What's Cropping Up? Blog

Articles from the bi-monthly Cornell Field Crops newsletter

Series: Phosphorus and the Environment, 1. An Introduction to Phosphorus

Mart Ros1, Karl Czymmek1,2, Quirine Ketterings1
Nutrient Management Spear Program and 2PRODAIRY, Cornell University

In 1999, “What’s Cropping Up?” featured a series of articles on phosphorus (P) and agriculture. At the time, P and water quality was a big topic. New York had just released its first Concentrated Animal Feeding Operation (CAFO) Permit and the United States Department of Agriculture Natural Resources Conservation Service (USDA-NRCS), the New York State Department of Environmental Conservation (NYSDEC), and the New York State Department of Agriculture and Markets (NYSDAM) and Cornell University personnel worked closely together to frame up New York’s version of the Comprehensive Nutrient Management Plan (CNMP) system. This system has continued to evolve and now serves the 4th generation of the New York CAFO Permit that takes effect in July 2017. The P and agriculture series was initiated in recognition of the role of P, not just as a necessary nutrient for crop growth, but also as a as a contributor of P in water bodies to such levels that it can support harmful algal blooms, excessive weed growth and other issues. Proper management of P as a resource is therefore essential, for both economic and environmental reasons.

The articles in the first series on P and agriculture made the case for the New York Phosphorus Index (NY-PI), a tool designed to help identify and better manage farm fields that are at high risk for P runoff. The first NY-PI, which was based on the principles set out in the series of articles, has served the state well, resulting in changes to rates, timing and application methods for manure, among other things. Through changes in fertilizer use and feeding practices on dairy farms, and changes in the way manure is handled, New York agriculture has significantly reduced P imports over the past ten years. Every pound of P not imported onto farms represents reduced risk of loss of P to the environment. Yet, we must continue to look for ways to improve P management on and off farms, to protect New York’s water resources.

Here, we initiate a new series of articles entitled “Phosphorus and the Environment”. Phosphorus as a topic of discussion had died down for a while, taking second place to nitrogen (N) in years with extreme weather. However, because concerns about harmful algal blooms have resurfaced over the past years, and P is usually the limiting nutrient in these freshwater systems, P has returned to the forefront for many. This series of articles will range more broadly than the first. We will address some basic soil P related issues, provide an update on statewide and Upper Susquehanna P balances, and have a closer look at whole-farm nutrient mass balances of dairy farms that are improving sustainability while maintaining or increasing productivity. We will discuss proposed revisions to the NY-PI as well, and touch upon some unconventional topics such as shoreline septic systems and characteristics of human waste, to keep P management at the landscape level in perspective. In this first issue, we provide a refresher on P basics and compare excretion of dairy cows to that of people in terms of total volume, N and P.

Phosphorus is an essential macronutrient which means that plants, animals, and humans cannot go without it; P is a structural element of DNA and it is used in energy transfer processes in plants. Within farming systems, where economic security is directly linked to crop yield, animal health, and milk or meat production, it is crucial these systems have sufficient levels of P. To ensure that farms have an adequate P supply to support healthy animals and crops, P often needs to be imported in the form of fertilizer and/or animal feed.

Phosphate rock is the main source for the P fertilizers that are applied on agricultural fields throughout the world. In its natural state, phosphate rock is not very soluble, making it somewhat ineffective as a direct fertilizer source. This is why phosphate rock is normally ground and treated with sulfuric acid to obtain more effective fertilizer sources like superphosphates. Phosphate rock is mined from pits, and the major part of the global supply is located within just a few countries, such as Morocco/Western Sahara, China, South Africa, United States, and Jordan. Over the past century, the global use of, and dependency on, P fertilizers has increased exponentially. In modern crop-based agriculture, the application of P fertilizers is often standard procedure. However, like other resources such as fossil fuels, sources for rock phosphate are finite. It is uncertain how long these sources will last and predictions about the size and availability of global P reserves vary widely. Some projections estimate that the world’s reserves could be depleted within the next 50 years, whereas others expect they will last for centuries still. Regardless of whether and when we will run out of easily mineable P, expectations are that fertilizer costs will increase over the coming decades. This, combined with the indispensability of P for agricultural productivity calls for careful use of the resource, which is prudent for long-term sustainability.

Figure 1. Phosphorus cycle among mineral, organic and inorganic pools in the soil. Plants require P in solution for optimal growth and production.

Distribution and application of mineral fertilizers and other P sources such as manure over the world varies greatly. This results in different P-related problems depending on location. In large parts of Africa and Australia, soils are very poor and contain little P. In these areas, low P inputs and strong binding of P to soil particles prevent plants from taking up enough P, which can strongly limit crop yield. In other parts of the world, such as temperate climate zones in North America and Europe, P fertilizers and manure have been applied consistently over a long period. For some farm fields, this has resulted in a substantial buildup of P in the soil beyond crop needs (often referred to as ‘residual P’ or ‘legacy P’), which increases the risk of losing the P to the environment. In the case of animal manure, continued excessive application of fertilizer beyond what is already applied with manure can result in unnecessary loss of P to the environment. The application management of manure and fertilizer P can also contribute to P losses, regardless of soil test P level.

One way to improve P management (in cases of P excess as well as deficiency) is through a good understanding of P dynamics in soils. The soil contains many different pools of P. Plants however, can only take up P from the soil solution pool (Figure 1). It needs to be dissolved and in its inorganic form (orthophosphate). The fraction of P that is in solution (and thus directly available) is usually very small. With adequate soil P levels, crops can source much of the needed P from small amounts released from the soil supply over the growing season. This can occur through several processes, like desorption of P from binding agents such as iron and aluminum oxides and clays, the dissolution of P from calcium phosphates, or the mineralization of organic matter. These processes that determine the availability of P to crops all depend on soil characteristics such as pH, organic matter content and soil structure. This is why proper soil management, in addition to P source management (for example, not importing P if it is not needed for animal or crop production), is key to sustaining a healthy, profitable business.

Managing P in soils, on the farm, across the landscape, and in streams and lakes is an extremely challenging job. We need a better understanding of P movement and how management impacts P uptake by plants and loss to the environment, so we can reduce the risk of P loss and improve agricultural production. In the meantime, farmers and other members of the community will be called upon to take the steps they can to reduce P loading to our waterways, where P can be too much of a good thing.

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