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Potassium Helps Sweet Clover

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From Chapter 17: Potassium Helps Put More Nitrogen into Sweet Clover

Some tests with sweet clover as the green manure crop in a rotation grown on Putnam silt loam on the South Farm of the Missouri Experiment Station in 1947 demonstrated that potassium as well as calcium is needed if this crop is to be a producer of considerable tonnage of vegetative bulk. Potassium also demonstrated its service in raising the concentration of nitrogen in the crop, and, presumably thereby, the fixation of nitrogen from the atmosphere.

Plan of the Study

The sweet clover studied is the legume crop in a four-year rotation of corn, oats, wheat, and sweet clover. The soil treatments on the respective three plots include the basic addition of calcium in limestone at the rate of 2 tons per acre every 8 years. In addition, there is a superphosphate application of 475 pounds per rotation and a potash application of 475 pounds as a 0–20–20 fertilizer per rotation on this plot.

The crop harvests for forage yields were taken, and the plants divided into tops and roots with the customary “stubble” included with the tops by dividing at the soil surface line of the plants. The data for the crop weights according to soil treatments are given in Table 1. The weights of the tops and roots may well be compared by the ratios given in the table when calculated with the roots taken as unity. In order to measure the yields and concentration of nitrogen, the plant parts were finely ground in a special hammer mill and the nitrogen determination made on the oven-dry weights of the samples. The data are presented for the nitrogen in the plant tops and in the roots as total harvests in pounds per acre and also in terms of the concentration of the nitrogen in the vegetation as pounds per ton. Then there are given the ratios of nitrogen per acre in the tops to that in the roots, and the ratio of the nitrogen per ton of tops to that per ton of the roots.


Root-Rot Less with Potassium Applied

Observations made on the sweet clover on approaching maturity showed a crop of heavier stems, more dense in growth, and taller as the additional soil treatments were applied (Fig. 1).

Fig. 1. — Relative yields of forage and differences in roots and stems of sweet clover varying in root-rot according to soil treatments.
Fig. 1. — Relative yields of forage and differences in roots and stems of sweet clover varying in root-rot according to soil treatments.

It was especially interesting to note that the nearly mature plants could easily be pulled up from the soil where lime only was used. They were pulled up with more difficulty on the plot with lime and phosphate, but it was impossible to pull them up by their roots where lime, phosphate, and potash had been used as soil treatments.

Examination of the roots led a plant pathologist to declare that the root-rot disease was prevalent on the plants grown where the potash had been omitted, but he considered very little of this disease evident on the plants grown with the added potash. Here is the suggestion that when the plant physiology was considered and provision was made for better nutrition through the addition of potassium to balance the generous application of calcium, there was little damage from the disease.

Roots More Efficient in Making Plant Tops Because of Application of Potash

From the data in Table 1, it is evident that there are wide differences in the efficiency with which a given amount of roots made their corresponding tops under the different soil treatments. In the production of total bulk, where only lime was used, each unit of roots produced 21/times its weight as tops, but when both lime and phosphate were added to the soil, each unit of roots made tops about 10% more efficiently. This is shown by the ratio of 2.59 in contrast to that of 2.25. When both phosphate and potash were used along with the lime, however, each unit of roots was 50% more efficient in making tops than when lime alone was used and 30% more efficient than when lime and phosphate were used. Potash was of greater effect when added to this combination than was the phosphate under the conditions of its addition.

Dry Matter and Nitrogen in the Tops and Roots of Sweet Clover with Different Soil Treatments
Dry Matter and Nitrogen in the Tops and Roots of Sweet Clover with Different Soil Treatments

Roots More Efficient in Putting Nitrogen into Plant Tops Because of Application of Potash

There was also a wide variation inw the efficiency with which a given amount of nitrogen in the roots was translocated to the tops under different soil treatments. In terms of total nitrogen per acre of roots and tops where the soil was limed, the nitrogen in the tops represented 2.3 times as much as that in the roots. Here the efficiency of the roots in putting nitrogen into the tops was about the same as the efficiency of the roots in making plant bulk, as shown by the similarity of the ratios. This is shown also by the fact that there was, coincidently, as much nitrogen in a ton of roots as in a ton of tops. When phosphate was used along with the lime on the soil, then the roots were about 50% more efficient in moving nitrogen into the tops, as shown by the ratio of 3.33 for the nitrogen in the tops to that in the roots. In terms of pounds of nitrogen per ton where lime and phosphate were used together, the figure for the roots was 30 and for the tops 38. However, when both phosphate and potash were used along with the lime, then the roots were 100% more efficient in putting nitrogen into the tops than with lime alone, as shown by the ratio of 4.74 for the former in contrast to the ratio of 2.30 for the latter. In terms of nitrogen per ton of roots grown with lime, phos­phate, and potash, the figure was 29, while for a ton of tops it was 41. Potash as a soil treatment in addition to lime and phosphate made for an increase in efficiency of concentrating nitrogen into the harvested tops which was twice that for the addition of phosphate to the lime as the soil treatment.


It is significant that potash used along with calcium and phosphate had the most outstanding effect of the three soil fertility factors concerned in these trials, not only in making for more plant bulk, but also in making for more total nitrogen in the crop per acre of tops and per acre of roots. Potash was the major factor also in making for a larger concentration of nitrogen in the tops while there was a lower concentration in the roots. All of this suggests that the potassium commonly associated with carbohydrate synthesis and metabolism in the plant can scarcely be divorced from the synthesis of proteins there. The carbohydrates, or what is so commonly emphasized as the product of photosynthesis formed under sunshine energy, may well be the raw material in terms of both the starting compound and the energy supply for the products of biosynthesis such as proteins, and the many other complexes elaborated and compounded by life processes rather than by those driven under sunshine power in the leaf. Here is the suggestion that calcium is helpful in the synthesis of nitrogenous compounds or proteins in the legumes and that this synthetic process demands, in advance, the carbohydrates for the synthesis of which potash is needed. On this soil, the maximum assemblage of nitrogen in sweetclover required potash as help in this performance.

While sweet clover is commonly considered the crop that can be established on most any soil by liming alone, after other and more desirable forage legumes have failed, one dare not forget that nitrogen delivery by this crop as well as its greater production of bulk call for other fertility elements beside calcium and phosphate. There may be many elements among these, but certainly there is the suggestion from these studies on Putnam silt loam, a prairie soil and a planosol in the common soil classification, that potassium is important when the higher nitrogen content of the sweet clover crop is considered. After our soils are once heavily limed this legume as a collector of nitrogen may need extra fertility elements in the soil, with potassium well near the top of the list.

Learn more about Albrecht on Soil Balancing, Vol. VII here.

About the Author:

Dr. William A. Albrecht, the author of these papers, was chairman of the Department of Soils at the University of Missouri College of Agriculture, where he had been a member of the staff for 43 years. He held four degrees from the University of Illinois. During a vivid and crowded career, he traveled widely and studied soils in the United States, Great Britain, on the European continent, and in Aus­tralia. Born on a farm in central Illinois in an area of highly fertile soil typical of the cornbelt and educated in his native state, Dr. Albrecht grew up with an intense interest in the soil and all things agricultural. Both as a writer and speaker, Dr. Albrecht served tirelessly as an inter­preter of scientific truth to inquiring minds and persistently stressed the basic importance of understanding and working with nature by applying the natural method to all farming, crop production, livestock raising and soil improvement. He always had a specific focus on the effect of soil character­istics upon the mineral composition of plants and the effect of the mineral composition of plants on animal nutrition and subsequent human health. Respected and recognized by scientists and agricultural leaders from around the world, Dr. Albrecht retired in 1959 and passed from the scene in May 1974 as his 86th birthday approached.

More By This Author:

Albrecht Papers Vol. 1-8 + Video, by Dr. William A. Albrecht

Albrecht’s Foundational Concepts, Vol. Iby Dr. William A. Albrecht

Check out the Albrecht collection for a full list of all his titles.

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