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Nitrogen rate and timing critical for organic crops

“The major challenge when using organic sources of nitrogen is knowing how much to apply and how fast it will be available to the crop,” says Robb Mikkelsen, U.S. western director of the International Plant Nutrition Institute.

Mikkelsen, who is based in Merced, Calif., explained how nitrogen and the other major nutrients, phosphorus and potassium, are taken up by plants during an organic production seminar in Modesto.

The Institute, composed of 16 member companies, focuses on science-based, agronomic education and research, including best management practices for fertilizers.

“If you don't get the amount and the release rate right, the crop is not going to be happy or you will have an environmental problem,” he said. If the nitrogen is released too slowly there's a deficiency, or if too fast there's leaching.

Another problem with mistiming application is the nutrient might be released from the soil too near harvest time to be useful to the crop.

For organic production, alfalfa meal, cottonseed meal, fish meal, or blood meal are all excellent sources that release nitrogen quickly.

Higher soil temperatures stimulate the release and need to be calculated with the amount applied and the release rate to determine the amount available for the crop after a given period of time after application.

In contrast, 100 pounds of nitrogen applied in a commercial soluble fertilizer means the total amount is available for the plant to take up.

In manures and composts the ratio of nitrogen to phosphorus varies, so that if sufficient nitrogen is applied, more phosphorus than the plant can use may accumulate in the soil.

Nutrient release rates have been found in research to be highly variable in dairy manure from one source to the next, in part due to bedding materials used. “Some manures released nitrogen, others tied up nitrogen, so chemical analysis by itself does not always tell you all you need to know,” Mikkelsen said.

Composts, depending on the processed used to make them, can be very slow to release nutrients.

Phosphorus is somewhat easier to manage than nitrogen. Sources for organic production include bone meal, but Mikkelsen warned that there's a good reason why fossilized dinosaur bones remain for eons: “They don't dissolve very fast.”

With California soils in particular, organic growers cannot expect to apply a lot of bone meal and have it available to their crop right away.

The key with bone meal used in conventional farming is the content of hydroxyapatite, which when combined with an acid, makes single super phosphate to provide a soluble form of phosphorus.

“In organic production, we have to find another way to get bone meal to dissolve,” he said. “You might add elemental sulfur or in some soils there is enough acidity to dissolve some of the phosphorus.”

Mycorrhizal fungi may assist the uptake of phosphorus when the plants are stressed for the element, but he said they cannot always be depended on to increase the phosphorus supply.

Another source, rock phosphate, Mikkelsen added, works elsewhere as a source of phosphorus but in California, soils are not acidic enough and have too little calcium to make it available for the crop's use.

Some green manures may provide phosphorus, but others may not, depending on the cover crop and the main crop. For example, a cover crop that furnishes nitrogen in a pistachio orchard may take up phosphorus at the expense of the trees.

“Potassium,” Mikkelsen said, “is the easiest of all to manage and doesn't require the chemistry of the others.” Kelp-based products are a source, but they are low-analysis, typically requiring heavy rates for organic operations to replace that mined by the crop.

Greensand, a soil conditioner composed of potassium with a hydrated silicate of iron, is another source, although it too can dissolve slowly. Wood ash can contribute potassium, but the amount depends on the type of wood burned and the pH of the soil.

“There are lots of organic sources of nutrients. Some are effective and beneficial, and others may not perform the way you'd like. You need to have additional management. You have to be looking at your soil and looking at your roots. You have to be on top of it all the time,” Mikkelsen concluded.

Turning to composts used in organic systems, Scott Deatherage of TM Composting, Inc., Bakersfield, said the turned-windrow process for composting dairy manure is the most common in the state but is being challenged by air quality regulatory agencies in the Central Valley.

The objective is to keep temperatures in the compost pile at about 140 F to kill weed seeds and plant pathogens. The 60-day process is designed to duplicate what takes about four years naturally.

In addition to eliminating weed seeds and pathogens, Deatherage said, composting provides a soil amendment carrying fungal and bacterial organisms for soil aggregation.

“Aggregated soils facilitate drainage and allow roots to penetrate, allowing water and air to penetrate. Anything that restricts root growth restricts nutrient uptake as well,” he said.

“The big question is are you mining or are you building soil. The only way to find out is soil sampling. Get a microbial analysis, a plant analysis, and do general plant observation and inspection.

“Remember, soil is not just dirt. It's a living, breathing asset, and we have to manage it like we do our tractors and the rest of our operations,” Deatherage said.

The interaction of organic fertility programs and IPM practices was outlined by Tom Quick, entomologist with GrowMore, Los Angeles.

The type of an organic fertilizer, its composition, and its timing of application can have an effect on pest pressures. “Nitrogen materials have an effect on soft-bodied pests and plant diseases,” he noted.

The advent of new nitrogen fertilizers after World War II prompted a flush of healthy, green crops in conventional crops, but the ample nitrogen triggered heavy populations of aphids and other sucking insects on those crops.

“The more nitrogen in the sap of the plant, the more food for the insects. One of the lessons we've learned over the years as organic farming has increased is when you are in balanced fields the insect pressure is less,” said Quick.

Seed corn maggots are attracted to the organic matter surrounding germinating seeds during January in California.

The solution, Quick said, is to monitor for the maggots, and delay a typical dry organic fertilizer application by a week or so to avoid encouraging the first spring generation of the pest. The second and third generations become less concentrated.

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