Soil Phosphorus Testing for Crop Production and Environmental Purposes

Increasing animal production in the state or its concentration in certain areas is increasing the amounts of manure being applied to the land. Often, the manure is applied at rates or at a frequency that exceed the phosphorus (P) needs of crops and even the amount removed in harvested products. Animal manure can supply the nitrogen (N) and P needed by crops as well as other nutrients. Due to its relative N and P content and potential N losses, however, continued use of rates that supply the N removed in com grain (and in soybean grain too) results in P accumulation in soils. Accumulation in excess of crops needs may increase the potential for eutrophication of surface waters. Eutrophication means that nutrient levels in water, especially P, are high and excessive algae growth occurs, which could create imbalances in the water ecosystem and the esthetic value or water bodies such as lakes or streams. This problem is compounded because soils of many grain crop production areas already have soil-test P levels that are at or above levels that optimize grain yields. The upper limit for amounts of manure that could be applied with minimum environmental pollution could be ultimately determined by the P level in the topsoil and the potential for soil erosion, water runoff, or P leaching through the soil profile that can reach surface (through tile flow) or groundwater. This issue is calling for better estimates of P in soils, especially in manured soils, not only for crop production purposes but also for environmental resources conservation. Thus, questions have arisen concerning soil P testing for agronomic purposes or environmental purposes. A major question is the relative importance of a soil test value in terms of potential losses of P with erosion and runoff and if upper soil P limits can be reasonably determined to minimize P pollution of surface water supplies. A discussion of these alternatives necessitates a preliminary discussion of some basic aspects of P in soils, of the various types of tests, and their potential use for environmental purposes in comparison with routine soil tests for crop production

Phosphorus and Potassium Management for 2020 after a Year with Highly Variable Yields

Very variable weather this season will result in variable corn and soybean yield within and between fields. Entire fields or portions of fields were planted later than usual or were not planted. This will result in variable removal of phosphorus (P) and potassium (K) with harvest. Reduced yields combined with continued low market values create uncertainty about P and K fertilization decisions for the 2020 season

Fertilization Can Help with Cool Soils and Late Planting Dates

With delayed spring weather and low or uncertain grain prices, farmers and crop consultants are asking questions about starter fertilizer for corn this spring. The placement of small amounts of plant nutrients in bands offset to the side and below the seed row or in the seed furrow increases the concentration of nutrients near seedling roots. Common starter fertilizers have nitrogen (N), phosphorus (P), and potassium (K) and sometimes sulfur (S) or micronutrients. Research in Iowa and the north central region has shown that early plant growth increases from starter fertilizer are common and can be large in corn but are uncommon and small in soybean

Upper Soybean Leaves Began ShowingPotassium Deficiency Symptoms Since EarlyAugust in Some Iowa Fields

Since early August, soybean in several fields began showing typical potassium (K)deficiency symptoms on leaves located in the middle to upper canopy. This is notsurprising in fields or portions of fields with soil-test values in the very low or low K soil-test interpretation categories that did not receive adequate preplant K fertilization.Potassium deficiency symptoms are well-known and very common in older leaves duringearly growth stages. Due to poorly understood reasons, during the last couple of decades Kdeficiency symptoms in upper soybean leaves also have become common at middle to latereproductive stages. Moreover, K deficiency symptoms can develop in upper leaves in well-fertilized soybean when no deficiency was observed at early stages, mainly when droughtconditions develop during late spring or summer

Using the Iowa Phosphorus Index for Agronomic and Environmental Management of Fertilizer and Manure Phosphorus

The Iowa phosphorus (P) index is a risk assessment tool that was developed to assess potential P loss from fields to surface water resources. In 1999, the United States Department of Agriculture Natural Resources Conservation Service (NRCS) issued a national policy and general guidelines on nutrient management to include risk assessments for P These guidelines apply to nutrient management where nutrients are applied with organic by-products and animal manure sources. The national guidelines suggested the use of soil-test P values, threshold limits, or a P risk index. The Iowa State Technical Committee chose the P index approach based on advice from an interdisciplinary task force that involved scientists, technical personnel, and representatives from various interest or commodity groups. All NRCS staff or third-party technical service providers use this guidance when providing financial or technical assistance to producers enrolled in several federal conservation programs. The Iowa legislature mandated the Iowa Department of Natural Resources (IDNR) to begin using the Iowa P Index in late 2003 for manure management plans for confined animal feeding operations. The specific rules and guidelines are being prepared by IDNR at this time. The purpose of this presentation is to provide a brief overview of how the Iowa P index can be used to identify reasons for high risk of P loss from fields and to choose among alternative management practices for improving agronomic and environmental P management

Soybean Potassium Deficiency Symptoms During Early and Late Growing Stages

Since early August, farmers and consultants have been reporting what they believed were potassium (K) deficiency symptoms in soybean leaves located in the middle or upper canopy. This is not surprising in fields or portions of fields with soil-test values in the very low or low K interpretation categories. Moreover, K deficiency symptoms could develop at these growth stages with drought conditions, even in fields with adequate soil-test K levels. Sometimes symptoms occur in late summer with rainfall events after a dry period. Potassium deficiency symptoms are very common and well known at early growth stages, but due to poorly understood reasons, in the last couple of decades deficiency symptoms in upper leaves at middle to late reproductive stages also have become common

Management Zones Soil Sampling: A Better Alternative to Grid and Soil Type Sampling?

Global positioning systems (GPS), yield monitors, various forms of remote sensing, new computer software, and variable rate technology are new tools available to producers. Intensive soil sampling and crop scouting methods that directly relate each sample or measurement with their geographical coordinates complement the new technological package. Soil testing is a diagnostic tool that adapts well to site-specific management because it can assess nutrient availability of different areas within a field. Intensive sampling, soil test mapping, and variable-rate application of fertilizers or manure can improve the efficacy of nutrient management compared with the conventional practice of applying a uniform rate across a field. However, the spatial variation of nutrients within fields makes soil sampling one of the most important sources of error in soil testing, and may also be a limiting factor for optimal use of these new technologies. Traditional soil sampling recommendations are changing to adapt to the new precision agriculture technologies. This presentation discusses soil sampling methodology and summarizes results of Iowa research conducted in assorted producers\u27 fields. This research complements other research (not discussed here) studying the effectiveness of variable-rate fertilization

Is Tissue Testing Useful in Identifying Corn and Soybean Fields Responsive to Phosphorus and Potassium Fertilizers?

Producers often ask questions about tissue testing to decide emergency in-season phosphorus (P) and potassium (K) fertilization for corn and soybean. Previous articles(June 2010 and July 2008) have discussed the viability of post-emergence application of P and K fertilizers. A large application of granulated P or K fertilizer to soil during the very early crop growth stages may result in some grain yield increase although an economic benefit for the current crop is not likely. The probability of an economic response to foliar fertilization is likely with insufficient preplant fertilization or when soil and climate factors (other than drought) limit nutrient uptake. Traditionally, farmers and crop consultants have used soil sampling and testing of visually affected and seemingly unaffected field areas to determine if there is a nutrient deficiency. This is an effective practice when crop growth is limited by low soil nutrient supply, but will not be effective when soil or climatic factors other than low soil nutrient levels limit early nutrient uptake

Fertilizer placement for ridge-till and no-till systems

Uncertainty about cost-effective methods of broadcast vs. subsurface fertilizer application and soil test interpretation for ridge-till and no-till systems may be a major factor in the slow adoption or abandonment of these systems by Iowa farmers. Broadcast fertilizer is cheaper than banding, but seems inefficient because fertilizers are not incorporated (no-till) or incorporated too late (ridge-till) for plant needs. From a soil and water quality perspective, the lack of incorporation of fertilizers, especially phosphorus fertilizer, could hamper system benefits in reducing nutrient losses in water runoff because, although reduced, the runoff contains more soluble nutrients

Why are potassium deficiency symptoms showing now in corn and soybeans?

Some corn and soybean fields, mainly in eastern and southern Iowa, are showing potassium (K) deficiency symptoms, even when growth stages range from V3 to V8. The symptom for both crops is yellowing of the leaf margins of the older leaves that usually begins at the leaf tip and extends down the margins toward the leaf base. With severe deficiency the leaf edges may become brown and necrotic, although the newest leaves usually have normal coloration. For further information of symptoms, see Corn leaf potassium deficiency symptoms (ICM 7/1/2002), Is it iron or potassium deficiency? (ICM 7/1/2002), and ISU Extension and Outreach publication Nutrient Deficiencies and Application Injuries in Field Crops (IPM 42)