Recall that under the rental share restriction a higher tenant input cost tends to come about, and that the crops chosen to be rotated will be selected more according to their gross value than according to their net worth. Equilibrium under the share restriction no longer implies the maximization of rental annuity. Instead, it will tend toward the maximization of yield value per acre of land per period of time.[6] With this reasoning, three types of crops added at the margin are detected under the share restriction, and I discuss each in turn below.
The Crop of the Internal Land Margin
The term "land margin" as it is used in land economics usually refers to "arable" land which is not cultivated. The internal land margin used here refers to the bits and pieces of land within a privately owned farm, the quality of which is so low that under a free market they are seldom utilized. In Taiwan, these margins include hill slopes and badly eroded fields, upon which something of value could be grown. Under the share restriction, the crop chosen for this marginal land was citronella, a wild perfume grass which was now widely introduced for organized cultivation.
As is shown in table 5, the crop area for citronella increased from 2.36 thousand hectares in 1948 to a peak of 16.87 thousand hectares in 1951.[7] In prefecture ranking, the response in crop area increase is clearly more significant for the prefectures in group I than for those in groups II and III. The yield per crop hectare increased slightly from 1948 to 1949, but generally declined rapidly in 1950 and 1951. The large decline in crop hectare yield after 1949 as the crop area continued to increase may be interpreted as the result of a gradual use of less suitable land margins for citronella cultivation, where the very low yield of marginal planting pulled down the average crop hectare yield.
Presumably, the decrease in crop hectare yield in 1950 cannot be explained by the exhaustion of better land margins alone. Take Sinchu, for example, where the total yield in 1949 (5.06 × 10.4) was only a fraction lower than that in 1950 (8.3 × 6.61). The 64 percent increase in crop area in 1950 led to only a 4.3 percent increase in total yield, suggesting that the yields from incremental crop area were so low that one doubts whether the land-margin-exhaustion interpretation would suffice. Planting at a rapid rate might have led to generally poor organization for citronella cultivation. Of course, there is also the possibility of random fluctuations owing to weather, etc. But, the further decline in average crop hectare yield in 1951, with further increase in crop area (hectare), again confirms the very low yield of marginal planting owing to the exhaustion of better internal land margins.
To the extent that the total yield on the given (physical) land increased as a result of increasing crop area (planting rate), as is clearly the case when we compare 1948 with any of the following three years (table 5),[8] the average product of land had increased (i.e., shifted upward). This implies that the marginal product curve of cultivated land also increased, owing to the increase in tenant inputs. Judging from the fact that the total output (crop area times the hectare yield) increased significantly more in prefectures of the higher rankings (ranked according to proportions of farms affected by the share restriction), the marginal product of land in tenant farms increased relative to farms unaffected by the restriction. To the extent that the average yield per crop hectare sharply decreased, as was clearly the case after 1949, the marginal yield of increasing tenant inputs in cultivating incremental crop hectare (area) must have fallen to a very low level. If the cost of tenant inputs required to cultivate each additional crop hectare remained constant or increased, the very low yield of tilling the incremental crop area confirms that the marginal cost of tenant inputs was higher than the associated marginal product. Relating this to the relative prefecture responses as predicted in accordance with their ranking, the evidence also confirms that the marginal product of farming resources other than land yielded lower returns in tenant farms than in owner farms. All this is implied by the hypothesis of increased farming intensity derived from my theory of share tenancy.
Crops of the Interplanting Margin
Other identifiable marginal crops behaved similarly. Here let me turn to the crops of the interplanting margin. Between the rows of a planted crop something else could be grown. If another crop is added in the space between the rows to grow at the same time, it is called interplanting. If another crop is added in between, but continues to grow alone after the initial crop is harvested, it is called relaying. To relay is to interplant partially.