Keith Reid is a Soil Fertility Specialist with OMAFRA, based at Stratford.
Email keith.reid@omafra.gov.on.ca

The nutrient values of fertilizer vs. manure

by KEITH REID

Nutrient Balance. The first and most obvious difference between the two is that fertilizer can be blended to meet the exact requirements of the crop, while the nutrients in manure come mixed together. This means that a farmer applying hog manure to supply all of the nitrogen needs of a corn crop will add more phosphorus than the crop will remove, resulting in a build-up of phosphorus in the soil. This is desirable in fields where the P soil test is low, but not in fields where the phosphorus level is already high.

Nutrient Concentration. The other obvious difference between these materials is the amount of material that needs to be applied to supply the nutrient needs of a crop. As an example, urea fertilizer contains 920 pounds of nitrogen per ton and all of it is considered to be available. Solid beef manure contains about 14 pounds of nitrogen per ton and, depending on the application timing, between 20 and 40 per cent is available to the crop in the year of application. This means that it takes between 165 and 330 tons of solid beef manure to equal the available nitrogen in one ton of urea.

This difference has huge implications for the cost of applying the nutrients, and in the potential for causing soil compaction during application.

Nitrogen. Nitrogen fertilizers are formulated so that most or all of the nitrogen is immediately available to the crop. This makes it easy to meter the fertilizer precisely to meet the needs of the crop. In contrast, the nitrogen in manure is a mix of ammonium nitrogen and organic nitrogen. The proportion of each depends on the species of livestock and the way the manure is handled. Solid manure will have more organic N, while liquid manure, particularly liquid hog or poultry manure, will have more ammonium N. The ammonium part is the same as fertilizer and is chemically identical to aqua ammonia.

If manure is not incorporated immediately, however, part of this fraction is lost to the air as ammonia gas, reducing the amount of nitrogen available to the crop. The organic part is broken down by microbes in the soil to release the nitrogen in forms available to the crop, but the speed of this process depends on the type of manure and the temperature and moisture content of the soil.

Phosphorus. The phosphorus from manure was thought to be only 40 per cent as available as phosphorus from fertilizer. Recent studies with liquid swine manure would suggest that the availability is much higher, and may even be equal to fertilizer P, when comparing both materials mixed evenly through the soil.

Again, the difference in the field will come down to the concentration of nutrients in the manure and the placement of the nutrient. A starter fertilizer band will have a much higher concentration of phosphorus than a broadcast application of manure, so it would be difficult to show a starter effect from manure. Other experiments have shown, as well, that some manure contains phosphorus that is less soluble than fertilizer P. What is clear is that, while the value of the phosphorus to the current crop may be variable, manure is a very good material for building up the reserve of available phosphorus in the soil.

Other Nutrients. The potassium in manure is in very soluble forms, so manure K is almost as available to crops as fertilizer K. The biggest headache has been that some manure is very high in potassium, which can lead to high soil K levels and mineral imbalances in forages for dry cows.

Manure also contains a wide range of secondary and micro-nutrients. These may be in the mineral or organic form and are slowly released to crops. It is rare to see micro-nutrient deficiencies in crops grown with manure, even in areas where these deficiencies are common in conventionally fertilized fields. If deficiencies do show up, they can generally be traced to restricted root systems from compaction during manure application.

Organic Matter. The hardest benefit to quantify from manure is the effect of the organic matter. The difference in crop growth in a dry year is obvious, with manured fields showing much less drought stress than neighbouring fields that did not receive manure. It has been much more difficult to translate this into a consistent, measurable yield difference and to put a dollar value on this part of the manure.

My personal guess is that the value of the organic matter is probably equal to the nutrient value of the manure, although it may be much higher. The amount of value will be greatest in soils low in organic matter to start with. This means there could be a much greater yield difference with manure applied to a field which has been "cropped out", than in a field which has received regular manure applications in the past. BF

Keith Reid is a Soil Fertility Specialist with OMAFRA, based at Stratford. Email keith.reid@omafra.gov.on.ca

So what's the problem with nutrients?

by KEITH REID

The problem is that a lack of clarity on what does, or doesn't, represent a risk will, inevitably, lead to public pressure for restrictions that aren't effective in reducing environmental risk.

There are a couple of key points that I would like to make before discussing the specific environmental risks from nutrients. The first is that nutrients absorbed by crops do not pose any environmental risk, so crop production practices that optimize the agronomic use of nutrients will also be environmentally friendly. Only when nutrients are applied beyond crop requirements is there a risk of off-site impacts.

The second point is that an excess of nutrients on a particular farm does not indicate that the farmer is a poor manager. The economic pressures of modern agriculture have meant specialization in either crop or livestock production. Instead of a traditional farm where crops are fed to livestock on the same farm that receives the manure, many new operations are importing feed that has been produced on cash crop farms. This means that there is a much greater nutrient loading in the manure than was the case in the past.

The most glaring example of this pattern is the northeastern United States, where corn and soybeans from the cornbelt states are fed to hogs, poultry and dairy cattle, resulting in a huge net import of nutrients into these states. Ontario is nowhere near this situation, with a very close balance between nutrient imports and exports, but there are local situations where this condition is repeated in miniature.

Nitrogen is not inherently toxic. Nearly 80 per cent of the air we breathe is nitrogen gas. There are two situations, however, where nitrogen can be a concern. The first is ammonia nitrogen in surface water. If the concentration of ammonia gets too high in rivers or lakes, it is toxic to fish and other aquatic organisms. Since ammonia binds to soil particles, it does not normally move off of the soil and into water. The most common cause of elevated ammonia is the runoff of raw manure into surface water. This is particularly the case with liquid manure, which has a large part of its nitrogen in the ammonium form.

The second situation of concern is where nitrate moves into groundwater. Where groundwater is used for drinking, this is a human health issue, particularly for very young children, since nitrate can bind to the hemoglobin in the blood and lead to "blue baby" syndrome. The problem is primarily in groundwater because the amount of biological activity in surface water will break down the nitrate quickly, while it persists in most aquifers.

The nitrate ion is negatively charged, so it does not stick to soil particles. This means that whenever there is water moving down through the soil profile, it can carry nitrate towards the aquifer below. This will occur most quickly in coarse-textured soil, and primarily during the winter and early spring when there is more precipitation than evaporation.

The other necessary ingredient for leaching to occur is nitrate in the soil, and this is a result of applying more nitrogen than the crop requires, or applying nitrogen in the fall. Careful management is required on sensitive soils to reduce the risk of this occurring.

Phosphorus, however, is not a human health issue. It almost never moves into groundwater, but when it is present in surface water at high enough concentrations it encourages excessive growth of algae and other aquatic vegetation. When this vegetation dies and decomposes, it uses up the oxygen in the water, resulting in the suffocation of fish, particularly the desirable sports fish. The algae also impart an unpleasant colour and odour to the water, reducing its usefulness as drinking water or for recreation.

Phosphorus binds tightly to soil particles, so most of the phosphorus entering the water is carried on eroded soil. Preventing soil erosion is one of the key methods for reducing phosphorus loading in surface water.

Concentrated sources of phosphorus, such as manure or fertilizer, can contribute significant loadings of dissolved phosphorus, so incorporating or injecting these materials will also reduce phosphorus losses.

As for other nutrients, potassium has not been identified as causing any environmental harm. So even though it is present in manure in large amounts, there is little to be gained by imposing limits on potassium applications. Some of the metals that are plant nutrients (such as zinc or copper) can present an environmental risk if they are present in high enough levels. Research is now being done to see if any problems are arising out of nutrient applications that contain these metals. BF

Keith Reid is a Soil Fertility Specialist with OMAFRA, based at Stratford.
Email keith.reid@omafra.gov.on.ca

Starter fertilizer: it really is worth the bother!

by KEITH REID

The basic concept of starter fertilizer is simply to put the fertilizer where and when the crop needs it most without "over-feeding." Most crops have a high demand for nutrients when they are small and actively growing, but the root systems are still small so a concentrated source of nutrient close to the seedling can really fill the gap.

An analogy with vitamin pills might help here. Young children should be able to get all the vitamins and minerals out of the food they eat, but when they are growing quickly or if their diet isn't perfect, they may run short on something. Vitamin pills make up for this shortfall while the demand is high, the same way that starter fertilizer carries the plant until the root system gets well established. The better the child's diet, the less they need the supplements; the higher the soil test, the less likely the crop is to respond to starters.

To carry the analogy further, if a child decides to swallow a whole bottle of vitamins, they can be toxic, just as too much starter fertilizer can be toxic to young plants. I won't go any further with this parallel, though, as it will soon start breaking down.

Most starter fertilizers have a high concentration of phosphorus. This is the nutrient required by seedlings in the highest concentration and also the nutrient that is tied up by the soil. Placing phosphorus in a band rather than mixing it with the bulk soil keeps a higher proportion in soluble form until the roots can take it up. Placing the phosphorus below the soil surface also protects it from surface run-off, which is an environmental benefit. Including a bit of nitrogen with the phosphorus increases the P uptake.

Potassium in starter fertilizer can increase the K availability relative to surface application in no-till, but it also increases the risk of fertilizer burn. Starters for no-till corn often provide more nitrogen to overcome the slow mineralization of nitrogen in cooler soils.

The response to starter fertilizer will vary from crop to crop, depending on when the crop has the greatest demand for phosphorus and how sensitive it is to fertilizer injury. The grass crops (corn and cereals) initiate the cob or the head while the crop is still in the seedling stage, so a shortage of phosphorus in the seedling can reduce the yield potential.

Soybeans, on the other hand, don't start to initiate flowers until the root system is well established, and continue to produce more flowers as long as the plant is putting out new leaves. Soys respond to the total amount of available phosphorus in the soil, but aren't nearly as fussy about having it early. In addition, soybeans are much more sensitive to fertilizer burn, so any increased growth from the starter is offset by a reduced plant stand.

The upshot is that corn and cereals will respond to starter fertilizer much more reliably than soybeans or field beans. There are some broadleaf crops that also respond to starters, alfalfa being the most notable example.

Much has been said about where to put starter fertilizer, and there are supporters for both seed placed and 2x2 bands. However, this argument only applies to crops planted in wide rows, since there isn't good technology yet for side-banding a starter in solid-seeded crops. Fortunately, the lower concentration within the band with a solid-seeded crop means that fertilizer burn is a much lower concern.

Placing fertilizer in a band two inches below and two inches to the side of the seed has been the traditional method of applying starter fertilizer, especially for corn. This placement will be intercepted soon after the nodal roots of the seedling start to grow downwards at an angle, giving the plant a boost early in its growth. It is also far enough away from the seed that relatively high rates of fertilizer can be applied without causing burn, which can be an advantage when most, or all, of the supplemental fertilizer for the crop is being applied at planting. This is also a drawback, since high fertilizer rates also mean frequent stops to fill the planter in a season when time is usually at a premium.

Seed placed fertilizer is available as soon as the first roots emerge, and does result in a consistent increase in early growth and vigour. The rates applied must be much lower to avoid fertilizer burn, but this also means that a larger area can be planted between planter fills. Both placements will provide yield increases; one recent study in southwestern Ontario showed that adding both seed placed and 2x2 bands gave greater yield increases than either on their own. Choosing between them will depend more on what fits with the rest of the cropping system than on which gives the biggest yield.

Beyond choosing a fertilizer that is high in phosphate, the main factors in choosing starter fertilizers are the salt index and the amount of nitrogen. Highly soluble fertilizers normally have a propensity to cause salt injury, so the amount of nitrogen and potash should be limited in starter fertilizers. High nitrogen fertilizers also commonly include di-ammonium phosphate or urea.

Both of these materials can release free ammonia in the soil, which is toxic at the high concentrations in a band. Monoamonium phosphate (MAP) or ammonium polyphosphates are safer forms of starter fertilizer. BF

Keith Reid is a Soil Fertility Specialist with OMAFRA, based at Stratford.
Email keith.reid@omafra.gov.on.ca