Those Lime-House Blues
David G. Leach, Brookville, Pa.
The publication in the Bulletin of several articles on the liming of rhododendrons has created a lively and constructive interest in the nutrition of these plants. Readers have learned how lime has benefited ailing specimens and at least one nurseryman has subsequently described to me how he made successful field use of lime to improve the growth and vigor of persistent leafed azaleas.
These reports are challenging and they pique our curiosity because they are contrary to the popularly accepted idea that lime is "poisonous" to rhododendrons. But readers of the Bulletin will probably be well advised to keep in mind that no overnight revolution has occurred either in soil science or in the nutrition of rhododendrons. The accounts of the successful use of lime are news, like the man biting the dog, because they are exceptional. The time is not yet at hand for all growers to throw away their ammonium sulphate and haul in the ground limestone. It appears that the liming of rhododendrons is about to become a fad so it may be useful to review the effects, good and bad, of such treatment on a scientific level. Readers can then apply the principles to their own use.
The beneficial effect of limestone on rhododendrons is probably due to one of three factors or a combination of them. Soils which are excessively acid have a portion of their fertility unavailable for the growth of plants. Below pH 5.5 the bacteria and molds which accelerate the decay of organic materials become inactivated to a progressively greater degree as the acidity increases. When decay slows in the soil, the reduction of organic residue to nitrogen compounds and other plant-stimulating essentials similarly declines. It is the virtual absence of decay bacteria in very acid soil which preserved the bodies of long-extinct mastodons found in the marshes of Siberia a few years ago.
When ground limestone is added to excessively acid soil the activity of the beneficial soil organisms quickens, to the advantage of rhododendrons growing in it. But most amateur field tests for acidity are inaccurate in a range of -0.5 pH, so it would be prudent to think twice before reducing the acidity of any soil which tests pH 4.5 or above by home methods, inasmuch as the optimum for the greatest variety of rhododendrons appears to be about pH 5.0. A test will usually be made for $1.00 by county agricultural agents which includes an accurate pH determination plus valuable additional information on the organic content and any fertilizer that may be needed.
A second beneficial effect of limestone in regions of heavy soil comes from its granulating effect upon clay. The requirement of rhododendron roots for good aeration is well known, and the improvement in texture from liming heavy soil may be beneficial if the site is somewhat deficient in this respect.
A third benefit from the use of ground limestone may come from the increased availability of any of the elements, major or minor, which may have been restricting growth. Phosphorus, calcium, magnesium and molybdenum may be tied up in insoluble compounds in acid soil. In a strongly acid soil potassium and several of the minor elements may not be assimilated freely. So limestone may benefit rhododendrons in some exceptional cases by altering the chemical form of the nutrients to soluble compounds or otherwise making it possible for plants to absorb them.
Limestone is calcium carbonate, to a chemist, and recent research by Dr. W. J. Haney at Michigan State University suggests that it is the carbonate rather than the calcium which is injurious to rhododendrons. In his experiments, calcium carbonate and ammonium carbonate were both harmful, but calcium sulphate and ammonium sulphate at equal concentrations were not harmful at all. And here is exposed a further pitfall in the liming of rhododendrons.
Under acid conditions at or below about pH 5.0. calcium carbonate diffuses into the air and drains out of the soil with rain water as a result of chemical change. But liming the soil in which many rhododendrons are commonly grown can decrease the acidity to the point, between pH 5.0 and pH 10.0, where the carbonates pick up a hydrogen ion and become bicarbonates. These are a specific enzyme poison which is toxic to rhododendrons, and the familiar yellowing of the leaves and loss of vigor occur long before the soil actually becomes alkaline. Only three pounds of ground limestone per 100 square feet will lower the acidity of average soil from pH 5.0 into the trouble zone pH 5.5.
Evidence presented by Dr. Melvin S. Colgrove, Jr., formerly of Oregon State College, suggests that it is a high level of the total bases (calcium, magnesium, potassium) which induces chlorosis, the jaundice of the plant world.
In his experiments, rhododendrons in sand culture showed iron deficiency leaf-yellowing when the calcium, at 50 parts per million: the potassium, at 59 p.p.m. and magnesium, at 12 p.p.m., reached a total of 121 parts per million of all three bases combined. But the revealing thing is that he subsequently increased the calcium to 75 parts per million, with the potassium and magnesium held so low that the total of all three was 96 parts per million, and no chlorosis appeared! Obviously, it was not the calcium content, but the total of all three bases combined which inactivated the iron in the sap and caused the leaf yellowing. So there can be a good deal of calcium in the soil if there is little magnesium and potassium, and there will be no trouble with the rhododendrons.
How does this help to explain, then, the use of limestone beneficially in some cases and the quick demise of rhododendrons when it is used elsewhere?
If the soil is low in magnesium and potassium at the same time that it is more acid than pH 5.0 (so that bicarbonates do not accumulate), the calcium in limestone will not induce chlorosis, and it may provide one or more of the benefits described earlier. But if the soil is high in potassium or in magnesium, or if it has just a moderate content of both of them, then the addition of the calcium from an application of limestone may raise the combined total of the three bases to an injurious level.
But the risk of the three bases reaching a damaging concentration is especially great with rhododendrons. Dr. John R. Havis at the University of Massachusetts has recently pointed out how potassium leaches out of the popular organic mulches and increases in the soil beneath. Azaleas and rhododendrons at the Waltham Field Station of the University of Massachusetts suffered severe injury when grown with a mulch which was especially rich in potassium. Barring direct root damage from excess, this might be called "potassium-induced chlorosis." If the leaf yellowing had appeared following the use of lime, it might be called "lime induced chlorosis," and the same could be said of magnesium. But all are probably aspects of the same problem: the addition of the one element raised the combined total of the three bases above the limit of tolerance.
And potassium can accumulate from waste by-products mixed into the soil to provide organic matter. Douglas Fir sawdust is seemingly widely used with only rare injurious side effects under West Coast conditions but in the East ground corncobs and sawdust, particularly, produce such high potassium levels as soil amendments that there is frequent injury on this account alone, without increasing the total bases still further through the addition of calcium from limestone. There is an almost universal impression that rhododendrons can not be harmed from the incorporation of organic waste material into the soil. This is entirely erroneous. Dr. W. J. Haney at Michigan State University stresses that the injurious effect is often merely delayed until the organic material is nearing complete decomposition and then the roots can suffer from the accumulation of salts, particularly potassium. Organic materials such as peat moss, already decomposed, present no problem.
Suppose that limestone is added to a soil only moderately acid, to the benefit of the rhododendrons. A subsequent application of one of the complete acid soil fertilizers containing potassium can then boost the total bases into the trouble zone. Or the decay and leaching of a mulch could cause the same effect. Such possibilities suggest caution unless there is some reason predetermined by a soil test to apply lime.
At pH 5.0 and below there is no reserve insoluble calcium carbonate. The total supply must come from exchange capacity and the decomposition of organic matter. The exchange capacity is large in the organic soils in which rhododendrons are commonly grown, but a large proportion of it is occupied by hydrogen ions, so that the supply of calcium and of other bases is decreased proportionately. But calcium is essential as a nutrient and roots can not survive without it. The foliage becomes chlorotic. The tips of new leaves become brown and are often twisted. Finally, terminal buds are killed and the older leaves die and curl downward. And it is a fact that most of the spectrographic analyses of the leaves of ericaceous plants in good growth show that the amount of calcium exceeds that of the major element, potassium. But if an actual deficiency of calcium exists, as may occur in peaty soils, it is much safer to remedy it with gypsum (calcium sulphate), thus avoiding the risk of bicarbonate poisoning.
In the light of the soil factors which affect their welfare, then, it is not so remarkable that limestone may occasionally produce beneficial results, considering the varying conditions under which rhododendrons are grown. A low level of potassium and magnesium or an exceptionally high acidity provide a situation in which limestone may benefit rhododendrons, sometimes to a dramatic degree.
But the point I want to make, while conceding the accuracy of the reports of beneficial limestone applications published in the Bulletin, is that such results are likely to be sporadic and exceptional. For generations, gardeners the world over have avoided using lime where ericaceous plants are grown, as a lesson learned by practical experience. Modern soil chemistry supports this time-honored taboo and laboratory evidence explains the reason for it, while also confirming that there are circumstances, however unusual they may be, in which limestone can be used to advantage on rhododendrons.