Wednesday, October 17, 2012

Growing Healthier Salad Greens Organically

Just about every gardener and grower knows that fertilizing lettuce, spinach, and other salad greens with nitrogen increases crop yields. But does dumping nitrogen on salad crops increase or decrease their nutritional content? And should if it does, should the nitrogen come from chemical or organic sources?

When the grower's objective is to produce leafy greens with the highest possible vitamin C content, there is no doubt that nitrogen fertilizer is a no-no in the week before the greens are harvested. Leaves use up vitamin C in the process of making the hormones that power mitosis, the creation of new cells. If a plant is rapidly growing because it has just been fertilized with nitrogen, it has to use vitamin C to use the nitrogen to make the proteins that become new leaf matter.

A quick spray of nitrogen on salad vegetables a few days before harvest may increase the weight of the crop, but it won't increase the nutritional quality of the crop. Darker salad greens such as Swiss chard (also known as silverbeet) and red-leafed lettuce and colorful cool-weather vegetables such as purple cabbage, purple potatoes, and purple cauliflower concentrate another group of antioxidants known as the anthocyanins.

In leaves, these red and purple plant compounds help leaves capture sunlight, and they have a variety of well-known benefits for people. Plants make both anthocyanins and vitamin C from simple sugars, but usually the leaf doesn't "run out" of the carbohydrates it needs to make these healthy antioxidants.

The problem, from the standpoint of growing vegetables with maximum human nutritional content, is that the anthocyanins are also used up rapidly when a plant is growing rapidly. Too much nitrogen fertilizer too close to harvest just isn't the best way to grow veggies with maximum nutrition. But the kind of nitrogen fertilizer used before harvest makes a difference, too.

 The standard advice to gardeners and growers is that is OK to use a chemical nitrogen fertilizer on fast-growing annual plants. After all, lettuce and spinach and chard are in the ground and harvested in less than two months, so how much harm could a little chemical nitrogen fertilizer do?

It turns out, a lot. Salad greens do grow when they are fertilized with chemical nitrogen fertilizers. So do soil bacteria. If you are concerned about E. coli splashing up from manures, immature compost, or runoff from your neighbors, chemical nitrogen fertilizers applied to the soil aren't working with you. They are working against you.

Chemical nitrogen fertilizers also raise the pH of the soil and disrupt the growth of fungi. During cool-weather, this may not be a major problem. But should drought strike, then the soil will have been deprived of the fungi it needs to hold water and conduct it to the roots of slower-growing plants.

There is a way around these concerns. If you are going to do a quick fix with nitrogen, then should consider making an exception to the rule of feeding soil, not plants. For a burst of growth in salad greens about half way between germination and harvest, feed the plants, not the soil.

The way to do this is with liquid, foliar feeding--preferably with products that have nitrogen in its nitrate form. This gives the plant the nitrogen it needs to make the amino acids it uses to build its proteins, but it also gives the plant a chance to reestablish its stores of antioxidants and vitamin C.

Foliar feeding is especially useful if you are gardening or farming on clay soils. It helps to do foliar applications of sulfates and phosphates, too, because the negative charges on the fine particles on clay and humus repel the negatively charged nitrates, sulfates, and phosphates out of the soil. There's no such problem when fertilizer is applied directly to the plant.

And if "chemicals" are a problem for you, there are many fine products based on plant and fish extracts that feed your greens so they deliver maximum nutrition. These natural products provide an additional component that most nitrogen fertilizers don't.

Derived from living plants or animals, they give your plants a dose of the amino acid glutamine. The glutamine helps your plants use nitrogen more efficiently, as does selenium in the selenite (but not selenate) form. And if you don't like the odor of natural products, there you have a great way to know when to apply them. Never put fertilizer on a plant if it would result in a stinky salad. Always use nitrogen at least a week before harvest.

Photo Credit: Chaojoker (own work), via Wikimedia Commons.

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