By Charles White
Crops need the macro-nutrients nitrogen (N), phosphorus (P), and potassium (K) to grow and thrive, among a host of other secondary and micro-nutrients, healthy soils, and of course good weather. These macronutrients can be supplied by fertilizers, by the reserves already in the soil, or by manures produced on or off the farm. With N and P fertilizer prices extending to the highest levels we have seen in recent years, now is a good time to consider the stock of nutrients already in the soil and the value that nutrients in manure can provide.
Soil testing is the traditional method of determining the availability of soil nutrient levels in relation to crop requirements. The chemical extracts used to measure the nutrients in the soil don’t actually extract a quantity of nutrients that is equivalent to the same pounds of nutrients in raw fertilizer, rather the extracted nutrients provide an index of the soil’s ability to supply nutrients to the crop over the growing season. The soil test levels are calibrated through field experimentation to determine at what soil test level there are sufficient nutrients already in the soil to be able to meet the needs of a crop without the need for a fertilizer addition. This soil test level is called the "critical level" and in the Penn State soil test interpretation categories (below optimum, optimum, above optimum), the critical level is at the bottom of the optimum range. For P, the critical soil test level with a Mehlich 3 extract is 30 ppm P, and the optimum range extends from 30-50 ppm. For K, the critical soil test level with a Mehlich 3 extract is 100 ppm K, and the optimum range extends from 100-150 ppm for most grain crops and 100-200 ppm for most forage crops.
The fertilizer recommendation philosophy used by Penn State and many other commercial labs call for building soil test levels into the optimum range and maintaining levels at that range by applying nutrients to match crop removal rates. This philosophy provides for good economic returns in the long run. In a year when fertilizer prices are high, you might consider reducing fertilizer application rates due to cost or lack of availability and if you followed the "build" component of this philosophy, you may have a buffer. Theoretically, if the soil test level of a nutrient is at or above the critical level (the bottom of the optimum range), then you could grow a full yielding crop this year without adding that nutrient at all. The exception to this is using a small amount of P in starter fertilizer, which can still provide a benefit due to relatively low availability in early spring when crops are planted. You can’t continue growing crops without fertilizer forever; eventually, soil test levels will fall below the critical level and yields will suffer, but you can weather through temporary price spikes and supply shortages if your soil test levels are in the optimum zone to begin with.
Manures are also a great source of N, P, and K, and for better or worse, the Mid-Atlantic region is blessed with an abundance of it. Do you calculate the value of the nutrients contained in that manure and then determine which fields to spread on to get the most value? Or have you been taking those nutrients for granted and just spreading blindly? This may be the year to take pencil to paper and figure out the most strategic allocation of manure possible, especially if you are in a situation where you can use manure to offset purchased fertilizer.
To get the best value from the nutrients in manure, you need to know both the manure nutrient analysis, as well as the soil’s requirements for nutrients based on a soil test. Manure nutrient analyses based on typical values for a given animal and manure type are provided in the Penn State Agronomy Guide and are available from the
Nutrient Management Education Program. For a more accurate nutrient analysis, you can have manure samples analyzed for their nutrient content at most labs that provide soil testing services.
The nutrients in manure only have an economic value if they replace fertilizer that would have otherwise been purchased, and you can’t accurately determine whether you need fertilizer without a soil test for each field (see discussion above about soil test critical levels). Once you have soil test results for each field (results from within the last 3 years should still be valid), you can then determine where P and K is needed and where it isn't. The most strategic and profitable place to apply manure are on fields that need all three nutrients, N, P, and K, as you are deriving value from all the nutrients in the manure. The next most profitable places for spreading would be fields that need just N and P, followed by just N and K or P and K, and finally fields that only require a single nutrient based on soil test recommendations.
The other way that soil testing can help to maximize the profitability of manure usage is by allowing higher manure application rates to meet the full N demand of crops, rather than just the P demands. The regulations governing manure usage in Pennsylvania require that to apply manure at a rate to meet the N requirements of a crop, which usually over applies P, a soil test must be taken, and the soil test P level must be lower than 200 ppm P, or a P Index assessment must be completed showing that the field is in the low or medium risk category for P losses. Some other restrictions may apply, especially during the winter manure spreading period. Nonetheless, armed with soil test P levels of less than 200 ppm and verifying compliance with all other applicable restrictions, you can apply manure at the N balanced rate, fully offsetting N fertilizer purchases. In this scenario, over applications of P and K will be stored in the soil for use by future crops in the rotation, perhaps offsetting future P and K fertilizer purchases. This can be a sustainable nutrient management system as long as you don’t continue applying N-based rates of manure every year.
Determining the value of manure as a replacement to fertilizer takes some thought and advanced planning to gather the necessary soil test information for each field. When nutrient prices are cheap, it may not be worth the effort to go through this process, and you might be able to maintain profitability managing nutrients with purchased fertilizers and manure applications based on convenience. But when nutrient prices get to the levels we're seeing now, a little bit of planning and strategic thinking can effectively offset fertilizer purchases and go a long way in reducing the costs of production and increasing the profitability of your operation.
Source : psu.edu