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The monitoring of soil moisture content and fertility levels at regular intervals on a crop during the growing season is a relatively new concept in crop production. This concept assumes there is an optimum level of moisture and plant nutrients in the soil and plant tissue for each stage of growth, and that the levels will change as the crop matures.
Levels of soil moisture and nitrogen and plant tissue nitrogen that have produced high yields and quality have been worked out for several crops. Examples of soil moisture, total profile nitrogen (1 foot ammonia nitrogen plus 5 feet nitrate nitrogen), and petiole nitrates for late potatoes and sugar beets sampled at 4 or 5 day intervals are given on the following two pages. For potatoes, sugar beets and most other field crops, moisture levels should be maintained between 70 and 90 percent of field capacity during the growing season. For seed crops such as grain, peas, beans, etc. the moisture level can be allowed to decline after the hard dough stage of seed development.
Optimum nitrogen level in the soil and plant tissue varies greatly with the crop and with the stage of development. For potatoes, about 250 lbs. of available nitrogen in the soil profile is adequate for germination and early development. Of this amount, at least 150 lbs. should be in the top foot. Enough nitrogen should be added during the growing season to maintain only a gradual decline during tuber development. Consumptive use of nitrogen during this development period is about 4 to 6 pounds of nitrogen per day. A carry over at harvest time of 75 to 100 pounds of nitrogen (15-20 lbs. per foot) is desirable. If the level drops below this amount, probably a deficiency and resulting yield depression occurred during final tuber development. Carry over levels much above this amount promote excess vine development, delayed maturity, and smaller tuber size which results in less yield. With these levels of soil nitrogen, the concentration of nitrate nitrogen in the petiole (petiole from fourth leaf back from tip of stem) will be about 28,000 to 32,000 ppm in the early development stage and will gradually decline to 6,000 to lOOOO ppm by the end of August. If either the soil or petiole nitrogen levels vary greatly from the proposed chart, the supplemental nitrogen additions can be increased or decreased. By evaluating both soil and petiole chemical analyses, better fertilizer application decisions can be made than by evaluating only one or the other. Observations indicate that those irrigation and fertilizer practices which produce maximum yield also give highest quality factors such as size, shape, high solids and least amount of such factors as regrowth, knobbiness and hollowheart.
For sugar beets, about 200 to 250 pounds of nitrogen in the soil profile is adequate for germination and early development. Again, the major portion of this amount should be in to the upper levels of the soil profile. This level should be maintained to be about 180 to 220 pounds on June 30. If leaching is kept to a minimum, this is adequate for the season to produce maximum yield. By August 30, the nitrogen level should be depleted to under 50 pounds In the profile (10 lbs. per foot) and by mid September nitrogen in the soil should be nil, under 4 lbs. per acre foot l ppm). Nitrogen levels above this amount will stimulate excessive top growth and depress sugar content without increasing yield. As with potatoes, those irrigation and fertilizer practices which promote maximum yield of sugar beets will also give highest quality as measured by sugar content
Optimum levels of nitrate nitrogen in the beet petioles (petiole of youngest manure leaf) at the beginning of the season, ranges between 2S,000 to 30,000 ppm. This should gradually decline to under 3,000 ppm by August 30 and under 800 ppm by mid September.
If soil or petiole nitrogen levels are much above these levels on a given date, consideration should be given to over irrigation to leach the excess nitrogen out of the root zone. If the nitrogen levels are substantially lower than these levels on a given date, a minor application of nitrogen can be made to promote a higher yield potential without depressing sugar content.
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