Plants, unlike humans, can only use nutrients when they are in the exact form that can be taken in by their roots or leaves. Nitrogen for example can only be utilized in one of two forms through the root systems: 1) the nitrate form, or 2) the ammonium form. When we look at phosphorus, we find the plant roots can only take up phosphorus when it is in the ortho-phosphate molecular form. Potassium can only be taken into plants when it is in the ionic potassium form.
But when we look at plant materials that exist in the soil such as humic acid, organic matter, fertilizers, and manure, we find that nutrients exist in many, many forms, especially the more complex nutrients like phosphorus which can exist in 3 or more different forms in the soil! The presence of these different forms of nutrients depends on the temperature and biological activity of the soil, and the presence of soil moisture. An ideal soil which is well balanced with organic matter and plant nutrients will have a constantly changing flux of nutrient forms which exist between the storage sites on the soil particle surfaces, and the inner soil particle areas known as soil pores.
Plant roots must first come in contact with nutrients in the soil. They do this by growing through the macro-pores of the soil and between the soil particles and by extending root hairs out from the roots to absorb these nutrients. Moisture that moves towards the roots also carries soluble forms of fertilizer.
So, as the plant sees it, it is looking for those forms of nutrients that are immediately available for uptake because anything else is not recoverable!
Feeding a plant is much like having an intravenous put into a person’s arm. For example, if you took a bowl of cereal and fed it to a person, they could digest it through their stomach, but it would take at least 20 minutes until nutrients began to move into this person’s blood system. If you put nutrients into a person through intravenous methods, the nutrients will move immediately into the blood stream. But if you took that same bowl of cereal and put it in a blender, you couldn’t feed it through an intravenous tube, even though it’s the same exact food that’s going into a persons stomach, because the nutrients are not in the form the blood stream can use! The reason this would kill a person is because of the difference between available and unavailable forms of nutrients. Intravenous materials that are feed into a person’s blood system must be in the form of nutrient that the person’s blood system can uptake and utilize, so the job of the stomach is to convert available food to recoverable, useable nutrients. Plants have no stomach and so therefore, they must take in through their roots only the form of nutrients that they can immediately utilize in their cellular physiology.