saturation of the soil by calcium as cation and by phosphate as anion as a factor in plant growth and in the movement of these ions from the soil into the crop as nutrient harvest.

Plan of Experiment

An extensive series of 2-gallon pots of surface soil of the Putnam silt loam was arranged to include treatments of one-fourth of the soil with calcium in amounts equivalent to that needed to saturate it completely; and but one-half that quantity. These same amounts of the calcium were also distributed through the entire soil. Additions of phosphate representing 100 pounds and 200 pounds of 38% phosphate per 2 million pounds of soil were applied in similar manner.

Thus, in the case of calcium, there were jars in which the upper fourth of the soils was completely saturated; some in which the deficit in calcium in this layer was remedied by but one-half; some in which only a light application was given to the entire soil body; and another in which it was given a heavier application. These latter two treatments amounted to roughly 600 and 1,200 pounds per 2 million pounds for a soil that had an initial pH of 5.6 and was originally only about half saturated with calcium. This soil was also low in soluble phosphorus and responds readily to such application by better crop yield. It gives, however, the best responses to lime alone, and to phosphates used in conjunction with lime. For purpose of convenience these treatments will be spoken of in the case of the limestone treatments as (a) partial saturation and (b) complete saturation; and in the case of phosphate treatments as (c) light dosage and heavy dosage in either case of treatment through the larger or the smaller soil volumes.

In mineral nature, this surface soil of the Putnam silt loam contains few, if any, “other than quartz” minerals which carry calcium. The subsoil is an impervious clay containing a high percentage of the beidellite clay colloid with an exchange capacity of more than 65 M.E. per 100 grams of clay. The surface soil used carried approximately 15% clay of which the exchange capacity combined with that of the organic matter gave it a total capacity of about 18 M.E. per 100 grams of surface soil.

Four crops were used in quintuplicate for each treatment. These included two grasses, bluegrass and redtop, and two legumes, sweet clover and Korean lespedeza. These selections were made in accordance with the generally accepted fertility demands by these crops, ranking those for bluegrass and sweet clover above those for redtop and Korean lespedeza.

The growths were harvested as forages at regular intervals to give five harvests of quintuplicate pots as carefully weighed amounts on constant moisture basis. Analyses were made for their contents of calcium and phosphorus to determine the fertility harvest for these nutrients applied in contrast to such from the untreated soil.

 

Experimental Results

Forage Harvests

The single outstanding result throughout the experiment is the much larger yields of forage and fertility harvest that resulted when the treatments were applied to only the smaller portion of the soil to give it the higher degree of saturation by the ion of the treatment. The increases in forage yields as percentage over the untreated soil are assembled in Table 1. Comparison of the second column of figures with the first under each separate crop shows the much larger percentage increases where the treatment was put into the smaller soil volume.

Table 1–Increases (percentage) in forage harvests from limestone and phosphate distributed through large and small soil volumes.

Soil treatment		Grasses				Legumes
		Redtop		Bluegrass		Lespedeza		Sweet clover
Kind	Magnitude	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.
Limestone	Partial saturation	15.5	15.5	-3.8	9.7	4.8	13.7	25.1	50.1
	Complete saturation	1.0	36.0	0.0	33.0	2.5	28.0	23.2	89.1
Phosphate	Light dose	8.8	4.3	9.0	34.4	20.0	23.0	22.8	51.0
	Heavy dose	14.3	12.3	15.7	18.0	21.4	22.4	27.2	70.8

 

In the case of limestone this increase held for the non-legumes as well as for the legumes. In fact the figures for the former were generally larger than those for Korean lespedeza, though not as large as those for sweet clover. The yields show clearly that, as measured by forage increases, the higher degree of saturation by calcium in a limited soil area was more effective than a moderate or less degree of saturation in a larger soil area. This raises the question, and answers it forcefully, whether the economical use of lime is not one of feeding the plant calcium more than one of neutralizing the entire soil area of the root zone.

Phosphate, like the calcium carbonate, also showed more influence on the crop yield when the treatment was concentrated into a part of the soil, though this illustration was not as pronounced, in general, as that of the effects by the limestone. In the case of sweet clover, the effects by phosphates were almost equivalent to those by limestone in terms of percentage yield increase. For both the single soil treatments of calcium and phosphate additions in general, the crop yield increases were larger as the treatment was used to give a higher degree of soil saturation.

 

Fertility Harvest of Calcium

Analyses of the crops for calcium when lime was applied show that the crop content of this plant nutrient as totals per acre responded with larger differences in the increases than was the case for the forage yields. The higher degree of soil saturation by the application of the calcium into a limited soil area gave increases as much as 2 ½ times as large as where this same amount was distributed through more soil. This is demonstrated clearly in Table 2. Again, the non-legumes demonstrated increases in calcium harvested from the soil which were even greater than the increases taken by the legumes. Redtop was superior in this respect to Korean lespedeza, and bluegrass to sweet clover. It suggests that because these crops manage to produce vegetation on soils low in lime, we have perhaps not been giving sufficient attention to the capacity of the grasses to take lime for their possible improved feeding value.

 

Table 2–Increases (percentages) in calcium and phosphorus harvests from limestone and phosphate distributed through large and small soil volumes

Soil treatment		Grasses				Legumes
		Redtop		Bluegrass		Lespedeza		Sweet clover
Kind	Magnitude	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.
Calcium Harvest
Limestone	Partial saturation	19.5	45.0	16.2	58.6	12.6	27.7	34.2	59.5
	Complete saturation	37.7	87.5	48.7	129.0	21.0	43.9	46.0	94.0
Phosphorus Harvest
Phosphate	Light dose	31.0	6.0	27.3	70.0	41.3	41.3	28.0	94.0
	Heavy dose	16.0	23.4	43.6	34.4	39.1	46.7	29.6	130.0

 

Fertility Harvest of Phosphorus

The total phosphorus harvested in the crops where the soils were given phosphates shows greater increases when the treatment was concentrated into the lesser amounts of soil for all but two of the eight cases as given in Table 2. Redtop and Korean lespedeza with the lower phosphate applications failed to give greater increases where the phosphate was applied in the surface soil only. In the other cases the differences were very significant and larger than any others in the case of sweet clover. In terms of forage, of calcium harvest, and of phosphorus harvest through the crop, this last crop showed the outstanding response to both calcium and lime applications into the limited soil area.

The crop responses rank these crops in the order as they are commonly arranged in fertility requirements. The bluegrass and the sweet clover showed greater response to the soil treatments than was true for the other two. They also removed larger amounts of calcium and phosphorus from the soil.

 

Fertility Harvest of Nitrogen

Since both non-legumes and legumes were included, the significance of concentrating the calcium and phosphates into less soil as these influence nitrogen fixation by legumes and nitrogen removal from the soil can be measured. The data assembled into Table 3 are in agreement, in principle, with those previously given. The increases in nitrogen harvested were much greater again where these soil treatments were used so as to provide them at higher degrees of soil saturation. Even for non-legumes the higher concentration within the soil of the same application of limestone was much more effective in delivery of nitrogen from the supply in the soil to the crop. Small dosages or lower degree of saturation gave negative increases or amounts in the crop below that in crops on unlimed soil. This suggests that the introduction of limestone encouraged microbiological competition sufficient to utilize the effect by the calcium to the detriment of the crop competing for the supply of nutrients even other than calcium. Similar situations were provoked by the addition of the phosphatic fertilizers for the non-legumes.

In the case of the legumes, the treatments all increased crop yields. The Korean lespedeza, however, was not correspondingly responsive to the higher applications of phosphate into the smaller soil volume. The sweet clover gives distinct evidence of the influence by both the calcium and the phosphate on the nitrogen increase by this crop, but especially of the effects when these treatments are concentrated into small soil volumes to give them higher degrees of nutrient ion saturation. Thus in this crop the higher nitrogen harvest, probably much through nitrogen fixation, agrees with the higher fertility delivery as calcium and phosphorus by the soil to the plant.

 

Table 3–Increases (percentages) in nitrogen harvests from limestone and phosphate distributed through large and small soil volumes.

Soil treatment		Grasses				Legumes
		Redtop		Bluegrass		Lespedeza		Sweet clover
Kind	Magnitude	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.	Large soil  vol.	Small soil  vol.
Limestone	Partial saturation	-26.4	15.0	-18.5	14.3	20.0	30.6	39.8	67.3
	Complete saturation	-13.0	23.1	-4.5	40.8	13.0	52.4	35.4	113.7
Phosphate	Light dose	-12.8	-6.2	-15.5	25.0	47.3	54.0	25.9	65.1
	Heavy dose	-10.0	-10.0	-5.8	0	50.5	41.8	34.1	80.8

 

Discussion and Summary

The data all emphasize the fact that more nutrients were delivered by the crops because of the higher degree of the soil saturation even of only a limited part of the soil. This area of soil was seemingly large enough to prohibit injury through excessive salt concentrations. These increased movements of the nutrients into the crops were roughly paralleled by increases in forage yields, though not directly so. Thus, there has resulted in most cases increased concentration of nutrients within the crops to give them higher forage feed value. Thus, the efficiency of the treated soils in terms of tonnage yield per unit of nutrient delivered is lower than the efficiency of the untreated soils, but it may be far more efficient in producing an animal feed of higher calcium, phosphorus, and protein concentrations. The increased use of nitrogen by the crop points to the significance of calcium and phosphorus in making this phase of plant metabolism operate effectively in case of the non-legumes as well as for legumes.

Since calcium and phosphorus are the two most significant soil needs in the corn belt, as shown by past agronomic experience, by soil development, and by crops in their ecological array, we may well look forward to their wider use. For more effectiveness in practice, however, limestone and phosphate should be applied in more limited soil areas rather than distributed through the soil zone. Possibly not only the concentration within limited soil zones should deserve consideration, but also some efforts toward retardation of their rate of adsorption for reaction with the soil. Effectiveness of granular forms of such soil treatments may be premised on the greater efficiency of the nutrients when in areas of higher degrees of saturation. Efforts to improve applications for such effectiveness should give results in terms of crop increases.

Since the very acid clay is active even to the point of removing calcium from the mineral lattice¹ and since a calcium clay is not so active in the removal of bases from plant roots,² perhaps the higher degree of calcium saturation in limited soil areas lessens the activity by the soil in adsorbing the anion phosphorus. If this is the case, then the applied phosphorus remains longer in the soil without reacting with it and may explain, in part, the greater efficiency of phosphates when used on limed soils³ or those liberally stocked with calcium.

These results suggest most forcefully that in liming and fertilizing the soil, attention must go to the degree of saturation of the soil. The use of such soil treatments will be more effective when applied in limited soil areas to feed the plant than when applied through greater areas to modify the soil condition.

 

References Cited:

1. Graham, Ellis R.: “Primary minerals of the silt fractions as contributors to the exchangeable base level of acid soils.” Soil Science (In press).
2. Jenny, Hans, and Overstreet, R.: “Cation interchange between plant roots and soil colloids.” Soil Sci., 47:257-272, 1939.
3. Albrecht, Wm. A. and Klemme, A. W.: “Limestone mobilizes phosphates into Korean lespedeza.” Jour. Amer. Soc. Agron., 31:284-286, 1939.