Increasing Nitrogen & Phosphorus Efficiency
Plants need both Nitrogen (N) and Phosphorus (P) to grow well. A lack of these two elements limits growth. Farmers apply N & P fertilizer, especially to corn, to increase yields, but there is a limit. Since the 1900s, N fertilizer applications have increased 5X or 500%, while P fertilizer inputs have increased 76%. Too much N fertilizer ties up soil P levels and other nutrients. A recent study looked at how farmers may be able to reduce N fertilizer inputs while increasing crop yields and enhancing P fertilizer use efficiency. P fertilizer stocks are running low in the USA (expected to be depleted by 2050), so farmers need to start thinking about how they can use existing stocks of P fertilizer better, both mined and in the soil, to enhance yields.
A large meta-study (combined results from 222 independent studies) looked at how N responds to P fertilizer. Overall, adding P fertilizer had a positive effect on plant N uptake, resulting in higher plant growth above and below ground. It also increased soil microbes, which increased soil N and plant-available N while decreasing the amount of N that leaches away. Soil P is a major limiting element to both microbes and plants, so when P fertilizer is added, it tends to increase the growth of living organisms (microbes and plants). Soil microbes are just soluble bags of fertilizer, releasing many plant-available nutrients—not only nutrients but also plant growth stimulators (enzymes, hormones, etc.), which enhance crop yield. The good news for Midwest farmers is that the biggest growth and yield response from P fertilizer seems to occur on slightly acidic soils.
For plant N uptake, soil microbes play a key role. In legumes and clovers, farmers know that Rhizobium bacteria take atmospheric N and convert it to plant-available N. Most farmers pull up soybean or alfalfa plants and look for the nodules where the N is made. Pink nodules indicate good and efficient N synthesis. Ideally, for higher soybean, alfalfa, and clover yields, farmers should strive to have as many nodules as possible on each root segment to enhance yield. Just recently (last 5-10 years), farmers and researchers have started to learn more about rhizophagy—literally, plant roots consuming bacteria for nutrients. Anywhere from 40-60% of the plant’s N needs come from small rod-shaped bacteria (Pseudomonas) that enter roots and get directly inside root cells. They lose their cell walls and spill out nutrients, which directly influence plant growth and yield. Not only soluble N (nitrates and ammonia) but also proteins, enzymes, and hormones—all beneficial for enhancing growth and yield.
Here is why good levels of plant and soil P become so important. P is used in all living organisms in the cell wall, to store and release energy (ATP), and as part of genetics (DNA, RNA). When microbes multiply (these soluble bags of fertilizer), they need P. The takeaway is that a little extra P greatly enhances N uptake and N nutrient use efficiency. P fertilizer additions slightly increased soil pH, increased available soil P (106%), but did not change soil organic matter levels. P fertilizer increased soil microbe populations by 10% (10.8% increase in good fungus and 9.5% in bacteria).
Too much N fertilizer can actually decrease P uptake and use. Too much N ties up copper but also increases magnesium (Mg) availability, which can make soils tighter and more compacted. The best example is the USA military’s use of ammonium fertilizer in WWII to harden airport runways for planes to land. Too much soil N makes soils really hard and dense.
For farmers, especially those growing corn, adding sufficient P fertilizer can enhance crop yield without causing P runoff. On N fertilizer, farmers are probably overapplying N. Stalk nitrate and tissue tests can optimize N fertilization. For N, weather and timing are critical—wet weather causes N to leach away or be lost by denitrification (N gas lost to the atmosphere). Research shows that split N applications and using Y-drops around tasseling time are very effective methods for N corn fertilization. Enhancing soil health allows microbes to do more work and supply crop nutrients cost-effectively, lowering fertilizer bills. For soil microbes to thrive, good soil tilth is required. Minimizing tillage can greatly enhance both N & P use efficiency. Building soil organic matter (SOM) stores plant nutrients and enhances microbe populations. Adequate P is needed for optimal plant N uptake, while too much N may be detrimental.