Spectrum Agronomic Library

Knowledge is key to using your analytic results to their fullest. The Spectrum Agronomic Library provides you with useful information that will help you to better understand the complex science of agronomy. Our agronomists will be continually adding original and reprinted articles, so check the library regularly for new information.

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Building Up and Drawing Down Soil P and K Levels

It is common to hear people discuss fertilizer recommendations in terms of crop removal, plus or minus soil test buildup as if they were somehow disconnected from each other. In fact, they are two aspects of the same subject. Most of us fall into the habit of thinking that we are fertilizing plants. Except for foliar fertilizer or tree trunk injection, we do not fertilize plants… we fertilize soil. Because of this, soil chemistry will determine how much of the applied nutrients the plants will be able to take up. If a soil is low in phosphorus (P) or potassium (K), it will tie-up or “fix” much of the applied fertilizer P and K (P2O5 and K2O) into forms that are not available to the plants. This nutrient fixation is simply another way of saying that the soil is trying to build itself up in these nutrients… whether that is your intention or not. The soil is a reservoir for the nutrients that have been applied or generated by other means over the years. The nutrients that a plant gets in any one season are likely to be ones that have been in the soil for many years. Therefore you can think of fertilization as putting nutrients into one side of a reservoir while the plants are taking them out of another end. What happens inside of this nutrient reservoir is soil chemistry and microbiology. These processes, along with weather, determine how much access the plants have to the nutrients within the reservoir.

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Plant Nutrients in Municipal Leaves

Application of collected municipal leaves to agricultural land improves soil quality and provides a solution to a disposal problem. Farmers are permitted (New Jersey Register, NJAC 7:26, 1.12. Nov. 7, 1988) to apply up to a 6-inch layer of leaves annually. Application at this rate, which is equivalent to approximately 800 cubic yards/acre or 20 tons/acre of dry matter. This will increase soil organic matter content, and improve soil tilth and water holding capacity.

A chemical analysis of 100 municipal leaf samples collected from across New Jersey shows that leaves are a valuable source of all crop nutrients (Table 1). Although nutrient concentration values vary considerably, the application of 20 ton/acre of leaves would add on average 400 pounds of nitrogen, 40 pounds of phosphorus, and 152 pounds of potassium. Assuming values of $.30/pound N, $.23/pound P, and $.18/pound K, the nutrients from this example are worth $156.56.

Application of leaves at 20 ton/acre would also add on average 656 pounds of calcium, 96 pounds of magnesium, 44 pounds of sulfur, 1.5 pounds of boron, 58 pounds of iron, 22 pounds of manganese, 50 pounds of chloride, 4 pounds of sodium, 0.3 pounds of copper, and 3 pounds of zinc. The actual amounts of nutrients applied can vary considerably as shown by the concentration ranges in Table 1.

Although leaves add agronomically significant amounts of nutrients, only a portion of the nutrients are available immediately after application for use by the crop. The increase in the soils total nutrient content will, however, contribute to the long term fertility of the soil as the nutrients are released over time.

Much of the nutrients in leaves are part of the organic structure of the plant tissue and require microbial decomposition to release them. The carbon-nitrogen ratio of an organic material undergoing decomposition is an important indicator factor in the rate of release of its nitrogen in available form. The average carbon nitrogen ratio of leaf waste is 50 and it ranges from 27 to 72. For comparison, the carbon-nitrogen ratio of compost is generally about 25.

The abundant carbon (carbohydrates which provide energy) content of leaves leads to extensive development of fungi and bacteria in the soil which uses up the supply of available nitrogen for the production of microbial cell tissue. As decay proceeds, the carbon-nitrogen ratio decreases and some of the nitrogen becomes available to plants. Because of the high carbon content of raw leaves relative to their nitrogen content, there will likely be very little of the organic nitrogen in leaves available to crops for a period of time after application. Observations of crops (including legumes) planted on soil to which leaves have been applied indicate that plants suffer from a temporary N deficiency unless additional N fertilizer is added.

Crops grown on soils the year after leaf application likely will need additional N fertilizer. Legume crops, such as soybean, may benefit from 20 to 30 pounds of starter N banded beside the row at planting. This will supply a readily available N source to be used by the legume until it forms nodules to supply N by fixation. An additional 50 to 100 lbs of N fertilizer is recommended for corn grown the first year after leaf application. The additional fertilizer that is required increases the cost of crop production on the soil the first year after an application of leaves.

The amounts of P, K, and other nutrients present in leaves are not easily translated into nutrient credits that may be used to reduce fertilizer application. These nutrients are relatively stable in soil and can be monitored simply through soil testing. As soil fertility levels increase as a result of leaf applications, take credit for these nutrients by fertilizing accordingly.

Of the three major nutrients, potassium is the most easily released from leaves and is the most readily available to crops in the first year after leaf waste application. A minimum nutrient credit of 50 lbs K2O per acre may be used for 20 tons of leaves.

Application of collected municipal leaves to soil should not significantly change its agricultural limestone requirement. Three years of municipal leaf application caused no decrease in the soil pH compared to unamended soils.

Table 1 Nutrient concentrations in municipal leaves (dry weight basis).
Nutrient Minimum Maximum Average Average
Concentration % Lb.ton
Carbon 36 52 47 940
Nitrogen 0.66 1.62 1.00 20.0
Phosphorus (P2O5) 0.02 (0.05) 0.29 (0.66) 0.1 (0.23) 2.0 (4.6)
Potassium (K2O) 0.09 (0.11) 0.88 (1.06) 0.38 (0.46) 7.6 (9.1)
Calcium 0.13 3.04 1.64 32.8
Magnesium 0.02 0.46 0.24 4.8
Sulfur 0.01 0.21 0.11 2.2
Nutrient Parts per million Lb/ton
Boron 7 72 38 0.076
Iron 46 9800 1461 2.922
Manganese 19 1845 550 1.100
Zinc 22 392 81 0.162
Sodium 36 325 110 0.220
Chlorine 68 3995 1264 2.528
Copper 2.8 31.5 8.1 0.016
Cobalt 0.9 10.9 2.7 0.005
Nickel 1.1 57.9 5.3 0.0106

Phosphites and Phosphates

With the cost of fertilizers and other crop inputs continuing to rise, there are some additional forms of fertilizer coming to the market place. One of the problems with the products coming on the market is there confusion with the traditionally used fertilizer materials. One of the products that is coming to light is the phosphite fertilizers manufactured using phosphorous acid (H3PO3). Not to be confused with the phosphate materials that are traditionally used, which are manufactured using phosphoric acid (H3PO4).

Rather than reinventing the wheel and writing an article of our own, there have recently been two very good articles written. One appeared in Better Crops in 2006, and the other appeared in New Ag International in September of 2007. Both of these articles do a good job of bringing to light the differences in these products, how they can be used as well as benefits of each product.

Phosphite Fertilizers: What Are They? Can You Use Them? What Can They Do? (290KB) By C.J. Lovatt and R.L. Mikkel, Better Crops/Vol 90 (2006, No 4) pgs 11-13.

Phosphites and Phosphates: When Distributors and Growers alike could get confused! (415KB) By Jean-Pierre Leymonie. Courtesy of New Ag International, September 2007 edition.

library/start.txt · Last modified: 2010/03/31 11:41 by wayland