When any nitrogenous organic or fertilizer substances are added to the soil, numerous bacteria, fungi, and other soil organisms attack the materials, breaking them down into the nitrate (NO3-) form. The soil conditions that especially favor this process are good drainage, good aeration, warm soil temperature, adequate moisture, good organic matter supply, and a satisfactory pH range of 6.0 to 7.5.

Conversely in some soils a process of de-nitrification may take place where nitrate reverts to nitrite, possibly back to the ammonia form, and even to gaseous nitrogen. Such conditions may prevail in tight, soggy soils with pH range below 5.8 and low organic matter supply.

Estimating Nitrogen Needs

The most important factor in determining the nitrogen need is the crop to be grown. When the desired crop yield goal has been established, the nitrogen requirement can be computed. Your soil test report reveals the organic matter content and from this can be calculated the pounds of estimated nitrogen release for the growing season, taking into consideration the soil type and its geographical location. This estimated nitrogen release should be added to the reported test nitrogen carried over or present in the full soil root zone as nitrate and as ammonia in the surface foot. This total is then subtracted from the total nitrogen required by the crop to attain a calculated and reasonable potential yield. The difference or net is the approximate amount of nitrogen to be applied as fertilizer, but does not include losses from leaching, volatization, erosion, etc.

Other Factors to Consider

The plus or minus effects (on the total nitrogen supply) of crop residues, manure applications, legumes, nitrogen sources and forms, and at what time during the growing season the organic nitrogen will be released are other factors to be considered, and should be included in calculations of crop nitrogen need.

Plant Residues

The table below indicates some approximate amounts of nitrogen added to or subtracted from the soil by various ground covers, mulches, and crop residues worked into the soil, and serve as an estimate or guideline for inclusion in the calculation of total nitrogen needs.

Nitrogen Carry-Over

The carry-over of nitrogen in dry years is a factor which must be considered in calculating the nitrogen needs. During periods of little or no leaching and low crop production, much of the applied nitrogen plus as much as one-half the amount of nitrogen from normal nitrification may be carried over to the next growing season. However, under conditions of high rainfall and severe leaching, no nitrogen carryover can be counted and in fact, mild to severe losses of applied nitrogen may be encountered.

Nitrogen and Irrigation

The necessity for nitrogen control, under irrigation, poses added problems as well as potentially greater benefits. Under irrigation an excess of water is generally applied to avoid the build up of soil salts. In addition many high yielding, high value, intensive production crops are sensitive to moisture stress and suffer yield reductions even though the soil is still relatively high in available moisture, or drops to low levels for only short periods of time. This leads to frequent irrigations when the additional soil holding capacity is very limited. Nitrate nitrogen goes into water solution readily and quickly and moves with the water penetration front.

Given the high solubility of nitrates, the need for slight amounts of leach water, and the need to prevent moisture stress, it becomes readily apparent that leach losses of nitrogen can be very serious if irrigation is not closely controlled. Excessive water application can completely destroy the best of fertility programs.

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