Potatoes are the most important root and tuber crop for humans in the world and are a significant economic crop for many farmers. More than a billion people consume potatoes each day. For example, a typical American consumes over 140 pounds of potatoes each year, far more than any other vegetable. Potatoes are currently grown in more than 125 countries, with China and India leading in production.
Proper management of phosphorus for potato production is a critical aspect to success. Potatoes have a relatively high phosphorus demand and a root system that is not particularly well-suited to phosphorus uptake. This topic was the subject of a special symposium that was recently published in the American Journal of Potato Research (Volume 91, Issue 2). This article summarizes some of their key conclusions.
Root Development
The essential role of phosphorus for plants is well-known, but special attention is given to potatoes due to their relatively low phosphorus recovery and efficiency. Potatoes have a rather low total requirement for phosphorus (22 to 40 pounds per acre), but a high requirement for available phosphorus in the soil, indicating low uptake efficiency. Potatoes are also somewhat inefficient in taking up other nutrients. For example, potatoes require six to nine times more available potassium in the soil to reach 90 percent of their yield potential than do crops such as wheat or sugarbeet.
Potatoes have a relatively shallow root system, with the majority of the roots found in the upper 14 inches of soil. Potato roots generally stop development 60 to 90 days after planting, linked closely with the maturation of the crop canopy and the end of new leaf development. As the plants divert resources to tuber bulking, root systems begin to deteriorate, although the nutrient uptake requirement is still relatively high. Potatoes have a relatively low total root length density (about one-fourth of that of wheat), and also have relatively few of the root hairs that are critical for phosphorus uptake. Root hairs account for 21 percent of the total potato root mass, compared with 30 to 60percent in other crops. One study suggested that root hairs account for up to 90 percent of the total uptake by plants when the soil phosphorus concentration is low.
Phosphorus Fertilization
A review of the behavior of phosphorus fertilizer described the major reactions as sorption, precipitation and organic interactions.
Sorption refers to the adsorption of soluble phosphorus to the surface of soil minerals. These reactions include fast and reversible reactions through a ligand exchange. They also include the slower penetration of phosphorus below the mineral surface. The sorption capacity of soil has a strong influence on the amount of fertilizer phosphorus required to meet the nutritional needs of potatoes. Soil tests account for much of the sorbed phosphorus that will become available for plant uptake during the growing season.
Precipitation occurs when added phosphorus fertilizer causes the soil solution to become oversaturated with phosphorus and various solid minerals begin to form. Which specific minerals that form and their persistence depends on many environmental and chemical factors. As time passes, the most soluble of these minerals may dissolve and less soluble minerals may re-crystallize.
Organic phosphorus can contribute to potato nutrition. Much of the phosphorus added to soil in manures and composts is in the form of orthophosphate, which initially behaves similarly to commercial phosphorus fertilizer. Soluble organic matter can inhibit phosphorus sorption in some soils and may also promote accumulation of organic phosphorus compounds, which can serve as a slow-release phosphorus source.
Placement of fertilizer phosphorus is critical for a potato plant to get the most benefit from its application, as the plant roots must grow into the soil zone influenced by the granule or droplet. Placing the fertilizer directly into the root zone increases the probability that a root will intercept the added nutrients. The failure of potato roots to intercept the fertilized micro-sites accounts for the relatively low first-year phosphorus recovery of 10 percent for broadcast fertilizer applications and 35 percent for banded applications. The unrecovered fertilizer phosphorus will contribute to the building of the general phosphorus concentration in the soil and can be used by succeeding crops.
Soil Testing
Soil testing is widely used to predict the need for additional fertilizer phosphorus to meet the demands of the potato crop. Although a variety of methods and extractants are used in different regions, they all determine if soluble phosphorus concentrations are below a critical level where additional phosphorus is required to achieve optimal yield. Soil testing should always be the first step in developing a phosphorus management program where it is available.
Plant tissue analysis is commonly performed to confirm phosphorus adequacy in the developing crop. Excellent resources exist to define the sufficiency of phosphorus concentrations during different stages of growth. When phosphorus deficiencies are found, in-season supplemental phosphorus applications are commonly made to alleviate any nutritional limitations.
The economically justified rates of phosphorus fertilization are much higher for potatoes than for many other crops. The positive yield response to phosphorus fertilizer often provides justification to applying phosphorus, even when the soil phosphorus concentrations are already high.
Phosphorus Stewardship
Since many potato production fields often have relatively high soil phosphorus concentrations, they require special attention to prevent any off-field losses. Potato fields are at risk for loss of soluble phosphorus in surface runoff, particulate phosphorus with eroding soil, and phosphorus leaching in coarse-textured soils. Research has shown the need for special conservation efforts on fields with a 6 percent or steeper slope. Implementing appropriate conservation practices in high-risk areas can minimize loss of phosphorus.
The following seven-point recommendation of best management practices has suggested:
- Begin a phosphorus management plan for potatoes with soil testing to determine the existing phosphorus concentration and establish the need for additional fertilization.
- Base fertilizer phosphorus applications on calibrated potato response data. Excessive phosphorus fertilizer applications should be avoided for economic and environmental reasons.
- Plan to apply a dose of phosphorus fertilizer at planting (with a minimum spacing from seed pieces). Some potato-growing regions recommend only banded phosphorus fertilizer application, while other regions also use broadcast/incorporated phosphorus fertilization along with banded fertilizer.
- The source of phosphorus fertilizer does not generally influence potato performance. Avoid placing ammonium-based phosphorus fertilizers too close to the seed piece at planting.
- Monitor petiole phosphorus concentrations for determining the need for in-season fertilizer applications. In-season foliar phosphorus applications will not satisfy the nutritional requirement of a severely deficient crop. Check the chemical compatibility of fertilizer and irrigation water prior to fertigation.
- Account for all phosphorus sources added to the field, including animal manures and composts.
- Adopt appropriate conservation practices to minimize the loss of phosphorus to water, especially on vulnerable fields with a high risk of loss and with very high phosphorus concentrations.