Fertility and Nutrients
Managing plant nutrients
By Dr. Thomas L. Jensen
There are three main sources of plant nutrients available to farmers. They are commercial fertilizers, livestock manure, and municipal sewage sludge or biosolids. There is a preconceived notion that a farmer will use only one of these sources to supplement nutrients supplied by soils. However, these sources can be used in combinations very successfully.
When considering the weight of material applied to soil, manures are the greatest, followed by fertilizers, and lastly biosolids (see Table 1). However, the actual amount of nutrients supplied with fertilizer is still highest because of their greater concentrations compared to organic sources. Typical liquid dairy manure has a fertilizer analysis of 0.4 percent N, 0.2 percent P2O5, and 0.5 percent K2O. A typical NPK fertilizer blend, used for a small grain crop, would have a blend analysis of 22-9-18 – or about 50 times greater concentrations of plant nutrients compared to liquid dairy manure.
Fertilizers are much less expensive to transport compared to manure in order to supply the equal amounts of nutrients to a field. For this reason, the amount of nutrients supplied annually worldwide through fertilizer is 20 times greater compared to that applied as biosolids and manure combined. Plus, only a small percentage of cropland receives manure. In Canada, only about four percent of cropland receives manure applications compared to 96 percent that only receives fertilizer. This excludes some organic crop production land that receives neither manure nor fertilizer.
One disadvantage of using only manure compared to a combination of manure and fertilizer is that the balance of nutrients can be less than ideal. For example, a barley crop yielding 100 bu/ac (5300 kg/ha) will take (per acre) 100 lb N, 42 lb P2O5, and 158 lb K2O from the soil. This equates to a N:P2O5:K2O uptake ratio of 2.7:1:3, which translated into a higher N requirement than can be supplied from typical manures. To better supply N, P and K to a barley crop, a farmer could apply additional N fertilizer along with manure to more closely approach the actual nutrient-use ratio of the crop.
Another aspect of manure use is the challenge of applying it at rates that match annual crop use. Solid beef manure may commonly have a nutrient content of 0.8 percent N, 0.32 percent P2O5, and 0.7 percent K2O. When considering the N and P in this manure, only about one-third is mineralized to a plant-available form in the year of application. It would be necessary to apply 19.5 tons of the beef manure to equal a common fertilizer application rate (per acre) of 100 lb N, 42 lb P2O5, and 33 lb K2O required for a 100 bu/ac barley yield goal. This amount of beef manure would also supply 270 lb K2O, as almost all of the K in manure is plant available during the year of application. Land that regularly receives manure as the sole nutrient source tends to have plant-available P and K that accumulates to levels in excess of crop need. Careful use of manure at rates not exceeding N requirements also limits N losses to the environment due to leaching and gaseous emissions.
What can work well for farmers is to apply manure roughly once every three years. In the year of application, manure is applied at rates that are close to crop requirement. For the next two years, only N fertilizer is required as residual P and K from the manure usually satisfies crop demand. Low rates of P and K might be used as seed-row starter fertilizer to get crops off to a good start under cool spring soil conditions. This approach works well to grow adequate yields while avoiding excess buildup of soil P and K.
It is recommended that soil nutrient content be monitored annually or at least every three years, by having representative soil samples analyzed, and the results interpreted by a qualified consulting agronomist or certified crop advisor. How often manure can be applied and what rates of fertilizer nutrients are required each year will need to be adjusted depending on the crop to be grown, crop yields, nutrient removal in harvested grain or forage, the manure type and its nutrient content, and weather conditions. By using a combination of manure and fertilizer applications, crop nutritional needs can be met while avoiding excess levels of nutrients in your soil.
Dr. Thomas L. Jensen is Director, Northern Great Plains, International Plant Nutrition Institute (IPNI). Reprinted with permission from IPNI Plant Nutrition Today, Spring 2012, No. 2.