A "rich crop" refers to crop yields, measured in kilograms per hectare of agricultural land.

Crop yields remained more or less constant in the ancient world until the advent of modern irrigation and fertilizers after the industrial revolution. Later, there was another marked increase we call the "Green Revolution" which occurred in the mid 20th century, mostly due to the scientific selection of strains bred for higher yields, increased drought resistance, and other local factors.

So what the author you're quoting seems to be saying is that crop yields in Egypt remained the same between 1,000 BC and 1950 AD.

While I cannot independently verify his statement, I find it believable for a few reasons:

• The Green Revolution arrived late in Egypt, in the 70's. So the related increase in yields didn't occur until then.

• Irrigation and fertilizers were not introduced early in Egypt because agriculture was confined to a narrow strip along the Nile. This land is naturally fertilized each year by the annual flooding of the Nile. The infrastructure needed to irrigate this small strip of land has existed since before 1,000 BC.

• The big irrigation projects such as the Aswan High Dam date to after 1950, so they are not a factor. Besides, they simply increased the area under cultivation, they did not increase the yields of land already under cultivation.

• Egyptian crop yields were uncommonly high for the ancient world, so there wasn't much room for improvement until the arrival of modern nitrogenous fertilizer and the Green Revolution.

Grain yields for ancient Egypt are quoted at around 2,000 kg per hectare. This is the highest in the ancient world. Ancient Greece was around 900 kg per hectare, ancient China around 1,200 kg per hectare, ancient India around 1,400 kg per hectare.

I can't find a published reason for this difference, but the obvious conclusion is the nature of the land. The most productive land in the absence of modern irrigation and fertilizers is an alluvial floodplain, where the river naturally replenishes the soil of nutrients lost to agriculture. The big floodplains associated with ancient civilizations were the Nile in Egypt, the Yellow River in China, the Indo-Gangetic plain in India. I don't include the Tigris/Euphrates of the fertile crescent because its floods are much less regular (once every few years instead of every year) so productivity is comparatively lower.

But in India and China, agriculture extended well beyond the floodplain into marginal areas, while in Egypt there is a sudden transition from rich farming land to desert. So the average yields in India and China might be expected to be lower because we include both the floodplain and its surrounding areas, while in Egypt we only include the floodplain.

So it sounds reasonable to me that since Egypt was already farming on land as good as you can get by natural means, it didn't see any increased yields until humans improved on the best nature can offer. I refer to nitrogenous fertilizers and modern plant breeding.

The yield estimates are from the book The Economics of Subsistence Agriculture by Colin Clark and Margaret Haswell. It's also an excellent read for information about factors that affected crop yields prior to the industrial age.