When plants absorb nutrition in a form other than simple ions from the soil solution, we need to reconsider nutrition transport pathways and mechanisms. A first step in connecting the dots is identifying as many dots as we are able. William Albrecht was passionate about the idea that nutrients should be available but not soluble, and he described how plants absorb nutrients into the roots even against the flow of water:
Nutrients are not washed into the plant by the transpiration stream: they enter under their own power1
In that contention that solubilities of high order are required for entrance the plant root, we are apt to believe also that such entrance is connected with the large amount of water moving from the soil into the root, passing through the plant, and evaporating to the atmosphere from the leaf surface. More water is moved through and transpired by the plant according as the evaporation rate from the leaves increases with the rise of the daily temperature, the wind, or air movement over the leaf surface, the lower humidity of the atmosphere, and the larger supply of water in the soil. But because there is a decided flow of water from the soil through the plant for evaporation to the atmosphere, that is not proof that the fertility elements are necessarily moving along that same course because of that current of water as transpiration. Calcium, magnesium, nitrogen, phosphorus, potassium, and all the other essentials are not swept into the plant because they are applied to the soil in water soluble forms of fertilizers and flooded in, as it were.
There are natural facts, some readily demonstrated in the laboratory, which refute such erroneous beliefs that the water solubility within the soil is a requisite for fertilizer availability and flow with the water into the growing crop. As the first fact, plants will grow and their nutrients will move normally from the soil into the roots without the evaporation of water from the leaves. A potted plant, enclosed in a water saturated atmosphere with carbon dioxide under a glass bell jar in the light, will grow normally. This fact tells us that while the transpiration stream is halted because the saturated atmosphere will not take any water of evaporation, the fertility elements are, nevertheless, flowing into the plant from the soil.
In research at the Missouri Station, some soybean plants were grown on soils of such low saturation of the clay by calcium, that the totals of nitrogen, phosphorus and potassium in the total crop of tops and roots were less than those of the planted seed. Such facts tell us that the fertility elements may flow out of the root, or in the reverse direction of the flow of the transpiration stream of water.
That same reverse flow of fertility can be demonstrated under the conditions used for the potted plant within the bell jar, or when there is no flow of transpiration. Such facts inform us that even in the absence of water movement within the plants, the nutrients will move either into, or out of, the plant, entirely independently of either the static or the flowing condition of transpiration water. Forces, other than the water flowing into the plant root, must move the fertility elements serving in connection with plant nutrition.
Still as another situation, the desert plants have shown according to research reports by Dr. Went, now Director of the Missouri Botanical Gardens, that nutrients go into the roots for nourishment of the plants when in the daytime the water is transpired to move from the soil to the atmosphere. Then, also, they go into the roots when at night time the atmospheric moisture of condensation moves from the plant back to the soil sufficiently for plant survival through such diurnal reversals in movement of the limited moisture supply.
These facts deny, categorically, any necessity of water solubility of nutrients for their flow into, or within, the plant for any delivery services of them by the transpiration. They tell us that the fertility, which is feeding – not watering – the crop plants, behaves according to certain laws of physico-chemical relations within the soil and plant, while the water movement behaves according to the meteorological conditions and the climatic situations controlling the conversion of water from the liquid to the gaseous form and vice versa.
Water solubility of plant nutrients in the soil is not the rule of nature for their services to plants. Rather, they are naturally insoluble there, by which condition they remain there against loss through leaching out of the soil. By virtue of that condition they are still there when the growing root comes along. But that fact does not deny their being available through other mechanisms than aqueous solution.