Chapter 3. Food Quality


A common practice is to fertilize with NPK fertilizer for maximum yield, with the assumption that a high nitrogen, phosphorus and potassium concentration in a plant produces maximum quality. But it is protein, not nitrogen, which affects quality. And quality depends on other essential minerals, which drop with unbalanced increases in these three.

Whether organic fertilizers produce higher quality than inorganic fertilizers is unsettled. What appears to be more important factors are the use of pesticides and the crop variey.

The quality of produce also depends on environmental factors such as water, soil structure, temperature and sunlight.

Table 1. Effects Of A Non-Nutrient Disorder On Crop Quality lists effects of an adverse environment on crop quality, and table 2. Effects Of A Nutrient Disorder On Crop Quality of a nutrient imbalance.

Food Quality

See [30], [53, chapter 10], [58, chapter 7], [73, chapter 7], [74], [76], [75], [77], [89] for the sources of most of the information in this chapter.

One must guard against the assumption that maximizing fertilizer use maximizes the quality of produce. This is especially necessary when the fertilizer referred to contains only nitrogen, phosphorus and potassium (NPK). Assessing protein by a measure of the nitrogen content is a particularly good example of why this is wrong.

The traditional way of reporting protein in animal feed is to multiply the nitrogen content by a constant to get what is called crude protein. The constant is 6.25, which is the average value of the nitrogen in protein. Nitrogen, however, may be present in a plant in forms and amounts that neither benefit nor are healthy for the human or animal consumer. Consequently, crude protein has no relation to food quality.

A nitrogen measurement includes:

  1. the true proteins, which are constructed from a variety of amino acids
  2. free amino acids which have not yet been integrated into true proteins
  3. mineral nitrogen, usually present as free nitrates but sometimes as nitrites or ammonium.

Only the true proteins are directly involved with the growth and health of the plant. Free amino acids constitute a reservoir for constructing proteins. Each protein requires a different combination of amino acids. In a satisfactory environment, a plant's amino acid reservoir is small because the plant creates proteins as soon as the proper amino acids are present.

A large reservoir of amino acids implies an amino acid imbalance. Some amino acids require magnesium, sulfur and/or trace elements in addition to nitrogen. If the soil is deficient in these, the plant will be deficient in amino acids which require them.

Amino acid balance in feed is not critical for domestic ruminants (cattle and sheep) because they manufacture all of the amino acids they need. But others must get certain amino acids in their food; for many of them, plants are the only source.

An excessive supply of NPK fertilizer, unsupplemented by other essential minerals, can lead to problems of protein formation in the plant. Vigorous plant growth stimulated by a fertilizer, especially nitrogen, can deplete the soil of other minerals. If the depleted mineral is needed for the production of a particular amino acid, that amino acid will not be produced.

Excess nitrogen favors the production of some amino acids. Nitrogen fertilization of corn tends to increase the amino acid Zein, producing a relative deficiency of the other amino acids and a large amount of free amino acids. Some amino acids are inversely related to the nitrogen content of the plant.

Also, an excessive amount of free amino acids causes a characteristic flavor and aroma which may attract insect pests to the growing plant [64].

Nevertheless, although an imbalance of free amino acids may produce a nutritionally inferior product, it is not dangerous to human or animal health. Nitrates and nitrites, however, are toxic to both ruminants and nonruminants when present at abnormally high levels. Metabolism in the body reduces nitrates to nitrites; nitrites produce an anemia in animals, particularly ruminants, and in humans, especially infants. Nitrites can also hinder circulation in peripheral blood vessels.

Toxic quantities of nitrates are not usually present in plants, because plants normally reduce them to ammonium, from which they produce amino acids. Under any of several conditions, however, nitrates may accumulate in the plant:

Consequently, acceptable food quality requires a small free amino acid content and a low level of nitrates. The only exception is that free amino acids may be high when the feed is for ruminants.

A simple nitrogen content converted to crude protein is no measure of these criteria.

Nor do phosphorus and potassium serve uncritically as measures of quality. An increase in the concentration of one nutrient in a plant leads to a decrease in another. An increase in phosphorus may cause a decrease in a trace element, and a rise in potassium is likely to result in a drop in calcium and magnesium.

The application of NPK fertilizer increases the vigor of plant leaves and roots, growth which increases the plant's ability to scrounge the soil for magnesium and trace elements not present in the fertilizer. Consequently, fertilized plants will remove more of these nutrients from the soil than unfertilized plants. But the additional amount taken up is not in proportion to the added growth, so that the concentration of minerals in the plant is lower.

This conclusion became evident in a survey of pastures in Austria, where intensively fertilized fields were compared with fields fertilized only with cow manure. The intensively fertilized fields produced an increase in plant yield but a decrease in dry matter content. Plants in the manured fields had higher concentrations of magnesium and trace elements. The manured fields also contained a greater diversity of plant species, and the milk cows were more fertile.

If NPK fertilizer is spread, the amount used should be adjusted according to the needs and availability of other nutrients; and it should take into account nonnutrient factors. One should take care, for example, against overfertilizing greenhouse crops in the hope of forcing growth. Experiments have demonstrated an accumulation of nitrates, especially in a heated greenhouse, owing to the lack of energy to metabolize the nitrogen.

Organic Versus NPK Fertilizers

The distinction between organic and NPK fertilizers is more controversial than the question of fertilizing for quality. One could defend any claim by citing at least a half dozen references, all produced by researchers of the highest authority.

Most of the studies which found no difference were done, however, at about 1940 or before. Since then, experiments have shown that produce grown with organic fertilizers (animal manure or compost) has a lower yield on a fresh weight basis but a higher dry weight yield (less water and more minerals) and a higher quality, whether measured by chemical tests or by the effect of the food on animal or human health. There is also evidence that the highest quality is obtained by a mixture of organic and NPK fertilizers, although the conclusions seem to be contradictory1.

There are also anecdotal references to the advantages of organic fertilization, so called because they do not meet the rigid requirements of scientific enquiry. In one such reference, a group of students were found to be perceptibly healthier after their lunches were prepared from an organic garden. In another, a doctor noted that a postoperative spread of cancer disappeared completely from five patients after they began eating organically grown foods.

We know that many diseases, including cancer, can be strongly influenced by the patient's mental attitude, a parameter which would be difficult to control in a scientific experiment. So we are left with a choice between anecdotes, which do not reveal all the controlling factors, and scientific experiments, which do not include them.

In any event, the best improvement in quality that can be expected with organic fertilization, so far as scientific enquiry is concerned, is perhaps 20%, possibly more in some situations. Though important, this is a small improvement compared to quality differences among different varieties of the same crop. The amount of vitamin C in different varieties of apples, for example, was found to vary more than 9-fold while the carotene content of carrots varied by a factor of 2-1/2 [74]

Furthermore, any benefits in quality that may be gained by careful production are often lost by improper storage. Nutritious elements can be destroyed by light, heat, oxygen and inadequate moisture control. Overcooking and disposal of cooking water also results in an excessive loss of nutrients [53].

In view of all the qualifications, the scientific evidence for or against the value of organic fertilizers on food quality is weak.

What is so damaging about this controversy is the tendency to ignore - and with that action to belittle - the far more important value of organic residues. This is its contribution of energy to an agricultural system. There is no substitute for it.

One issue that has not entered into this discussion is the use of pesticides. That is even more contentious than the question of organic vs commercial inorganic fertlizers. On the one hand is the claim that the acute toxicity of the pesticide residues consumed by the average person in one year is less than that of the aspirin in one tablet or the caffeine in one cup of coffee2.

On the other hand are the precautions that are necessary in the application of these pesticides and the existence of some evidence of manifestations after years of accumulations from exposure to small amounts at any one time[84].

Furthermore, documentation does exist showing that insects and disease can be controlled at least partly with organic residues and sound practices. The following references are an uneven survey of the subject.

Effects of Environment and Culture on Quality

Many people, when they see something wrong with their crops, immediately assume that fertilizer is lacking, and they instinctively ship off a soil sample to be tested for nutrients. More often than not they are wrong, and they are almost always wrong when only one crop out of several is adversely affected.

Experience shows that fertilizers are not the only determinant of plant growth. There are years in which mountain laurel blooms more profusely than any other. In other years, locust trees are never more fragrant, or horse chestnut more beautiful. In some years, tomatoes bear a record crop, no matter how they are abused; in others squash will not bear at all. Whenever crops do not meet expectations, we should look first of all to the environment for causes and only consider fertilizers as a last resort. Nutrients are often as detrimental in excess as in deficiency; if we pour on the fertilizers without thought, we risk additional problems.

Excluding nutrient availability, the most common influences on the success of crops are water, soil structure, temperature, sunlight, pollination and crop variety. Sometimes these produce effects which are typical of nutrient imbalances. Poor drainage or overwatering may result in stunted plants with yellow leaves, typical of a nitrogen or trace element deficiency. Underwatering may cause plants to wilt or blossoms to fall off. Insufficient sun may lead to weak, spindly plants, also true of an excess of nitrogen. A cold spring or low temperatures at any time may cause a phosphorus deficiency, even though soil phosphorus is adequate.

Poor germination is not caused by a soil nutrient deficiency, because seeds contain enough nutrients to sprout and produce an initial set of leaves. More likely causes are insufficient or excessive water, cold soil, a crust on the surface of the soil, or animals. Lettuce seeds fail to germinate if the soil is too warm. If soil water is marginally low, the presence of dry soluble fertilizers may inhibit seed germination indirectly by absorbing some of this water and limiting its availability to the seeds.

Some disorders are due to plants growing too closely together.

If plant flowers are numerous and healthy, nutrients may affect the quality of the fruits that do form. But the lack of fruiting is not likely to be due to a nutrient imbalance, and fruit quality may not be. Likely alternate causes are poor pollination, low moisture (causing flowers to drop prematurely) or low or high temperatures.

Some of the effects of an improper environment or culture are shown in more detail in table 1. Effects Of A Non-Nutrient Disorder On Crop Quality . Also a Cooperative Extension agent should be familiar with problems of this kind or can recommend a state specialist. Usually a telephone conversation suffices.

If a problem is not likely to be traceable to an environmental cause, then a nutrient imbalance or a disease is possible. As a guide, table 2. Effects Of A Nutrient Disorder On Crop Quality is a summary of some of the effects of nutrient deficiencies or excesses on the quality of produce.

1 See [76]. The nutritional tests in the article indicate that produce fertilized with manure or compost alone was of superior quality to produce fertilized with a manure-NPK mixture; however reference is made to another study showing the mixture to be better in all respects. Moreover, the manure used was acknowledged to be of poor quality, having a low nitrogen content, which may have been the reason why it was fortified    [return to text]

2 [20]. The argument is based upon a comparison of the LD 50 values for pesticides and for aspirin and coffee. The author stated that more recently introduced pesticides have even lower LD 50 values. The LD 50 is a specification of the quantity of a substance which is likely to kill half of an exposed population.    [return to text]

© 2013 Robert Parnes

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