Liming Alone will not Cure a Sick Soil
LEGUMES, AN INDISPENSABLE forage crop, are unfortunately not grown universally. We have ascribed their frequent failure to the acidity of the soil, a diagnosis which is open to question.
Acidity, a common soil condition in the temperate zone, occurs when rainfall washes away fertility or when much vegetation has leached the soil of its supply of nutrients. Thereafter, the soil grows mainly carbonaceous or woody vegetation. Soil acidity is in reality, then, a shortage of plant nutrients. How can we restore the soil to its original fertility?
Liming as an agricultural art was known even to the Romans, and to Benjamin Franklin, who used gypsum or land plaster on clover. The growing agricultural science of the early years of this century brought back liming as a general practice, particularly under the encouragement of the new soil-testing service.
This service was guided by the belief that limestone, hydrated lime, or quicklime fought soil acidity by counteracting the high concentration of hydrogen ions in the leached soil. The ease and speed with which acidity could be detected and measured encouraged widespread testing of soils—we discovered soil acidity everywhere! We likewise discovered that in acid soils productivity was, in general, lower as the degree of acidity was higher. From this we concluded that the presence of large amounts of hydrogen was the cause of poor crops. So we began fighting the acidity by neutralizing the hydrogen ions with carbonate of calcium.
We are just now coming around to a better understanding of how Nature grew crops on acid soils before we did. At the same time, we are beginning to understand what limestone really does when it makes better crops. For the study of the physiology of plants, and of the colloidal behavior of the clays which grow them, indicates not only that soil acidity is not detrimental, but is in reality beneficial to growth!
Heavy liming drives out not only acidity. It eventually removes all other fertility and may load the soil so heavily with calcium that only that element is offered as plant nourishment. Plants will then, though growing on a neutral soil, starve for additional nutrients. Neutralizing soils by calcium saturation does not, therefore, make them productive, for this is the condition of the semiarid soils of some of our western states.
Calcium has been a good fertilizer for legumes on acid soils for some years. It has served directly as a nutrient, and it has served indirectly by helping other nutrients, as well as nitrogen, to get into the plant roots more abundantly. But once the need of leached soils for calcium has been met, the need for other nutrients becomes evident—potassium, for instance, and nitrogen. Thus fertilizers are coming into prominence for use on soils which a few years ago were treated only with limestone. Perhaps these are the facts behind the age-old rhyme that “lime and lime without manure, makes father rich but son poor.”
Soil acidity, therefore, is not the sole problem in growing legumes. The production of these crops is a matter of ample soil fertility among which calcium from lime is only one nutrient. Our soils need a well-balanced supply of all plant food elements to remain productive.