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Root Rot of Sweet Clover Reduced by Soil Fertility
Published in Better Crops With Plant Food, February 1948.
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Root rot was prevalent in Missouri on numerous fields of second-year sweet clover during the cool, wet springs of 1946 and 1947. This “disease” was also observed on some of the sweet clover plots under fertility control in the four-year rotation of corn, oats, wheat, and sweet clover on the South Farm of the Missouri Experiment Station. Other adjacent plots on the Farm made normal growth and were apparently not infected.
Various investigators have reported that different plants growing on potassium-deficient soils are distinctly susceptible to root rot. Ginsburg1 reported that soybeans suffering a potash deficiency develop very few lateral roots and those are only near the root base. These roots decay easily. According to Eckstein, Bruno, and Turrentine2 potassium deficiency has a greater influence on the root development of biennial and perennial plants than on annuals.
Close observations were made in 1947 of the sweet clover on the plots at the Missouri Agricultural Experiment Station in order to ascertain, whether there was any correlation between the levels of the soil fertility and the susceptibility of the sweet clover to this condition often spoken of as the disease of root rot. An attempt was also made to determine the effects of the “disease” on the growth of the plant and on the production of nitrogen within the crop.
The plots on the South Farm of the University of Missouri where the sweet clover was studied have been in the above rotation since 1938. The soil is a Putnam silt loam, typical of the clay-pan soils of Northeast Missouri where the “disease” was reported most prevalent.
Previous Soil Treatments Represented
A highly pure calcium carbonate limestone of ten-mesh, mill-run fineness had been applied to all plots at the rate of two tons per acre. On other plots additional treatments of phosphate alone or this in combination with potash had been made. The plots and the soil treatments used in these more careful observations and study were as follows:
Plot l–Limestone–2 tons each eight years.
Plot ll–Limestone–2 tons each eight years plus 425 pounds of 20 per cent superphosphate in each rotation.
Plot lll–Limestone–2 tons each eight years plus 425 pounds of 0-20-20 in each rotation.
The phosphate and the phosphate with the potash were applied as follows: 150 pounds in the row with the corn; 150 pounds drilled with the wheat, and 125 pounds drilled with oats. All crop residues were left on the land.
It was easily observed that the sweet clover on the plots receiving only limestone was badly infested with root rot. Only the roots near the surface of the soil were alive. The others were dead and had partially or even fully decomposed. The plants growing on the plots which received both limestone and superphosphate were more deeply rooted. They had much more top growth than those growing on the plots receiving limestone alone. Those growing on the plot receiving lime and both phosphate and potash had more root growth and much more top growth. The differences in weights of plant parts, in nitrogen content, and in the root rot are shown in Table I. The plants growing where only limestone was used could be pulled up by hand. The plants growing on the other plots were deeply rooted and could only be removed with the help of a tile spade.
Yields and Nitrogen Content
Fractional samples of both roots and tops were harvested when in full bloom, air-dried, weighed, and the nitrogen content determined. These data are included with Table I.
Acknowledgement is given to Taylor Howard for collecting samples and to Wm. Mierke for making nitrogen determinations.
Table I.–Hay Yields, Total Nitrogen, and Root Rot of Sweet Clover as Affected by Soil Treatments
Treatments | Pounds Per Acre | Root Rot | |||
Roots | Tops | Total Plant | Nitrogen | ||
Limestone
Lime & Phosphate Increase over Limestone Lime, Phosphate & Potash Increase over Limestone |
2,072
3,428 1,356 3,644 1,572 |
2,864
5,594 2,730 7,570 4,706 |
4,936
9,022 4,086 11,214 6,278 |
80.0
162.0 82.0 240.4 160.4 |
Severe
Moderate — Slight — |
Discussion
The yield of air-dry roots harvested from the plots receiving limestone and phosphate exceeded that where limestone alone was used by 1356 pounds or 65.5 per cent. Where both phosphate and potash were used along with the limestone, the harvest of roots exceeded that from where limestone alone was used by 1,572 pounds or per 75.8 per cent. Where limestone and phosphate were used, the yield of air-dry tops exceeded that where limestone alone was used by 2,730 pounds or 95.3 per cent. Where limestone, phosphate, and potash were used, the yield to air-dry tops exceeded that over limestone alone by 4,706 pounds or 16.3 per cent. In total growth of roots and tops where limestone and phosphate were both used, the yield of air-dry material was 4,086 more pounds or 83.3 percent higher than where limestone alone was used. The yield of roots and tops where limestone, phosphate, and potash were used was 6,278 more pounds or 127.3 per cent higher than where limestone alone was used.
The total nitrogen in the tops and roots combined increased over 100 per cent as a result of the addition of phosphate and about 200 per cent as a consequence of the addition of both phosphate and potash. The root rot infection varied from severe where limestone was used to moderate where limestone and phosphate were used, to slight where limestone, phosphate, and potash were used. The differences in the condition of the roots and the size are shown in Figure I illustrating the growth trom equal areas.
Fig. 1. Root rot infection varied from severe where limestone was used (left),to moderate where limestone and phosphate were used (centers, to slight where limestone, phosphate, and potash were used (right).
Conclusions
The increased growth of roots and tops, and the larger amount of total nitrogen produced on the plots that received lime plus phosphate, and lime plus phosphate and potash, indicate that the additions to the soil of these inorganic plant nutrients reduced the susceptibility of the sweet clover to the root rot “disease.”
These facts raise the question whether the root rot is a primary matter or whether it is one secondary to the deficiency of plant nutrients. There is the strong suggestion from these observations that the trouble of such a weakened root system is merely a consequence of the insufficient fertility and a condition to be remedied, not by an attempt to exterminate the microorganism attacking the roots, but by feeding the plant more potash in a more fertile soil by which the plant roots are made non-susceptible to the root rot. Are we not about ready to say that the increasing incidence of what we call “disease” is the result of the decreasing fertility supply in our soils?
References Cited:
- Ginsburg, Joseph M., Soil Science, 1925, 20 pp. 1-13, “Composition and Appearance of Soybean Plants Grown in Cultural Solution Each Lacking a Different Essential Element.”
- Eckstein, Oskar, Bruno, Albert, Turrentine, J. W.: “Potash Deficiency Symptoms,” 1917, p. 36.