7.4: Flow Patterns at Farm Scale

With relatively undisturbed forests or grasslands, the nutrients used by plants are mostly cycled back with leaf litter and the periodic dieback of roots. Carbon flows are different from the cycling that occurs when nutrients such as nitrogen, phosphorus, potassium and calcium are taken up from soil by plants, used and then returned to soil. Carbon enters the field as plants use atmospheric CO₂ to carry out photosynthesis, providing the basis for all the various chemicals needed for their growth and reproduction. The portion of the plant that remains after harvest is thus added to the soil as “new” carbon in the form of organic residues; commonly this represents as much or more organic matter than was decomposed by organisms during the year.

When considering the whole farm, there are three main nutrient flow patterns, each one with implications for the long-term functioning of the farm and the environment: 1) imports of nutrients are less than exports, 2) imports are greater than exports or 3) imports are equal to exports.

Figure 7.5. Nutrient flows and cycles on (a) a crop farm, (b) a grass-fed beef or other integrated crop-livestock farm, and (c) a dairy farm.

Imports are less than exports. Farms with a negative nutrient balance are “living off capital” and drawing down the supplies of nutrients from minerals and organic matter. This can continue for a while, just like a person can live off savings in a bank account until the money runs out. But at some point, the availability of one or more nutrients or organic matter (carbon) becomes so low that crop yields decrease. If this condition is not remedied, the farm becomes less and less able to produce food, and its economic condition will decline. This is clearly not a desirable situation for either the farm or the country. Unfortunately, the low productivity of much of Africa’s agricultural lands is partially caused by this pattern of nutrient and carbon flows, as increasing populations put pressure on farmers to increase land-use intensity, fertilizer prices are high for poor farmers and little attention is paid to soil organic matter. In previous times under the system of shifting cultivation, agricultural fields would have been allowed to return to forest for 20 or more years, during which time there would have been a natural replenishment of nutrients and organic matter. One of the greatest challenges of our era is to increase the fertility of the soils of Africa, both by using fertilizers and by using ecologically sound practices that increase soil health.

Imports and exports are close to balanced. From the environmental perspective and for the sake of long-term soil health, fertility should be raised to, and then maintained at, optimal levels. The best way to keep desirable levels once they are reached is to roughly balance inflows and outflows. Soil tests can be very helpful in fine-tuning a fertility program and making sure that levels are not building up too high or being drawn down too low (see Chapter 21). This can be a challenge and may not be economically possible for all farms. Farms that exclusively grow grain or vegetables have a lot of nutrients flowing onto the farm and relatively high annual carbon and nutrient exports when crops are sold (Figure 7.5a). Nutrients usually enter these farms as either commercial fertilizers or various amendments and leave the farm as plant products. Some cycling occurs as crop residues are returned to the soil and decompose. But a large outflow of carbon and nutrients is common on farms that sell considerable volumes of grains and vegetables per acre. For example, the annual export of nutrients is about 135 pounds of nitrogen, 25 pounds of phosphorus and 35 pounds of potassium per acre for corn grain and about 150 pounds of nitrogen, 20 pounds of phosphorus and 130 pounds of potassium per acre for grass hay. An acre of tomatoes or onions usually contains over 100 pounds of nitrogen, 20 pounds of phosphorus and 100 pounds of potassium. Generally, 50–60% of the carbon is harvested and exported off the farm, which in the case of corn grain amounts to about 3 tons of carbon per acre per year. But, of course, the whole point of farming in a modern society is to produce food and fiber for the non-farming public. This by necessity implies the off-farm export of carbon (sugars, starches, proteins and so on) and crop nutrients.

It should be fairly easy to balance nutrient inflows and outflows on crop farms, at least theoretically, but carbon cycling is difficult. In practice, under good management, nutrients are gradually depleted by crops until soil test levels fall too low, and then they’re raised again with fertilizers. But leftover residue (carbon-based plant material in aboveground residue and roots) from annual crops doesn’t normally replace the organic matter lost during the year of cropping. Replenishing extra soil carbon occurs only when applying organic fertilizers like manure or compost, through intensive cover cropping, or by adding perennial hay (grass/legume) crops to the rotation.

On integrated crop-livestock farms that produce their own feed, imports and exports of nutrients should be relatively small relative to the land farmed and close to balanced. Few nutrients or carbon leave the farm (they leave only as sold animals) and few are brought onto the farm (Figure 7.5b). Most of the nutrients on this type of operation complete a true cycle on the farm: They are taken up from the soil by plants, which are eaten by the animals, and most of the nutrients are then returned to the soil as manure and urine. And most of the carbon fixed by plants stays on the farm with crop residues and animal manure. A similar flow pattern with few nutrients coming onto the farm and few leaving occurs on a grass-fed beef operation that uses little to no imported feed.

It is easier to balance nutrient imports and exports on a mixed crop-livestock and grass-fed beef farms than on either a crop farm or a livestock farm that depends significantly on imported feeds. So if all the feeds are farm grown, adding an animal enterprise to a crop farm may lower the nutrient and carbon exports (Figure 7.5b).

Imports are larger than exports. Animal farms with inadequate land bases to produce all needed feed pose a different type of problem (Figure 7.5c). As animal numbers increase relative to the available cropland and pasture, larger purchases of feeds (containing nutrients) are necessary. As this occurs, there is less land available, relative to the nutrient loads, to spread manure. If the excess manure is not moved to another farm, the operation may exceed the capacity of the land to assimilate all the nutrients, and pollution of ground and surface waters occurs. For example, in a study of New York dairy farms, as animal density increased from around 1/4 of an animal unit per acre (1 AU = one 1,000-pound animal, or a number of animals that together weigh 1,000 pounds) to over 1 AU per acre, the amount of N and P remaining on farms increased greatly. When there was 1/4 AU per acre, imports and exports were pretty much in balance. But at 1 AU per acre, around 150 pounds of N and 20 pounds of P remained on the farm per acre each year.

Many dairy farms do not have the land base to grow all their needed feed and tend to emphasize growing forage crops. But the cows also need grain supplements and this situation involves additional sources of nutrients coming onto the farm. Concentrates (commonly mixtures containing corn grain and soy) and minerals usually comprise a larger source of nutrient inputs than fertilizers. In a study of 47 New York dairy farms, an average 76% of nitrogen came onto the farms as feeds and 23% as fertilizers. The percentages were pretty much the same for phosphorus (73% as feeds and 26% as fertilizers). Most of the nutrients consumed by animals end up in the manure, from 60% to over 90% of the nitrogen, phosphorus and potassium. A portion of carbon even comes onto the farm in purchased concentrate feed, and sometimes as bedding for the cows. The nutrients and carbon in manure that came from farm-grown feed sources are completing a true cycle. But the portion of nutrients in manure that originally entered the farm as purchased feeds and mineral supplements are not participating in a true cycle. These are completing a flow that might have started in a far-away farm, mine or fertilizer factory and are now being transported from the barn or feedlot to the field.

Compared with crop farms, where a high percentage of the crop grown is sold, fewer nutrients and carbon flow from dairy farms per acre and more stay on the farm, either completing a true cycle (soil to plant to animal to soil) or completing a flow (imported concentrate feed and minerals to cows to manure to soil). Because of the additional feed imports, nutrients will tend to accumulate on the farm and may eventually cause environmental harm from excess nitrogen or phosphorus. This problem of continual nutrient buildup exists for any animal farm that imports a significant percentage of its feed. The reliance on perennial forages plus imported feed and minerals, and certain types of bedding material, may increase carbon (soil organic matter) levels in the soil until they reach the soil’s saturation level. To put it another way, these farms don’t have an adequate land base to produce all their feed and therefore also have an inadequate land base on which to apply their manure at environmentally safe rates. The ultimate situations of this kind are found with animal operations that import all feeds and have a limited land base to use the manure; these have the greatest potential to accumulate high amounts of nutrients. Contract growers of poultry, with tens of thousands of chickens and few acres of land, are an example of this.

If there is enough cropland to grow most of the grain and forage needed, the result will be low amounts of imported and exported (as animal products) nutrients. It is therefore easier to rely on nutrient cycling on a mixed livestock-crop farm that produces most of its feed than on a farm growing only crops. An alternative is exchanges among neighboring farms. Since crop farms tend to have nutrient and carbon deficits, and livestock farms have excesses, transferring the excess manure or compost offers opportunities for more cycling and less environmental losses, as well as for improving soil health on the recipient farm (see Chapter 12).

The situation of imports greatly exceeding exports does not only occur on animal farms without sufficient land to grow all the needed feed. Organic vegetable farmers commonly import composted manure to supply nutrients and maintain or increase soil organic matter levels. In a survey from 2002 through 2004 of 34 organic farms from seven states in the Northeast, approximately half of the fields were found to have excessive levels of phosphorus. Other ways need to be found to add organic matter through on-farm practices such as the use of green manures, cover crops and rotations with perennial forages.

N Balance As An Environmental Indicator

Higher nutrient imports than exports is not limited to livestock-based operations, especially with nitrogen. Most grain farms in the developed world import more nitrogen than they export through their crops, meaning the N balance is positive. As we discuss in Chapter 19, nitrogen is difficult to manage and some losses as nitrate leaching and N₂O gaseous losses are unavoidable. The extent of losses is heavily dependent on how the farm manages the nitrogen through good timing and rates of applications, and through using the best product formulations and placement methods when applying commercial fertilizers. Recent research explored the use of the N balance as a simple and easily measured metric for sustainable N use. It is calculated as N inputs through nutrient additions minus N outputs through crop harvest on a seasonal basis. Optimum N balances are generally between 0 and +50 pounds per acre. If the N balance is below 0, the soil is being mined of nitrogen. If it is above 50 pounds per acre, there is excess that causes environmental damage. The 50 pounds per acre allowance reflects the fact that N use is never 100% efficient and some modest losses are often unavoidable under current practices. It is difficult for farmers to reach the optimum N balance range if they don’t carefully manage the nitrogen through the 4R practices (Chapter 18) and through the use of cover crops to catch excess nitrogen at the end of the season (Chapter 10). Better rotations that include crops that leach very low amounts of nitrate will reduce the average losses over the period of the rotation.

Distribution on the farm. A farm may aim to balance imports and exports of nutrients and carbon, but it also needs to aim for an optimum distribution onto its fields. For a portion of the year livestock farms typically concentrate their animals in barns or lots where the feed is brought in and the manure accumulates. It then needs to be returned to the fields, which in some cases may be distant from the barns/feedlots and more difficult to reach, especially with adverse weather. In the past, fields around barns received much more manure and typically had excess nutrients compared to those farther away. But with regular soil testing and good manure management planning, farms can balance nutrients and carbon for each individual field. Moreover, livestock farms that use well-planned rotational pasture systems, common in places like New Zealand, don’t have manure transportation issues and prevent nutrient concentration.

Unintended Nutrient Losses

Potential problems can occur even when fertilizer imports and crop exports are more or less in balance.

This chapter considers the planned flows of nutrients and carbon purchased from off the farm as fertilizers, lime and feeds, and leaving the farm in the agricultural products sold. But what about the unintended losses of nutrients? When imports are greater than exports, nutrients accumulate on the farm and significant amounts may be lost by leaching to groundwater or in runoff waters with resulting environmental damage. Nitrogen and phosphorus are the main nutrients that flow from farms that become water pollutants.

However, even when imports and exports are approximately the same, significant unintended losses may occur. This is a concern with many crops but especially with corn production, and it becomes a substantial regional problem when a large portion of the land is devoted to this crop. With high-yielding corn, the amount of N fertilizer applied may be similar to the amount taken up by the corn: perhaps 150–160 pounds of N applied in fertilizer versus 160 pounds in the corn grain. But a lot of the N applied is tied up in organic matter or lost by leaching or denitrification. On the other hand, soil organic matter decomposes during the season and can provide a lot of nitrogen to plants. Potential pollution problems arise when the combination of soil-derived and fertilizer-derived available nitrogen greatly exceeds the crop’s need.

When corn goes through its two-month-long growth spurt, it increases rapidly in height and then fills its grain. During this phase it needs to take up large quantities of nitrogen each day, usually as nitrate. This means that high concentrations of nitrate in the soil solution are needed during this period. The problem occurs when such large amounts are supplied from fertilizer or manure applications, and when the soil is also continuing to supply even more nitrate from decomposing organic matter. In most years there’s a lot of nitrate left over after corn harvest that can leach into groundwater during the fall, winter or early spring. Rates of loss depend on the amount of precipitation during these periods but can be in the range of 30–70 pounds per acre (excess nitrate may also be converted into the greenhouse gas nitrous oxide).

The best ways to decrease soil nitrate at the end of the season and to limit nitrate losses are to 1) precisely predict the crop’s seasonal needs, accounting for all N sources including fertilizer and organic; 2) time N application close to when the crop needs it; and 3) plant a quick-establishing cover crop such as cereal rye to catch excess N. But, rotations with corn appearing less frequently would also help reduce nitrate pollution of water.