On land where fish, fish-scrap, or guano, has been used freely for some years, and the crops exported from the farm, we may expect a relative deficiency of potash in the soil. In such a case, an application of unleached ashes or potash-salts will be likely to produce a decided benefit.
Clay or loamy land is usually richer in potash than soils of a more sandy or gravelly character. And on poor sandy land, the use of fish or of guano, if the crops are all sold, will be soon likely to prove of little benefit owing to a deficiency of potash in the soil. They may produce good crops for a few years, but the larger the crops produced and sold, the more would the soil become deficient in potash.
We have given the particulars of Lawes and Gilbert’s experiments on barley. Mr. Lawes at a late meeting in London, stated that “he had grown 25 crops of barley one after the other with nitrogen, either as ammonia or nitrate of soda, but without potash, and that by the use of potash they had produced practically no better result. This year (1877), for the first time, the potash had failed a little, and they had now produced 10 or 12 bushels more per acre with potash than without, showing that they were coming to the end of the available potash in the soil. This year (1877), they obtained 54 bushels of barley with potash, and 42 bushels without it. Of course, this was to be expected, and they had expected it much sooner. The same with wheat; he expected the end would come in a few years, but they had now gone on between 30 and 40 years. When the end came they would not be sorry, because then they would have the knowledge they were seeking for.”
Dr. Vœlcker, at the same meeting remarked: “Many soils contained from 1½ to 2 per cent of available potash, and a still larger quantity locked up, in the shape of minerals, which only gradually came into play; but the quantity of potash carried off in crops did not exceed 2 cwt. per acre, if so much. Now 0.1 per cent of any constituent, calculated on a depth of six inches, was equivalent to one ton per acre. Therefore, if a soil contained only 0.1 per cent of potash, a ton of potash might be carried off from a depth of 6 inches. But you had not only 0.1 per cent, but something like 1½ per cent and upwards in many soils. It is quite true there were many soils from which you could not continuously take crops without restoring the potash.”
“In all of which,” said the Doctor, “there is nothing new. It does not help us to determine whether potash is or is not deficient in our soil.”
“That,” said I, “can be ascertained only by actual experiment. Put a little hen-manure on a row of corn, and on another row a little hen-manure and ashes, and on another row, ashes alone, and leave one row without anything. On my farm I am satisfied that we need not buy potash-salts for manure. I do not say they would do no good, for they may do good on land not deficient in available potash, just as lime will do good on land containing large quantities of lime. But potash is not what my land needs to make it produce maximum crops. It needs available nitrogen, and possibly soluble phosphoric acid.”
The system of farming adopted in this section, is much more likely to impoverish the soil of nitrogen and phosphoric acid than of potash.
If a soil is deficient in potash, the crop which will first indicate the deficiency, will probably be clover, or beans. Farmers who can grow large crops of red-clover, need not buy potash for manure.
On farms where grain is largely raised and sold, and where the straw, and corn-stalks, and hay, and the hay from clover-seed are retained on the farm, and this strawy manure returned to the land, the soil will become poor from the lack of nitrogen and phosphoric acid long before there would be any need of an artificial supply of potash.
On the other hand, if farmers should use fish, or guano, or superphosphate, or nitrate of soda, and sell all the hay, and straw, and potatoes, and root-crops, they could raise, many of our sandy soils would soon become poor in available potash. But even in this case the clover and beans would show the deficiency sooner than wheat or even potatoes.