We all know that plants need nutrients to grow. But don’t they get those nutrients from the soil? Why do farmers need to apply fertilizer?
You might hear Iowa farmers talking about ‘applying manure’ or ‘dragging anhydrous’. What they are really talking about is the application of fertilizers to fields with the hopes of increasing crop productivity. All plants need a variety of nutrients to grow and be healthy. A lack of any one nutrient might cause symptoms like yellow leaves or brown spots or other unhealthy symptoms like wilt or susceptibility to diseases like mold or insects.
Plants need a whole host of nutrients to stay healthy. They need micronutrients like boron (B), carbon (C), chlorine (Cl), copper (Cu), hydrogen (H), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), oxygen (O), and zinc (Zn). Recycling plant matter is an excellent way of providing micronutrients to growing plants. They also need secondary macronutrients like calcium (Ca), magnesium (Mg), and sulfur (S). But plants need the most of primary macronutrients which are N-P-K. These nutrients are usually lacking from the soil because plants use large amounts for their growth and survival. The three primary macronutrients are nitrogen (N), phosphorus (P), and potassium (K).
Nitrogen is part of all living cells and helps transfer energy in plant cells. It is part of chlorophyll which makes the plants green and allows them to produce food through photosynthesis. Nitrogen supports quick plant growth and maintains strong leaves and good fruit production. Nitrogen can be fixed from the air through the nitrogen cycle or it can be added to fields in the form of fertilizer.
Phosphorus is also an essential part of photosynthesis. It helps the plants form oils, sugars and starches. Phosphorus aids in turning solar energy into chemical energy and helps the plant withstand stress.
Potassium benefits plants in building protein and producing high quality fruit. It also helps plants be more resistant to diseases.
Soil can hold some of these nutrients in place so that they are available to the plants when the plants are ready to use them. So often times farmers will apply manure (high in organic matter and nitrogen) or anhydrous (high in nitrogen). Because the soil can hold these nutrients, farmers can take advantage of slower seasons like the fall (after harvest) or spring (before planting) to apply fertilizer. But soil can’t hold an infinite amount of these nutrients. If there is too much nitrogen it can leach into waterways with a big rainstorm. Nitrogen in water can be a problem for wildlife and humans that rely on that water.
Farmers try very hard to only apply the correct amount of fertilizer. Too little and the corn or soybeans won’t grow well. Too much and the nitrogen will be wasted and potentially run off into the watershed. Precision application can use soil testing data to apply fertilizer only to the parts of the field that need it.
Anhydrous ammonia application in the fall should be done after the soil temperature is below 50 degrees Fahrenheit (usually around the first week of November). This prevents nitrogen losses from leaching. In the spring it is best to apply nitrogen within two weeks of planting the crops to avoid loss.
Once the crop is growing it may need some additional fertilizer (nitrogen) to maximize growth and yield. Even legume crops like soybeans (that have nitrogen fixing bacteria) sometimes need some extra nitrogen to help them grow. In that case it is important to apply the fertilizer as close to the period of maximum crop growth as possible. This ensures that it is available for the plant and won’t leach into the waterways.
Applying fertilizer takes a lot of scientific understanding of plant physiology, the nitrogen cycle, soil testing, and good management decisions. But with good management of fertilizer, crops can produce their maximum yield and we can still protect water quality here in Iowa. This is one small way that farmers celebrate Soil and Water Conservation Week – April 24 through May 1, 2016!