Why is phosphorus needed on farms?

Celebrating the 350^th Anniversary of Phosphorus’ Discovery!

Did you ever wonder why your home and garden fertilizer has specific ratios of nutrients? Well, the story goes back even further – to the discovery of those elements! Phosphorus is one of the main “ingredients” for healthy plant growth. This year, 2019, is the 350^th anniversary of the discovery of the element phosphorus, so the Soil Science Society of America (SSSA) and the American Society of Agronomy (ASA) are celebrating Phosphorus Week. We’ll have guest bloggers on Soils Matter this week, covering various topics. This second blog covers the importance of phosphorus in agriculture.

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Phosphorus is essential to all life. To overstate its role is impossible. Phosphorus provides the chemical backbone for many of biology’s most important processes:

• replication of organisms (DNA, RNA),
• energetic currency of metabolism (ATP, NADP), and
• gatekeeper for moving compounds across cell walls (phospholipids).

So, without phosphorus, crops would not grow into healthy plants. If the end-product of crop production is grain or beans, the yield would be lower from phosphorus-deficient plants. And, like humans, plants can get sick when they have nutrient deficiencies. Too little phosphorus will stunt plants, restricting both root and shoot growth. Other signs of phosphorus deficiency are revealed as purple, red or dark green stripes and spots on the leaves of different plants.

Fertilizing crops through the soil

Just like people, crops require good nutrition. Nearly all phosphorus acquired by plants during crop production comes from the soil. In 1861, nearly 200 years after Brandt’s discovery of phosphorus, French Georges Ville first reported the essential nature of phosphorus to plants. He said, “A judicious agriculturist must take account of what the soil receives and what it loses.”

Crops obtain phosphorus from the soil solution, the water component of soils. They do this directly through their roots, or indirectly by freeing phosphorus bound to minerals. Long ago, researchers estimated that the optimal concentration of inorganic phosphorus in the soil solution is only several parts per million. This is a seemingly infinitesimal concentration that is the equivalent to a five-course meal for a plant. To free phosphorus that is bound to soil minerals into the soil solution, some crop roots exude compounds that dissolve phosphorus. Still, other crops’ roots combine with naturally-occurring soil fungi that take care of this process and even bring phosphorus to the root.

Phosphorus availability to crops

Most soils bind a large fraction of the phosphorus that is applied to them. The phosphorus that is bound is unavailable to the plants. To determine how much fertilizer is needed to grow a crop, growers take soil samples from their fields. They send the samples to a testing lab. Ultimately, this information is used to develop fertilizer recommendations that are sent from a testing laboratory to a farmer.

Transforming land to agriculture using phosphorus

Modern agricultural economies have been built from the recognition of phosphorus’ importance to crops. As recently as the 1980 and 90s, Brazil, using lime and phosphorus, converted the soils of its Cerrado region into one of the world’s great crop production zones. This helped launch Brazil’s national economy into prosperity. On farms, phosphorus arrives in the form of fertilizer or manure for crops. Most of this phosphorus is destined for farm soils.

Another source of phosphorus comes through farm animals. The forages that they eat are plants – that contain phosphorus. Roughly two-thirds of the phosphorus in animal feed pass through them! It ends up in the urine and dung that, mixed with bedding, become manure. This makes manure a fundamental – and important – byproduct of animal farming.

Phosphorus stewardship

The discovery that phosphorus fertilizer could increase yield and crop health had a downside discovered decades later: the unintended consequence of runoff. Phosphorus in waterways is a serious problem, one that was firmly recognized in the 1960s. And, once the problem was detected, agronomists and soil scientists researched solutions.

How can we use phosphorus to ensure a healthy food supply, while protecting the environment? Sustainable phosphorus management (in addition to other nutrient management) became a primary field of study.

Phosphorus is also used in nursery plants. Research continues on the best ways to manage run off in this industry – where the public buys trees, bushes, vegetables, etc. for their own gardens.

Perhaps the most fundamental step forward has been the acknowledgement that phosphorus must be managed comprehensively. Armed with a soil test, a farmer can apply fertilizers and manures to soils at rates that are appropriate for crop production: not too much, not too little. This is the Right Rate, the first of the “4 Rs” of nutrient stewardship: Right Rate, Right Form, Right Placement and Right Timing. Today, these R’s bring into account not only the requirements of crop growth, but also the protection of water quality and conservation of an important natural resource.

In animal agriculture, the amount of phosphorus in feed has dropped dramatically. Research developed new cultivars and feed additives that improve the acquisition of phosphorus from feed. And, on farms, watershed conservation programs can keep phosphorus on the land and in the soil, so that it is available to crops and doesn’t wash off into our waterways.

By Pete Kleinman, United States Department of Agriculture

Read the other blogs in our phosphorus series!

The discovery and general uses of phosphorus

What are the sources of phosphorus for crops? 

What are the challenges regarding phosphorus use?

Ten things we all can do to manage phosphorus better

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