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

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

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

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)