nitrogen was so much easier with the growth of clover-like plants and legumes as
contrasted with the growth of cereal crops.
Hellriegel and Wilfarth included the knowledge that if you grow gramineae in a
field for sequential years without the supply of nitrogenous fertilizer, then yield of
the graminea will decrease every year. The same reduction in yield happens if
cultivation of legumes is repeated for a long time. But, by adding nitrates
(in addition to the necessary mineral fertilizer), the decline in harvests can be
reduced in the case of the gramineae although not in the case of legumes. That
indicated the lack of productivity observed by subsequent plantings of legumes was
not due to a shortage of nitrogen availability. It also had been known from previous
studies by other scientists that growing either lupines or red clover (Trifolium
pratense) as crop resultingly would increase the level of soil nitrogen.
Hellriegel and Wilfarth found that if legume plants were otherwise supplied with
nitrogen, then nodule formation and growth of the plants were not necessarily
interdependent. Their plants were able to grow normally, produce flowers and
produce fruit without starting a single root nodule. Thus, they concluded that legume
plants were able to beneficially absorb and process the nitrates available to them in
soil even absent root nodules. The legumes did, however, have a supplemental
second source from which they could cover their nitrogen requirements if the
amount of nitrogen compounds already available in the soil was not sufficient.
This second source provides elemental nitrogen from the atmosphere. While the
legumes do not per se have the ability to assimilate free nitrogen from the air, the
participation of living microorganisms in the soil is absolutely necessary in order that
this second source can provide for needs of the plant. That second source was linked
to both the availability of soil microbes and to root nodule formation. Root nodule
formation was always accompanied by a nitrogen gain for the plant, which did not
derive from the original nitrogen content of the soil at the start of the tests. Their
statements included certainty that the given observations did not support assumption
for the root nodules to serve the plant merely as reserve containers for storing
nitrogen-containing nutrients through the time of plant fruiting. Instead, it was
much easier to combine the concept of nitrogen storage with the view that the
nodules are assimilation organs of the plant. Resultingly, the papilionaceae were,
following harvesting, able to leave in the soil a significant amount more nitrogen
than had been present in the soil when the papilionaceae had begun to grow.
Hellriegel and Wilfarth concluded from their studies that the root nodules of
legumes seemed to be plant organs. They attributed nitrogen accumulation by the
legumes to involve a symbiosis between the legume and microorganisms which
promoted development of the legumes by promoting nitrogen uptake. Hellriegel and
Wilfarth presumed, although admittedly without supporting fact, that some types of
fungi could convert free nitrogen of the air into different compounds and that these
different forms of bound nitrogen were unequally assimilable for the different types
of higher plants. Hellriegel and Wilfarth believed that in order to make free nitrogen
available to the legumes for nutritional purposes, it is not enough just to have any
microorganisms in the soil, but it is necessary that certain species of the latter enter
into a symbiotic relationship with the former. Their final presumption was that the
14 Discovering the Symbiotic Nature of Microbial Life: Summarizing Milestone. . .
225
contrasted with the growth of cereal crops.
Hellriegel and Wilfarth included the knowledge that if you grow gramineae in a
field for sequential years without the supply of nitrogenous fertilizer, then yield of
the graminea will decrease every year. The same reduction in yield happens if
cultivation of legumes is repeated for a long time. But, by adding nitrates
(in addition to the necessary mineral fertilizer), the decline in harvests can be
reduced in the case of the gramineae although not in the case of legumes. That
indicated the lack of productivity observed by subsequent plantings of legumes was
not due to a shortage of nitrogen availability. It also had been known from previous
studies by other scientists that growing either lupines or red clover (Trifolium
pratense) as crop resultingly would increase the level of soil nitrogen.
Hellriegel and Wilfarth found that if legume plants were otherwise supplied with
nitrogen, then nodule formation and growth of the plants were not necessarily
interdependent. Their plants were able to grow normally, produce flowers and
produce fruit without starting a single root nodule. Thus, they concluded that legume
plants were able to beneficially absorb and process the nitrates available to them in
soil even absent root nodules. The legumes did, however, have a supplemental
second source from which they could cover their nitrogen requirements if the
amount of nitrogen compounds already available in the soil was not sufficient.
This second source provides elemental nitrogen from the atmosphere. While the
legumes do not per se have the ability to assimilate free nitrogen from the air, the
participation of living microorganisms in the soil is absolutely necessary in order that
this second source can provide for needs of the plant. That second source was linked
to both the availability of soil microbes and to root nodule formation. Root nodule
formation was always accompanied by a nitrogen gain for the plant, which did not
derive from the original nitrogen content of the soil at the start of the tests. Their
statements included certainty that the given observations did not support assumption
for the root nodules to serve the plant merely as reserve containers for storing
nitrogen-containing nutrients through the time of plant fruiting. Instead, it was
much easier to combine the concept of nitrogen storage with the view that the
nodules are assimilation organs of the plant. Resultingly, the papilionaceae were,
following harvesting, able to leave in the soil a significant amount more nitrogen
than had been present in the soil when the papilionaceae had begun to grow.
Hellriegel and Wilfarth concluded from their studies that the root nodules of
legumes seemed to be plant organs. They attributed nitrogen accumulation by the
legumes to involve a symbiosis between the legume and microorganisms which
promoted development of the legumes by promoting nitrogen uptake. Hellriegel and
Wilfarth presumed, although admittedly without supporting fact, that some types of
fungi could convert free nitrogen of the air into different compounds and that these
different forms of bound nitrogen were unequally assimilable for the different types
of higher plants. Hellriegel and Wilfarth believed that in order to make free nitrogen
available to the legumes for nutritional purposes, it is not enough just to have any
microorganisms in the soil, but it is necessary that certain species of the latter enter
into a symbiotic relationship with the former. Their final presumption was that the
14 Discovering the Symbiotic Nature of Microbial Life: Summarizing Milestone. . .
225