The only other nitrogen-replacement method available to farmers up to the nineteenth century was to grow cover crops of legumes such as clovers, alfalfas, peas and beans to capture atmospheric nitrogen through the bacteria growing on the roots. This practice was once part of the farmer's bible, but is slowly falling into disuse. Modern farmers find it much more profitable to apply one dose of low-cost liquid ammonia-the work of a few minutes-than to devote every second or third year to growing cover crops that bring no cash return. (In mentioning this modern trend, I am by no means giving it unqualified endorsement. I cannot help but feel that in our rush for cash income we are exhausting basic fertility in soils, using up elements which seem less critical than nitrogen, yet, when gone, will cause decreases in yields just as surely as will a nitrogen deficiency.)
Fortunately, today's gardener does not have to raid manure piles or grow cover crops in order to maintain soil fertility. For the price of a couple of movie tickets, the home grower can replace all the nutrient elements he removed in a year's harvest. The average garden plot is so small and fertilizer cost is such a minor factor that any elaborate organic system of conserving nitrogen would be pointless (except, of course, that a program of conserving and augmenting organic supplies in soils is essential for many other reasons).
NITROGEN RESERVES IN SOIL
The "furrow slice" (the depth to which a horse-drawn plow could "bite") was set years ago at seven inches. In richer, heavier soils, nitrogen tends to accumulate in this upper layer. On rich Midwestern prairie loams, a furrow slice may contain as much as 7,500 pounds of nitrogen per acre. The next lower seven inches may hold only half as much, while the nitrogen content seven inches lower is down to 25 per cent of that of the furrow slice.
In lighter soils, this accumulation pattern is reversed. The more sand and gravel a soil contains, the deeper into the soil the nitrogen tends to move. This is important to know when handling such a soil.
It suggests the importance of double digging and trenching (two soil-improvement operations described in Chapter Twelve) to bring the richer layer to the surface. Too, it suggests the value of growing deep-rooted plants which can penetrate to the layer where nutrients have accumulated.