Considering crop rotation in nutrient balancing

A function of 4R stewardship is to maximize the timing and placement of nutrients for crop efficiency and environmental protection. Crop rotation becomes an important consideration if application timing can match the crop that can maximize the utilization of nutrients.

Nutrient balancing compares the nutrients applied to a crop rotation to nutrients removed over a crop rotation. In order to balance nutrients, soil fertility levels, yield goals and crop needs are required information to determine proper fertilizer application rates. Corn and wheat have greater response to applied nutrients (especially seed-placed phosphorus), and applying to crop needs allows producers to follow the principles of right rate, right time and right place.

A recent news article (Ontario Watershed Study Shows Over-Application of Phosphorus) reported on research that indicates phosphorus is over applied by 40% for corn and wheat. If 50 lbs of phosphorus is applied to each of corn and wheat in a rotation, then 40% would represent 40 lbs P₂O₅ or about a 1 ppm soil test increase in soil test phosphorus levels. The research quoted in the article gives no indication of soil fertility levels or crop rotation, both of which are essential to nutrient balancing.

In Ontario a general guideline is that it takes approximately 35 lbs of P₂O₅ to increase/decrease soil test levels by 1 ppm and it takes about 20 lbs of K₂O to increase/decrease potassium levels by 1 ppm. There are variations based on soil characteristics, however these amounts give producers an indication of how closely they are increasing or decreasing soil fertility in their fields.

Nutrient management planning, crop needs/recommendations are always the first priority, especially on soils with low fertility levels. For livestock farms with adequate soil fertility levels and where nutrient recommendations are low, manure application rates are set to match the nutrients removed by a crop. Manure is most often applied to meet the nutrient needs of a corn crop, followed by wheat and forage crops. Manure and fertilizer application does not often occur on a soybean crop since soybeans do not make efficient use of the manure nitrogen (the root nodules supply the plant's nitrogen needs), however the soybean crop does remove significant nutrients from the field. Manure nutrient management plans generally have manure and/or commercial fertilizer applied 1 or 2 times in the rotation to meet the crop needs for a full rotation of crops. Therefore if 40% more phosphorus is applied to 1 crop in the rotation, it may be providing nutrients for the crop year where no nutrients are applied. From an environmental perspective this means manure and fertilizer are applied less frequently and from an economic perspective it allows a grower to follow 4R principals that maximize economic nutrient efficiency.

The tables below show the crop needs and crop nutrient removal for a corn-soybean-wheat rotation with average yield goals. In one scenario, the soil fertility levels are low, where nutrient needs are higher than crop removal, while in the second scenario the soil fertility levels are adequate and fertilizer needs are lower and are often met with a commercial fertilizer starter.

Typical example of nutrient balancing for a 3-crop rotation

Over the rotation the nutrients recommended will help ensure maximum economic yields, while the nutrients removed give an indication of how quickly fertility levels will decrease (or increase). Soil testing gives an indication of nutrients available for a crop, and are only a fraction of a "soil bank" of nutrients which are influenced by many physical and biological factors. Responsible crop production should not mine the nutrients from the soil. Healthy soils with good infiltration capacity, good aggregate stability and diverse microbial populations will be the best defense for minimizing environmental impact from nutrient loss.

Human nutrition is important to overall health and the Canada Food Guide gives suggested best practices. Similarly, food production requires good crop nutrition. Nutrient Management planning tools help Ontario producers to manage and apply nutrients based on soil fertility levels, crop rotation and crop needs accounting for field conditions that could impact environment. Tools, such as the NMAN software, allow producers to see what impact decisions such as timing of application or application rate could have on nutrient availability.