Chapter 4. Nutrient Requirements


Tables 3. Estimated Fertilizer Requirements - Field Crops - 6. Mineral Requirements Relative To Nitrogen show the nutrient contents and typical fertilizer use for a variety of crops. Although they represent averages, an acquaintance with local yields should lead to an estimate of nutrient requirements.

The tables follow the usual convention in expressing quantities of phophorus as phosphate (P2O5) and potassium as potash (K2O).

Several options are available for estimating the soil supply.

A careful inspection of plant roots can reveal information about conditions affecting growth.

Fertilizer Uptake

A reasonable first step in choosing the amount of fertilizer that is likely to be necessary for growing crops is to determine how much of a particular nutrient is used by the crops. Considerable information is available, but it is so inconsistent that one is tempted to give up the attempt. The only hope is that plants can be grouped into types having similar requirements, and by taking enough averages, one might obtain a reasonable estimate of plant requirements.

Tables 3. Estimated Fertilizer Requirements - Field Crops and 4. Estimated Fertilizer Requirements - Vegetables And Fruits are summaries of yields, nutrient uptake and fertilizer use in growing field crops and vegetable crops for market1.

In both tables, the first column of data states the range of crop yield and the average yield in tons/acre. The yields of some crops are often reported as bushels/acre, but using tons for some crops and bushels for others may be confusing. Instead, a conversion factor is shown for those crops which are reported in bushels.

So, for example, a bushel of corn weighs 56 lbs and a bushel of potatoes 60 lbs. To find the average yield of corn in bushels, one divides 8000 (the average number of lbs/acre - 4 x 2000) by 56 to get about 140 bushels of corn/acre; a similar calculation for potatoes (14 x 2000 / 60) leads to an average yield of about 470 bushels of potatoes/acre.

Also, yields are listed for crops in their normal marketable state. The consequence is that the moisture content is not taken into account. For example, hay yields are for field-dried hay, and vegetable yields represent fresh weight.

The remaining columns show the nutrient removal by the crops and residues. The quantity of nutrients is expressed as lbs per ton of harvested crop. For example, the average amount of nitrogen in field corn is about 51 lbs/ton. With an average yield of 4 tons/acre, the total amount of nitrogen removed by the grain is 51 x 4, or 204 lbs/acre. The average amount of nitrogen removed by the residues is 29 x 4, or 116 lbs/A. The total amount of nitrogen removed by both crop and residues is 51 + 29, or 80 lbs/ton of crop; with a 4 ton harvest, the average total nitrogen removed is 320 lbs/acre.

Most people know the yields that are likely in their area; so they could approximate the removal of nutrients in proportion to the yield. They should realize, however, that variations are due not only to the yield, but also to varietal differences in the crop, differences in the soil, fertilization, and the weather. Average nutrient requirements shown here are probably satisfactory for most crops, but clearly they are not absolute.

As an example of how the tables might be useful, suppose we want to know the potassium requirement of alfalfa grown in New England. From table 3. Estimated Fertilizer Requirements - Field Crops , we take the average nutrient content, 43 lb/ton, and multiply it by a typical yield for New England of 5 tons/acre, to get 215 pounds potash/acre. For comparison, the average fertilizer rate is 28 lb/ton, which for 5 tons/acre, results in a fertilizer use of 140 lb potash/acre. These two estimates are consistent if we assume that the average soil supplies about 75 pounds potash/acre. This is not much potassium - many soils furnish more than that - and some farmers may spread more potassium fertilizer than they need.

As another example, let us estimate the nitrogen requirement of potatoes. The average tuber yield is 14 lb/ton. The average indicated yield of 14 tons/acre would result in a total nitrogen requirement of 308 lb/acre. At an average fertilizer use of 19 lb/ton, the total average fertilizer applied to get the same yield is 266 lb/acre. Both figures represent a lot of nitrogen, more than can reasonably be supplied organically. Quite possibly potatoes are over-fertilized, and a lower total nitrogen rate, perhaps 15 lb/ton, is more reasonable, making the total nitrogen requirement 210 lb/acre. If the soil supplies 50-100 lb/acre, then the fertilizer requirement, assuming no losses, is about 100-150 lb/acre, still a lot to supply organically, although a heavy application of manure or a year of clover might be enough. A deep, rich topsoil, however, will furnish more than 100 lb of nitrogen/acre and bring the project closer to practicality.

Potatoes are a drain on the soil; growing them organically at high yields requires considerable effort. This is also true of corn.

Tables 3. Estimated Fertilizer Requirements - Field Crops and 4. Estimated Fertilizer Requirements - Vegetables And Fruits are unnecessary for backyard gardeners and most market gardeners who grow mixed crops. As a more useful guide, table 5. Average Nutrient Requirements For Vegetables summarizes table 4. Estimated Fertilizer Requirements - Vegetables And Fruits by ignoring the variations and using average values for the total nutrient content of vegetables. The nitrogen content of beans, peas and peanuts is not shown, on the assumption that the seeds of these plants are inoculated before being sown, and they should fix whatever nitrogen they need. The nutrient requirements are expressed first as lbs/acre, and then, in parentheses, lbs/1000 sq ft. The average requirements for all vegetables leads to an estimate of 81 pounds of nitrogen/acre, 31 pounds phosphate, 122 pounds potash, and 17 pounds sulfur.

Table 6. Mineral Requirements Relative To Nitrogen shows the balance that should be maintained among the four principal nutrients. The phosphate/nitrogen, potash/nitrogen and sulfur/nitrogen ratios are based upon the average values in table 4. Estimated Fertilizer Requirements - Vegetables And Fruits .

Generally, the phosphate/nitrogen ratio is about 1/3 for hay and slightly higher for other crops. The potash/nitrogen ratio is approximately 1 for hay and grains, and it varies between 1 and 2 for vegetables. Sulfur is usually about 1/10 to 1/5 of the nitrogen content. The high phosphate/nitrogen ratio shown for peppers is probably inaccurate, based upon only one experiment; the low phosphate/nitrogen ratio for parsley is also suspect.

1 Data for yields and nutrient removal are from [14], [23], [30], [46], [51], [70], [72], [49], [61], [1]. Fertilizer use is from [68] and is a summary of data from states where the crop is economically important.    [return to text]

© 2013 Robert Parnes

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