Successful vegetable growers are skilled at producing vegetables with the high quality demanded by the market place. There are many aspects of production that lead to successful production of vegetables, but nutrient management is a cornerstone practice upon which all other aspects of production sit. Nitrogen is a key nutrient that is taken up in larger quantities than all other nutrients except for potassium. Nitrate and ammonium are the forms of nitrogen that vegetables predominantly absorb; however, in warm soils, ammonium rapidly converts to nitrate with the help of soil bacteria which results in nitrate being the dominant form of nitrogen found in agricultural soils during the summer production season. Unfortunately, nitrate is susceptible to leaching by water from irrigation or rain.
Once nitrate leaves the root zone of the soil and makes its way to surface or groundwater, it is considered a contaminant. Nitrate levels are regulated in drinking water and agriculture is often pointed to as a major source of nitrate contamination of drinking water.
Given the potential high value of vegetables, growers have to balance providing for the needs of their crops and taking care to minimize nitrate losses from production fields. There is no question of the value and necessity of nitrogen fertilization, but the associated issues with nitrate runoff and leaching to groundwater have become front-burner issues for vegetable producers because of the enactment of water quality regulations.
In California, the Porter-Cologne Water Quality Control Act established authority of the Regional Water Quality Control Boards to oversee water quality regulation in the state. Nitrate levels in ground and surface water are regulated by this agency and are mandated not to exceed the drinking water standard of 10 ppm nitrate-nitrogen. It is very difficult for growers to meet these regulations at times. For instance, during wet springs with unexpected rainfall, it can be difficult to maintain sufficient nitrate in the root zone to achieve maximum yield.
On the Central Coast of California, water quality regulations were passed earlier this year and it is still not entirely clear how they will be enforced. Certainly growers will also be required to spend more time and money monitoring nitrogen use in their production fields. Furthermore, growers are increasingly utilizing a number of techniques and technologies to help them improve nitrogen use efficiency and thereby assist them in complying with water quality regulations.
Soil tests that measure levels of residual soil nitrate are a key practice to help growers reduce nitrogen fertilizer inputs. This technique is particularly useful in detecting situations when there are robust levels of soil nitrate that can offset nitrogen fertilizer applications. For instance, in the coast production district of California, leafy green vegetables are often double cropped and the second crop of the season can often take advantage of residual soil nitrate left by the first crop.
There is a great deal of research underway evaluating fertilizer technologies such as nitrification inhibitors, slow release fertilizers, foliar applications of nutrients, as well as other techniques to increase the nitrogen use efficiency of applied fertilizers. Rotations with deep rooted crops or cover crop that can take up residual soil nitrate can be very helpful in mitigating nitrate losses to groundwater. However, economically viable rotational crops are not always available and only provide a limited mitigation measure in many areas.
Vegetable growers are pulled by so many factors, it is difficult to find the time for the extra management activities required by the water quality regulations. As a result, growers are increasingly working with certified crop consultants to develop nutrient management plans and to conduct routine soil nitrate tests to better time and manage nitrogen fertilizer decisions. All of these measures cost extra money and it is unclear if consumers are willing to pay their share of the costs of improved nitrogen management.
Growers have confronted many challenges in the past. New water quality regulations are just one of the many new issues. The good news is that there are resources and techniques that can help growers improve nitrogen use efficiency and benefit water quality.