Thaddeus’ Ruminations: Feeding the Organic Plant
By Thaddeus Barsotti
Published: May, 2007
Keeping the right amount of plant food in the soil and in a form available for a plant to use is one of the main responsibilities for any farmer. This is a task for every farmer, but a task made more difficult for the organic farmer.
When we talk about plant food, we are more specifically talking about nitrogen – which is the key to building proteins and amino acids in any living organism. An important clarification to be made is that when we refer to nitrogen is that there are many different forms of nitrogen in the environment at any time, none of which are available for plants to use.
The Nitrogen Cycle is the key to understanding how to feed a plant.
Organic Nitrogen is nitrogen that is held in plant tissue in the form of proteins or amino acids. It can also be fixed by legumes (e.g. beans, peas, alfalfa, clover and vetch). These legumes take the Nitrogen gas (N2) from the environment and convert it into Organic Nitrogen through fixation by Rhizobium bacteria in the nodules of the legumes. This process can fix up to 300 pounds of nitrogen per acre per year into the soil (the reason most organic farmers grow cover crops of legumes).
Once nitrogen is in its organic form, Mineralization occurs. Mineralization represents the conversion of organic nitrogen into mineral nitrogen (i.e. ammonium) by decomposition from microbes. But the ammonium (NH4) is still not available for the plant to use. The next step is Nitrification, which is the biological transformation from ammonium to nitrate (NO3). Nitrate is the form of nitrogen that plants use – plant food.
Plants uptake the nitrate to grow, they immobilize the nitrogen by putting it into proteins or amino acids (Organic Nitrogen). Eventually this plant dies and the Organic Nitrogen is broken down to be used again. This process can take up to one year. The challenge of the organic farmer, who most often applies only Organic Nitrogen is timing the application and rates of uptake, so that it breaks down into nitrates for the plants to use at the right time.
Conventional farmers are able to apply manufactured nitrates to crops in a liquid form. Think about that. The moment they think their crops need nitrates, they inject nitrates into the plants irrigation water and the plants are able to use that nitrogen the moment they get it. Organic growers have to time the decomposition of Organic Nitrogen so that there are enough nitrates in the soil when the plant is ready to use them. This is the main reason that conventional agriculture is able to yield larger volumes per acre of a crop compared to organic agriculture.
It is not surprising to realize that conventional farmers tend to apply significantly too much nitrogen to their fields – if a little is good that a lot must be better, right? The nitrates that they apply are generally too much for the plants to use and the excess nitrates leach into through the soil to contaminate the ground water.
