Sweet Clover Responds to Potash Fertilizer

Soil improvement by means of legumes is not merely a matter of distributing legume seeds and hoping that this kind of crop will build up the land. Legumes can take nitrogen from the air to add it to themselves only when they find plenty of their other fertility needs supplied by the soil. Lime has become well recognized as one need that we must satisfy by applying it to the soil. Phosphorus is also accepted widely as a soil treatment to improve legume crops. Sanborn Field,^* with its carefully recorded experience, is indicating that we may well be putting potassium on the list with the lime and phosphate as an essential help to get stands of sweet clover for soil improvement.

On one of the plots where wheat had been grown for 25 years with manure applied annually, and then for the same number of years without manure, the cropping and soil treatments were changed in 1938 to a 2-year rotation of corn and wheat with sweet clover sandwiched in as a green manure crop for the corn. The soil treatment of lime was put on the entire plot.

Superphosphate at the rate of 150 pounds per acre and muriate of potash at 50 pounds were applied with the drilling of the wheat. When the corn was planted, an additional 50 pounds of muriate of potash were put on only the right half of the plot. The seeding of sweet clover that follows the wheat is now demonstrating the value of this extra potash by the successful stand of this legume in contrast to its failure where this additional potassium was not applied.

The significance of the extra potassium as a soil treatment for sweet clover after its applications in only three rounds of the rotation is evident from the growth of this crop in the stubble in late July. Though there were small sweet clover plants where lime, phosphate, and lesser amounts of potassium were applied, the contrast between them and those where potassium was more generously used is so marked that one would not be encouraged to expect much green manure effect by the sweet clover for the corn next spring in that part of the plot where the smaller amount of potassium was applied.

With wheat grown on this plot for 50 years, with all the straw as a relatively concentrated carrier of potassium taken off annually, and with no manure going back to return potassium in the straw as bedding or in the animal urine, this plot has developed a distinct shortage in its potassium delivery for sweet clover in a 2-year rotation. This shortage occurs for a crop not commonly considered sensitive to potassium deficiencies when it may be seen growing on a pile of crushed limestone. The shortage in this plot, however, is so severe that the spring-seeded sweet clover was starved out by the first of August except where extra potassium was supplied.

Here in this plot and its soil treatment there is evidence that our legume program, which we commonly grant needs help in the form of lime and phosphate as soil treatments, may well be looking forward to other helps such as potash fertilizer if we are to nourish these legume crops properly so they can fertilize our soils by means of the nitrogen they take from the air and by their organic matter when they are turned under as green manure.

To date no studies of the chemical composition of sweet clover in relation to soil treatment have been made, such as have been carried on with lespedeza to connect the soil treatments with the improved feeding value of the forage. When potassium now used in a more limited way is tried by more farmers, their observations on animal choices of sweet clover with different fertilizers may help to make sweet clover of better feeding value, in addition to giving it the unusual green manure value it already has.

 

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*This experimental field of the Missouri Agricultural Experiment Station is one of the oldest fields in the United States. It has been in service now for 55 years.