WHY ISN’T THERE a heavy white clover growth every year? Why does a heavy crop of the showy legume seem to appear in seven-year cycles? These are some of the old questions brought to life following this spring’s heavy white clover growth.
And these are logical questions and rot just a fancy in the minds of older observers who have noted that volunteer white clover comes into bluegrass pasture and lawns in periodic years.
Those who have noted the white clover cycle have different theories regarding its regular appearance. Some say it’s connected with glaciated, windblown, and other fertile soils where bluegrass grows well. Others believe white clover years are normal ones following drouth years.
It’s known that soils under sod gather increasing amounts of calcium through weathering processes over a period of years. This would supply a basis for better nutrition and, thereby to the appearance and growth, of legumes.
So, cycles of calcium increase and depletion may bring about white clover years. A soil under sod may spend a number of years building up its calcium reserve to the point where white clover can take advantage of the reserve and make a sudden growth. But, at the same time, this growth depletes the calcium reserve to the point where it will not be able to feed a second year’s clover crop.
This may be especially true where soils have a reserve of weatherable calcium minerals in the surface or subsoil. These could be north Missouri River hill soils continually getting new deposits of unweathered soil from dry western areas or glaciated soils.
Also, it’s known that clay, a result of weathering, is a partially reversible particle. That is, clay will absorb and hold fertility elements while moist but will lose its grip through severe drying and may not re-absorb the elements readily when wetted again. This would leave the fertility elements available to legumes following dry years.
In regard to the idea that white clover years follow one of drouth, looking at a study of soil samples collected at Bethany from 1931 through 1938, the final year of the study, 1938, was considered a white clover year.
Many Missourians remember 1936 for its disastrous heat and drouth. There had also been an almost equally serious drouth in 1934 and, in 1937, rainfall at Bethany amounted only to 22 inches as compared to the 50-year average of 35 inches at that location.
So, under near normal moisture conditions in the spring of 1938, white clover made one of its periodic heavy appearances. Conditions were similar to those of 1938 in parts of Missouri this spring. Previous years had been dry and white clover made another of its periodic appearances.
Going back to the seven-year soil study, samples were taken annually from a number of plots. One of the plots was in continuous bluegrass soil. It received no soil treatment, had no crops removed, and showed little erosion.
The sod plot was studied carefully to measure time changes accurately. This plot served as the basis for measuring changes in other plots undergoing fertility treatments, surface soil losses, and other processes. The sod plot was the “standard” in the study.
Soil changes during the study supply Albrecht’s theory for white clover cycles in continuous bluegrass. The study also suggests similar cycles of greater bluegrass growth. However, bluegrass cycles aren’t as noticeable as white clover cycles since the grass has no such flash of color as does white clover.
During the study period, calcium content of the soil’s exchange capacity increased from 57 to 71 percent. Since other soil tests have shown that a soil’s calcium level should be brought up to 75 percent for better legume crops, it seems highly probably that an increase in calcium content might be enough to invite sudden growth of white clover.
When an increasing magnesium content was considered along with the calcium, the exchange value varied from a low of 75.5 and a high of 91 percent during these years. The high figure represents a soil well-stocked with the two essentials for protein-producing legume crops.
Since no calcium or magnesium was applied, the arrival of white clover in 1938, suggests that either reserve soil minerals, or those recently blown in, were weathered enough to bring the active fertility up enough to meet the clover’s needs.
Of particular interest was the soil’s increase in organic matter and nitrogen as a result of growing bluegrass. Organic matter jumped from 3.5 percent of the dry soil to 4.3 percent during the seven years while nitrogen increased from .17 to .21 percent.
This nitrogen increase might be expected to give bluegrass sod a push. But, while the organic matter of the soil was increasing, it wasn’t becoming any less woody. This prevented the soil from increasing its rate of decay or of freeing its nitrogen, phosphorus, potassium, calcium, or any other nutrient.
Thus, during the absence of white clover, the bluegrass sod was building up more permanent organic matter that would slow its growth. It was increasing inorganic soil essentials that encourage white clover since l egumes need little nitrogen from the soil as they can build their own protein-rich organic matter from nitrogen in the air.
This protein-rich, highly decayable organic matter dropped back to the soil at the end of the white clover year. This provided extra and active nitrogen and hastened the decay of woody organic matter to set free and activate fertility elements. In turn, this invited lush bluegrass growth that competed with the clover crop.
So, the richer soils staying under sod are in a continual process. The sod builds up the soil’s organic matter and accumulates minerals through the weathering process. Clover comes in at periodic intervals to take advantage of the weathered minerals, gives the organic matter the nitrogen needed to speed up decay which, in turn, boosts bluegrass growth.
The years needed to build up the calcium level to the point where it can be used by white clover suggests itself as the time interval between white clover years. This interval may well include drouth years for whatever changes in the soil or cycle they bring about.
Such are the theories about white clover years on the more fertile north Missouri soils. They are presented to be confirmed or disclaimed by further studies of soil fertility in relation to white clover growth.