Balance is the Key

by Desiree Jans, Ph.D.

Intensive agriculture has been criticized for concentrating nutrients beyond the capacity of the land and plants to absorb them. Excessive use of any nutrient source, be it fertilizer, manure or compost, can lead to pollution of groundwater and surface water by nitrates and phosphorus. For the health of the soil, plants, animals and people, we must develop enterprises that are sustainable and harmonious with the environment. But what does this mean in practical terms?

In nature we generally don’t see these problems. A diverse mix of plants and soil organisms are growing or active at different times of the year, absorbing any nutrients that become available. Most nutrients are released slowly from the soil at times that match plant needs. Very little nutrient is lost to leaching or run-off. The key characteristics of this balanced system are: plant cover from spring through fall, moderation of nutrient supply, and timing of nutrient supply.

In an agroecosystem we can think of balanced nutrient management as:

Crop Nutrient Demand = Soil Nutrients + Crop Residue Nutrients + Fertilizer, Manure, or Compost Nutrients

The first step is to determine crop nutrient demand. Good crop rotations can go a long way toward improving nutrient use and minimizing risk. A well-planned crop sequence makes the most effective use of fertility conditions left by the previous crop. Some crops, like corn, are heavy feeders and dig deeply into a soil’s nutrient reserves. This is great if you have excess fertility but otherwise heavy feeders can put a strain on the soil nutrient balance. Medium and light feeders, such as oats or barley, work well in organic rotations, as do legumes. Because legumes are able to fix their own nitrogen from the atmosphere, they actually build up the soil’s reserves of nitrogen. When designing a rotation, nitrogen-fixing crops should alternate with high nitrogen demanding crops.

The next step is to figure out what nutrients the soil can supply to the crop. Soil testing will reveal the amount of available minerals, such as phosphorus, potassium, and sulfur, in the soil. Because soil nitrogen is so mobile, it is not part of our standard soil analysis. It’s assumed that most of the plant available soil nitrogen will have moved past the root zone between a fall soil sampling and a spring planting. Nutrient leaching is more likely to be a problem in sandy soils and in soils with low organic matter content, especially if there is no plant cover. Building soil organic matter helps to reduce nutrient loss and maintains a diverse and healthy soil biology that is better able to cycle nutrients.

The next factor in the nutrient management equation accounts for nutrients released from crop residues. As crop residues decompose, nutrients in the tissues are slowly made available to the crop. Forage legumes and green manures are essential in organic rotations and can add substantial nitrogen. Incorporating a stand of alfalfa or clover can add 100-200 kg of nitrogen per hectare to the soil. About half of this nitrogen will be available to a crop growing the year after incorporation; the rest will become slowly available over the next 3-5 years.

If nutrient demand and supply are still unbalanced, then the final step is to use soil amendments to make up the difference. Organic farmers strive to work within a closed agroecosystem in which nutrients are recycled on the farm. This often includes using manure and compost. Although raw manure contains more available nutrients, composted manure has lower potential risk of causing environmental pollution and is better for building long-term soil fertility. In either case, the amendment should be analyzed for nutrient content so that the appropriate application rate can be calculated. If applying raw manure, extra care is needed to apply the manure as close as possible to the time that the crop will use it. This may mean making a split application.

It’s also necessary to strike a balance between manure production and land capacity to utilize the nutrients. De-intensified operations pose less risk for nutrient leaching and runoff. That’s a strong argument for preserving valuable farmland rather than trying to squeeze more out of a crowded land base.

Because farming practices can have such wide reaching impacts, it’s important to look at a farm as part of a larger community. By balancing nutrient supply and demand on the farm, surrounding water quality is protected and the enterprise is one step closer to being sustainable and harmonious with the environment.

 

Desiree Jans, Ph.D, is a Web Course Instructor for the Organic Agriculture Centre of Canada (OACC). Please send comments or questions by phone to 902-893-7256 or by email to oacc@nsac.ca.

 

Posted December 2008