Soil — a dynamic and complex system of minerals, air, water and organic matter — is the lifeblood of anyone’s farm and of everyone’s food supply.
The primary nutrients used by plants are nitrogen (a functional part of chlorophyll and essential in the formation of amino acids and proteins), phosphorus (essential in energy transfer, photosynthesis, respiration and cell division) and potassium (required for the synthesis of both carbohydrates and proteins). Calcium, sulfur, magnesium, boron and other essential nutrients assist in the vegetative and reproductive phases of plant growth and ultimately harvest.
For plants, the predominant source of these elements is soil, the degree to which is largely dependent on the parent material of the soil (what lies beneath — bedrock, etc.) and influenced by climate, environment and human interventions. Farming practices too can have a significant impact on the quality of agricultural soils.
Conventional tillage (plowing with a moldboard plow that turns over deeper layers of soil) is the most severe form of tillage. Although this technique helps control weeds, it also exposes the soil to wind, rain and erosion. “Conservation tillage” and “no-till” farming leave at least 30 percent of the previous crop’s residue on the soil surface, where the next crop is planted.
Another soil-building practice is alternating the kinds of crops planted in a field. Crops are rotated by botanical family to alleviate pest and disease pressure and replace nutrients taken up by the previous crop. Vegetable crops within the same botanical family follow a four-year crop rotation so as not to perpetuate diseases and pests to which those plants are susceptible.
Planted between growing seasons, cover crops serve several purposes. They act as nutrient sponges that take up and bind in place nitrogen and other nutrients left over from the previous crop. They also build organic matter, assist in weed management, improve drainage, keep nutrients from moving to waterways and decrease sediment loss and erosion.
The Nutrient/Nutrition Connections
Plants that remove significant amounts of nutrients (called nutrient uptake) at harvest tend to have good levels of that nutrient (called nutrient removal) as a food. For example, potassium is deposited in the tuber of the potato plant during the reproductive phase of tuber growth — making potatoes a good source of dietary potassium.
However, the correlation between soil nutrients and those present in the plant is not exclusive. Insufficient potassium in the soil may result in smaller potatoes with lower starch content, but not necessarily lower potassium. And plants grown in soil with sufficient nutrients still can have nutrient deficiencies in the foods they produce; incorrect soil pH, drought or root system damage can limit a plant’s ability to take up nutrients.
Some research suggests that nutrients in fruits and vegetables have declined over time due to genetic dilution of breeding cultivars for higher yields. Other research has shown significant variances in mineral and flavonoid composition of fruit and vegetables across growing seasons, indicating that cultivar selection, growing conditions and crop production practices such as fruit thinning (removing a portion of fruit from a plant so the remaining fruit ripens more evenly) may be strong contributors.
According to the Food and Agriculture Organization of the United Nations, the most cropland acreage using conservation agriculture practices is in the U.S., Argentina and Brazil, respectively. Data from the U.S. Agricultural Resource Management Survey show that in 2009, nearly 36 percent of U.S. cropland planted to eight major crops (barley, corn, cotton, oats, rice, sorghum, soybeans and wheat) had no-till operations.
Leading the trend are farming families — particularly farms throughout the Northern Great Plains, Heartland, Mississippi Portal and Eastern Uplands regions — who are striving to produce healthy foods and maintain healthy soils for future generations.
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Jennie Schmidt, MS, RD, is a member of a third generation family farm practicing Continuous Quality Improvement on their soils.